Chemistry · 2020. 4. 4. · Chemistry Ordinary Level Marking Scheme (400 marks) Answer eight questions in all. These must include at least two questions from Section A. All questions

Page 1 of 52

L.33/34

Pre-Leaving Certificate Examination, 2017

Chemistry

Marking Scheme

Ordinary Pg. 4

Higher Pg. 29

2017.1 L.33/34_MS 2/56 Page 2 of 55 examsDEB


Chemistry

Ordinary & Higher Levels

Table of Contents

Ordinary Level Higher Level Section A Section A

Q.1 .................................................. 4 Q.1 .................................................. 29

Q.2 .................................................. 7 Q.2 .................................................. 31

Q.3 .................................................. 9 Q.3 .................................................. 33

Section B Section B Q.4 .................................................. 11 Q.4 .................................................. 35

Q.5 .................................................. 13 Q.5 .................................................. 38

Q.6 .................................................. 15 Q.6 .................................................. 40

Q.7 .................................................. 17 Q.7 .................................................. 42

Q.8 .................................................. 19 Q.8 .................................................. 44

Q.9 .................................................. 20 Q.9 .................................................. 46

Q.10 .................................................. 22 Q.10 .................................................. 48

Q.11 .................................................. 25 Q.11 .................................................. 51

examsDEB



Chemistry

Ordinary & Higher Levels

Explanation

Conventions Used

1. A dash – before an answer indicates that the answer is a separate answer, which may be considered as independent of any other suggested answers to the question.

2. A single forward slash / before an answer indicates that the answer is synonymous with that which preceded it. Answers separated by a forward slash cannot therefore be taken as different answers.

3. A double forward slash // is used to indicate where multiple answers are given but not all are required.

4. Round brackets ( ) indicate material which is not considered to be essential in order to gain full marks.

5. ‘etc.’ is used in this marking scheme to indicate that other answers may be acceptable. In all other cases, only the answer given or ‘words to that effect’ may be awarded marks.

6. In calculations, 3 marks are deducted for a mathematical error but no further penalty is incurred if the problem, otherwise correct, is completed.

7. Answers which are given in this marking scheme should not be considered as the only possible answers that may be accepted. Answers which are synonymous with or equivalent to those in this marking scheme are also acceptable.

Current Marking Scheme

Assumptions about these marking schemes on the basis of past SEC marking schemes should be avoided. While the underlying assessment principles remain the same, the exact details of the marking of a particular type of question may vary from a similar question asked by the SEC in previous years in accordance with the contribution of that question to the overall examination in the current year. In setting these marking schemes, we have strived to determine how best to ensure the fair and accurate assessment of students’ work and to ensure consistency in the standard of assessment from year to year. Therefore, aspects of the structure, detail and application of the marking schemes for these examinations are subject to change from past SEC marking schemes and from one year to the next without notice.

examsDEB



Chemistry

Ordinary Level Marking Scheme (400 marks)

Answer eight questions in all. These must include at least two questions from Section A.

All questions carry equal marks (50).

SECTION A

Answer at least two questions from this section.

Section A Question 1 (50 marks)

1. A group of students prepared a sample of soap in a school laboratory. A mixture of about 3 g of animal fat, 2 g of sodium hydroxide (NaOH) and 25 cm3 of ethanol was refluxed for about 20 minutes. The apparatus was then rearranged and the reaction flask heated to remove the ethanol by distillation. The residue in the distillation flask was dissolved in a little boiling water and the contents of the flask were emptied into a beaker containing brine. The soap precipitated out of the mixture as a solid. It was removed from the brine and washed with a small amount of ice-cold water. The diagrams below show the arrangements of some of the apparatus used in the preparation.

(a) Identify the arrangement of apparatus (A or B) which is used for (8)

(i) the reflux stage of the preparation, (4m)

– (apparatus) B

(ii) the distillation stage of the preparation. (4m)

– (apparatus) A

examsDEB

Apparatus A Apparatus B


Section A Question 1 (cont’d.)

(b) Draw a rough sketch of either arrangement of apparatus in your answer book and clearly indicate which part of the condenser should be connected to the cold tap. (6m) (6)

– drawing should show water entering through the lower intake point

or

(c) What is the purpose of the ethanol? (6m) (12)

– acts as a solvent for the animal fat

What material was added to the reaction flask before heating is commenced?

Any 1: (3m) – anti-bumping chips // – small pieces of glass // – small pieces of pumice stone // – porcelain chips (bits, pieces) // etc.

Why was this material added to the reaction flask?

Any 1: (3m) – to prevent bumping // – to help boiling take place more smoothly

(d) Why was only a small amount of boiling water used to dissolve the residue remaining in the distillation flask after the distillation? (6)

Any 1: (6m) – to maximise yield // – to minimise loss

(e) Describe how to remove the soap from the brine. (2 × 3m) (6)

– filter solution of brine and residue – wash the precipitated soap with more brine to remove any NaOH still present

Waterin

Water in

2017 LC Chemistry [OL]



(f) Name an alternative to animal fat with which a different kind of soap could be made. (6)

Any 1: (6m) – coconut oil // – olive oil // – palm oil // – vegetable oil // etc.

(g) How would you test the hardness of a water sample using the soap you made? (2 × 3m) (6)

– place a sample of the soap in a test tube containing water and shake – observe the lathering properties of the soap sample: - soft water easily forms a lather with soap - hard water does not form a lather / forms a scum with soap




2. To prepare 250 cm3 of a standard solution of sodium carbonate (Na2CO3), 2.52 g of solid anhydrous sodium carbonate was weighed out on a clock glass using an electronic balance. The entire solid was carefully transferred from the clock glass into a beaker and a small amount of deionised water was added. When the solid was dissolved, the solution was transferred to a 250 cm3 volumetric flask and deionised water was added until the bottom of the meniscus rested on the graduation mark. The volumetric flask was stoppered and inverted 20 times. Some of the equipment used is shown below.

BA

(a) Name the pieces of equipment A and B. (2 × 3m) (6)

A – funnel B – wash bottle

(b) What is meant by the terms (i) standard solution, (ii) primary standard in volumetric analysis? (12)

(i) standard solution (6m)

– a solution of known (exact) concentration

(ii) primary standard

Any 1: (6m) – a substance from which a solution of exact concentration can be made // – a substance which is stable, available, pure and water soluble

(c) Why is deionised water used to make up the solution? (6)

Any 1: (6m) – so as not to add any other species / contaminants / other substances / anything which

might react // – to ensure purity (accuracy)

(d) Why is the volumetric flask stoppered and inverted 20 times? (4m) (4)

– to completely mix the solution / to make the solution homogeneous

** Award 3m for ‘help dissolve completely’.




(e) What is meant by describing the sodium carbonate used as anhydrous? (4m) (4)

– (it) contains no water of crystallisation

(f) The solution contains 2.52 g of anhydrous sodium carbonate (Na2CO3) in 250 cm3 of solution. Calculate the concentration of the solution in moles per litre. (9)

– (3m)

(3m)(3m)

106

452.2

– = 0.095 M

** Award 3m for correct calculation of Mr.

(g) The resulting solution was used to titrate a solution of hydrochloric acid (HCl) of unknown concentration. (9)

Name a suitable indicator for this titration.

State the colour change at the end point.

** Colours must match indicator named. ** Award 3m if colours correctly named but given in reverse order.

Indicator Colour change

Any 1: (3m) Any 1: (2 × 3m) – methyl orange // – orange (yellow) to – red (accept peach or pink)

– methyl red // – yellow to – red

– phenolphthalein // – pink (purple, violet, red) to – colourless

– thymolphthalein // – blue to – colourless

– thymol blue // – blue to – yellow

– cresol purple // – purple (pink, violet) to – yellow

– neutral red // – yellow-brown (yellow, brown) to – red

– phenol red // – red to – yellow

– bromothymol blue – blue to – yellow




3. (a) Describe how you would carry out a flame test on a sample of salt. (12)

Any 4: (4 × 3m) – using clean / description of cleaning // – platinum (nichrome) / soaked (dipped) // – wire (probe) / splint (lollipop stick) // – dip wire (splint) into salt // – place salt in (into, on, at edge of, at top of) flame // – note (observe) colour

(b) Copy the table into your answer book and complete it, matching the metallic element with the colour that it imparts to a Bunsen flame. (3 × 3m) (9)

Flame Test Results

Flame colour Orange-Yellow Green Lilac

Metallic element – sodium Any 1: – barium // – copper

– potassium

(c) What precaution would you take to prevent cross-contamination during the flame test experiment? (5)

Any 1: (5m) – use a clean platinum wire / splint each time to avoid cross-contamination // – make sure the platinum wire is thoroughly cleaned by dipping it into HCl before each test // – use separate mortar and pestle to crush salt

(d) Name the reagents used to test for (i) chloride (Cl–), (ii) nitrate (

3NO ) and

(iii) sulfate ( 24SO ) ions in aqueous solution. (24)

(i) chloride (Cl–) (4m)

– silver nitrate solution

(ii) nitrate ( 3NO ) (4m)

– iron(II) sulfate and concentrated sulfuric acid

(iii) sulfate ( 24SO ) (4m)

– barium chloride solution




(d) (cont’d.)

State the observations made that clearly confirmed the presence of each of the above.

(i) chloride (Cl–) (4m)

– a white precipitate forms

(ii) nitrate ( -3NO ) (4m)

– a brown ring forms

(iii) sulfate ( -24SO ) (4m)

– a white precipitate forms (which will not dissolve with the addition of hydrochloric acid)



SECTION B

Section B Question 4 (50 marks)

4. Answer eight of the following (a), (b), (c), etc. Award 1 bonus mark to each of the first two fully correct answers.

(a) Identify one natural product that is extracted from plant material by steam distillation. (6)

Any 1: (6m) – clove oil (eugenol) // – rose oil // – oil of lavender // – citrus / orange / lemon oil // etc.

(b) What term describes a chemical reaction for which the value of H is positive? (6m) (6)

– endothermic

(c) State two characteristics of ionic compounds. (6)

Any 2: (2 × 3m) – contain a network of ions in a crystal lattice // – usually hard and brittle // – have high melting and boiling points // – usually solid at room temperature // – conduct electricity in molten state or when dissolved in water as the ions are free to move

(d) Give one industrial source of hydrogen gas. (6)

Any 1: (6m) – steam reforming of methane / natural gas // – electrolysis of water

(e) Give an example of a useful product of organic synthesis. What use is made of this product? (6)

** Example (3m), Use (3m). ** Use should correspond to example named.

Any 1: (2 × 3m) Product Use

– aspirin – painkiller // – penicillin – antibiotic // – ibuprofen – painkiller // – morphine – painkiller // – paraquat – weedkiller // – DDT – insecticide // – malathion – insecticide



Section B Question 4 (cont’d.)

(f) What is the volume in litres of 6 × 1023

gaseous molecules at s.t.p.? (6m) (6)

– 22.4 litres

(g) Why is the domestic water supply chlorinated? (6m) (6)

– to disinfect / kill bacteria / germs

(h) Write the arrangement of the electrons in the main energy levels of a calcium (Ca) atom. (6m) (6)

– 2,8,8,2

(i) Calculate the percentage by mass of iron in iron(II) sulfate (FeSO4). (6m) (6)

– 56

×100152

= 36.84%

** Allow for rounding.

(j) A 250 cm3 sample of mineral water has a sodium content of 0.018 g. Express the concentration of sodium in parts per million (p.p.m.). (2 × 3m) (6)

– 0.018×1000

250= 0.072 g/l

– 0.072 × 1000 = 72 p.p.m.

(k) A State one method by which nitrogen is fixed in nature. (6)

Any 1: (6m) – lightning // – bacteria (micro-organisms) // – volcano

** Accept ‘legumes’ or ‘clover’.

or

B What happens in the process called galvanising? (3m)

– metal (iron) dipped in molten zinc / iron coated with zinc

Why is galvanising carried out?

Any 1: (3m) – prevents corrosion // – stops rusting // – sacrificial metal




5. (a) The nucleus of an atom was discovered by a scientist, pictured on the right, in an experiment in which he directed a beam of particles from a radioactive source at a thin sheet of gold.

(i) Name the scientist. (5m) (5)

– (Ernest) Rutherford

(ii) What type of radioactive particles did he use? (5m) (5)

– alpha particles / α-particles

(iii) State three observations he made and the conclusion he drew in each case. (3 × 4m) (12)

** Observation (2m), Conclusion (2m). ** Conclusion should correspond to observation.

Observation Conclusion

– alpha particles deflected – particles passed close to small, positive charge or mass

– alpha particles reflected – particles collided with small, very dense mass (nucleus)

– alpha particles went straight through – most of the atom was empty space

(b) The following scientists have made an important contribution to our knowledge of atomic structure and the elements: (12)

Bohr Dalton Boyle Thomson

Identify the scientist from the above list who:

(i) described atoms as small, indivisible particles, (3m)

– Dalton

(ii) discovered that electrons were sub-atomic particles, (3m)

– Thomson

(iii) defined an element as a substance that could not be broken down into simpler substances, (3m)

– Boyle

(iv) proposed that electrons move around the nucleus in orbits. (3m)

– Bohr




(c) (i) Define relative atomic mass (Ar). (5m) (5)

– average mass of an atom of the element relative to one-twelfth the mass of an atom of carbon-12

(ii) Define atomic radius (covalent radius). (2 × 3m) (6)

– half the distance between the nuclei of two atoms of the same element – that are joined together by a single covalent bond.

(iii) How does atomic radius change across a period in the periodic table? (5m) (5)

– (atomic radius) decreases




6. (a) Crude oil is a mixture of hydrocarbons, which are separated into various fractions and are used as fuels and for other purposes. (14)

(i) Name the process that is used in an oil refinery to separate the fractions in crude oil. (4m)

– fractional distillation

(ii) What physical property of the molecules causes them to be separated in this way? (4m)

– boiling point

(iii) Name the fraction that is used (i) in diesel-powered motor vehicles and (ii) in surfacing roads.

(i) in diesel-powered motor vehicles (3m)

– gas oil

(ii) in surfacing roads (3m)

– fuel oil (and residue)

(b) (i) Give two situations in which there may be a hazardous build up of methane gas. (9)

Any 2: (2 × 3m) – in coal mines // – in dumps // – in fuel tanks or fuel tankers // – in sewage-effluent treatment systems // – in livestock sheds / slurry tanks on farms // etc.

(ii) Why are the levels of methane in the lower atmosphere a cause of environmental concern?

Any 1: (3m) – methane is a greenhouse gas // – methane causes / contributes to global warming

(c) (i) Explain what is meant by the octane number of a fuel. (2 × 3m) (18)

– the measure of the ability of a fuel – to resist or to cause ‘knocking’ / autoignition

(ii) What is the significance of the compound 2,2,4-trimethylpentane in relation to octane numbers?

Any 1: (6m) – it is a reference hydrocarbon // – it is assigned an octane number of 100




(c) (cont’d.)

(iii) Draw the structural formula of 2,2,4-trimethylpentane. (6m)

–

(d) Methane (CH4) is commonly used as a fuel. When 3.2 g of methane undergoes complete combustion, 178 kJ of energy is released. The equation for the reaction is: (9)

CH4 + 2O2 CO2 + 2H2O

Calculate the heat of combustion of methane. (3 × 3m)

– moles of CH4 16

2.3 = 0.2 moles

– 0.2 moles - 178 kJ of heat

– 1 mole = 2.0

178 = –890 kJ/mol

** Deduct 2m if negative sign is not given.

CH3 CH3

CH3CH3

CH3

H

H

C C

H

C




7. In 1884 the Swedish chemist Arrhenius proposed a new theory of acids and bases.

(a) Define an acid according to the theory of Arrhenius. (4m) (4)

– a substance that dissociates in water to produce H+ ions

(b) Give an example of (i) a common household acid, (ii) a common household base. (6)

(i) a common household acid

Any 1: (3m) – vinegar (ethanoic acid) // – lemon juice (citric acid) // – car battery acid (sulfuric acid) // etc.

(ii) a common household base.

Any 1: (3m) – cleaning products (ammonia) // – oven cleaner (sodium hydroxide) // – indigestion tablets (magnesium hydroxide) // etc.

(c) Explain the term neutralisation. (2 × 3m) (9)

– a reaction between an acid and a base – to form a salt and water

Give one everyday example of neutralisation.

Any 1: (3m) – baking soda to neutralise bee stings // – toothpaste to neutralise food acids // – Alka-seltzer / antacid product to neutralise excess stomach acid // etc.

(d) (i) Define pH. (13)

Any 1: (4m) – the negative logarithm to the base 10 of the hydrogen ion concentration measured in

moles per litre // – pH = –log10[H

+], where the square brackets indicate concentration in moles per litre

(ii) Describe how you could measure the pH of a solution. (3 × 3m)

– dip / spot / add / insert probe // – universal indicator / pH meter // – compare to chart to read pH / read meter




(e) The concentration of a solution of sulfuric acid (H2SO4) is 4.9 grams per litre. (18)

(i) What is the concentration of the solution in moles per litre?

– 4.9

=98

(3m)

(3m) 0.05 M (3m)

(ii) Calculate the pH of the solution. (3 × 3m)

pH = –log10[H+] – [H+] = 0.05 × 2 = 0.1 – pH = –log10[0.1] – = 1




8. Study the reaction scheme below and answer the questions that follow.

X Y C2H5OH

A

CH3CHO

BCH3COOH

C

(a) Give the IUPAC names for A, B and C. (3 × 4m) (12)

A – ethanol B – ethanal C – ethanoic acid

(b) Name the homologous series to which B belongs. (5m) (5)

– aldehyde

(c) Which of the compounds A, B and C is found: (8)

(i) at a concentration of 6% to 15% (v/v) in wine, (4m)

– A / ethanol

(ii) at a concentration of approximately 6% (v/v) in vinegar? (4m)

– C / ethanoic acid

(d) Which of the compounds A, B and C contains only tetrahedral carbon atoms? (6m) (6)

– A / ethanol

(e) Conversions X and Y are the same reaction type. (10)

(i) What term is used to describe this type of reaction? (4m)

– oxidation (dehydrogenation)

(ii) What reagents could be used to bring about both of these conversions? (2 × 3m)

– sodium dichromate / Na2Cr2O7 – sulfuric acid / H2SO4

(f) Describe how you would test product B with Fehling’s reagent. (3 × 3m) (9)

– add a little B to Fehling’s solution in a test tube – heat (warm) gently – red precipitate (precipitate of copper(I) oxide, cuprous oxide, Cu2O) formed




9. (a) State Le Châtelier’s principle. (4m +3m) (7)

– when a system at equilibrium is subjected to a stress, such as a change in temperature, pressure or concentration

– the system will alter to oppose / minimise the effect of the stress

(b) Explain the term chemical equilibrium. (7)

Any 1: (4m + 3m) – when the rate of forward reactions – is equal to the rate of the reverse reactions

or

– when the concentrations of reactants and products – are constant

(c) Ammonia is prepared by reacting nitrogen (N2) gas with hydrogen (H2) gas according to the following balanced equation. (36)

N2 + 3H2 2NH3 H = –92.4 kJ

(i) What is the name given to the process used to produce ammonia? (6m)

– Haber process

(ii) What does the symbol tell us about the reaction? (6m)

– the reaction is an equilibrium process / forward and reverse reactions occurring simultaneously

** Allow ‘reversible reaction’.

(iii) Write the equilibrium constant (Kc) expression for this reaction. (6m)

– Kc = 3

22

23

]][H[N

][NH

** Award 3m for top or bottom correct.

(iv) Using Le Châtelier’s principle, state whether you would use (i) a high or low temperature and (ii) a high or low pressure, in order to favour the production of ammonia.

Give a reason for your choice for each condition.

** Condition given (3m), Reason given (3m). ** Reason should correspond to the condition given.

Condition (2 × 3m) Reason (2 × 3m)

– low temperature – because forward reaction is exothermic – high pressure – fewer molecules on the right




(c) (cont’d.)

(v) Is the ammonia (NH3) gas produced soluble or insoluble in water? (3m)

– soluble

Give a reason for your answer. (3m)

– (ammonia) is polar



Section B Question 10 (2 × 25 marks)

10. Answer any two of the parts (a), (b) and (c).

(a) (i) What is the function of a catalytic converter in modern cars? (6)

Any 1: (6m) – to reduce harmful emissions // – to convert environmentally harmful gases into harmless gases

(ii) Name or give the symbol of two metallic elements used as catalysts in a catalytic converter. (6)

Any 2: (2 × 3m) – rhodium / Rh // – platinum / Pt // – palladium / Pd

(iii) Name an element that poisons the catalyst present in a catalytic converter. (3)

Any 1: (3m) – lead // – arsenic // – sulfur

(iv) Identify the two gaseous products formed when nitrogen monoxide (NO) and carbon monoxide (CO) react together in a catalytic converter. (2 × 3m) (6)

– nitrogen / N2 – carbon dioxide / CO2

(v) What term describes the type of catalysis that occurs in the catalytic converter of a car? (4)

Any 1: (4m) – heterogeneous // – adsorption




(b) (i) Describe how you would measure the free chlorine concentration in swimming pool water or bleach using a colorimeter or a comparator. (4 × 3m) (12)

Colorimeter

Any 4: – take known volume of water // – add iodide and acid // – to release iodine / colour develops // – add sample to cuvette // – insert in colorimeter // – take reading // – compare value with calibration graph

Comparator

Any 4: – take known volume of water / fill the compartment(s) with water // – add DPP tablet(s) // – crush / stir // – colour develops // – add sample to cuvette // – compare colour with colour card

(ii) Explain the principle on which the technique you have described above is based. (4m + 3m) (7)

– colour developed – is directly related to the concentration

(iii) Explain the importance of regularly monitoring free chlorine levels in swimming pools. (6m) (6)

– to maintain appropriate levels / to ensure chlorine levels don’t get too low (high)




(c) A mass of 16.25 g of zinc reacted with hydrochloric acid to produce zinc chloride and hydrogen gas according to the following equation.

Zn + 2HCl ZnCl2 + H2

The hydrogen gas liberated was collected.

(i) How many moles of zinc were used up? (4m) (4)

– 16.25

65 = 0.25 (moles)

(ii) How many moles of hydrochloric acid were needed to react fully with this amount of zinc? (6m) (6)

– 0.25 × 2 = 0.5 (moles)

(iii) What mass of zinc chloride was produced? (6m) (6)

– 0.25 × 136 = 34 g

(iv) What volume of hydrogen gas (measured at s.t.p.) was produced? (9m) (9)

– 0.25 × 22.4 = 5.6 litres




11. Answer any two of the parts (a), (b) and (c).

(a) (i) Name the scientist, pictured on the right, who was awarded a Nobel Prize for her work on the isolation of two radioactive elements. (6m) (6)

– Marie Curie

(ii) One of these radioactive elements is named in honour of the country of her birth. Name both radioactive elements. (2 × 3m) (6)

– polonium – radium

(iii) Radioactive sources can emit alpha (α-), beta (β-) and gamma (γ-) radiation. Arrange these three types of radiation in order of increasing penetrating power. (7m) (7)

– alpha (-), beta (-), gamma (-)

** Award 3m if correctly ordered by decreasing penetration power.

(iv) Cobalt–60 (60Co) is an isotope of cobalt which emits γ-rays. State one use made of this type of radiation. (6)

Any 1: (6m) – cancer treatment // – food irradiation // – irradiation of medical instruments




(b) (i) Name and give the formula of a calcium compound that causes temporary hardness in water. (6)

Name (3m) Formula (3m)

– calcium hydrogen carbonate – Ca(HCO3)2

(ii) Water with temporary hardness forms a limescale when boiled. (12) Write the name and formula of the chemical limescale. (2 × 3m)

– calcium carbonate – CaCO3

Describe a test that would identify its presence in limescale.

Any 2: (2 × 3m) – add hydrochloric acid to the solid // – fizzing observed // – gas produced (CO2) turns limewater milky

(iii) What is the purpose of flocculation in water treatment? (4m) (7)

– to cause the small, suspended particles present in water to stick together and form larger, heavier particles called flocs

Identify a substance that causes flocculation. (3m)

– aluminium sulfate / Al2(SO4)3




(c) Answer part A or part B.

A

Nitrogen and oxygen are the major component gases in air. (25)

(i) How is oxygen gas produced commercially from air? (4m)

– liquefaction / fractional distillation / diffusion

(ii) State one commercial use of oxygen and one commercial use of nitrogen.

Commercial use of oxygen

Any 1: (3m) – breathing apparatus, e.g. scuba tanks, medical oxygen tanks // – rocket fuel // – to remove impurities from iron // – cutting torches / tools // – to combat pollution in lakes // etc.

Commercial use of nitrogen

Any 1: (3m) – food packaging // – flushing oil tankers // – medicine // – quick-freezing food // – storing bull semen // – ammonia synthesis // etc.

CFCs are believed to be the main cause of damage to the ozone layer.

(iii) What do the letters CFC represent? (3m)

– chloroflurocarbons

What use was made of CFCs before they were banned for general use?

Any 1: (3m) – air conditioning unit gases // – refrigerant gases // – making expanded polystyrene // – aerosol propellant




(c) A (cont’d.)

(iv) What is the formula for ozone? (5m)

– O3

State one consequence of damage to the ozone layer.

Any 1: (4m) – increased incidence of UV light reaching the surface of the Earth // – contributes to global warming / climate change // – increased risk of skin cancer // – damage to ecosystems, e.g. bleaching of coral reefs, damage to phytoplankton

in oceans // etc.

or

B

(i) Give two general properties of transition metals. (25)

Any 2: (4m + 3m) – form coloured compounds // – act as catalysts // – have variable valency // etc.

Iron can be made using a blast furnace, similar to the one shown on the right

(ii) Identify the three materials added to the top of the blast furnace. (3 × 3m)

– iron ore / named ore of iron – limestone / CaCO3 – coke / charcoal / carbon

(iii) What is the molten iron produced in the blast furnance converted to? (6m)

– steel

(iv) What use is made of the slag co-product? (3m)

– cement

Exhaustgas outlet

Hot airSlagoutlet

Molten ironoutlet

Three materials added




Chemistry

Higher Level Marking Scheme (400 marks)

Answer eight questions in all. These must include at least two questions from Section A.

All questions carry equal marks (50).

SECTION A

Answer at least two questions from this section.

Section A – Question 1 (50 marks)

1. In the analysis of domestic bleach containing sodium hypochlorite (NaClO), exactly 25 cm3 of the bleach solution was placed in a 250 cm3 volumetric flask and diluted to the mark with deionised water. A 25 cm3 portion of this solution was placed in a conical flask to which dilute sulfuric acid (H2SO4) and potassium iodide (KI) were added.

The resulting solution was titrated against a 0.1 M sodium thiosulfate (Na2S2O3) solution. The three titration figures recorded were 25.4 cm3, 25.0 cm3 and 25.1 cm3, respectively.

The equations for the reactions are:

ClO– + 2H

+ + 2I

– Cl

– + H2O + I2

2S22

3O + I2 S42

6O + 2I–

(a) In order for the reaction to take place hydrogen (H+) ions are needed. The source of these H+ ions is H2SO4. Why should nitric acid (HNO3) not be used? (3m) (3)

– (nitric acid is) an oxidising agent

(b) Give two reasons why excess potassium iodide is used. (2 × 3m) (6)

– to ensure all the hypochlorite ions react / iodide ions kept in excess to ensure that the amount of I2 formed depends on the amount of hypochlorite present / hypochlorite is the limiting reactant //

– to keep the iodine in aqueous solution

examsDEB



(c) Why is the bleach diluted? (5)

Any 1: (5m) – if undiluted, an excessive amount of KI and Na2S2O3 would be required to get a reasonable

titration figure – if undiluted, very concentrated KI and Na2S2O3 solutions would be required to get a

reasonable titration figure // etc.

(d) What sequence of colours was observed in the conical flask from the start of the titration until the end point was reached? (4 × 3m) (12)

– red-brown (I2 liberated in the conical flask) – straw yellow (upon addition of Na2S2O3) – blue-black (upon addition of starch indicator) – colourless (at the end point)

(e) Outline three operations involving the conical flask and its contents during the titration. (9)

Any 3: (3 × 3m) – swirl to mix // – allow time after addition from burette for reaction to occur // – wash down sides of the flask with deionised water // – place flask on a white tile

(f) Calculate the concentration of NaClO in moles per litre (9)

(i) in the diluted bleach sample, (2 × 3m)

–

2

05.251.0

1

251 M

M1 25 = 1.2525

– M1 = 25

2525.1 = 0.05 M

(ii) in the original bleach sample. (3m)

dilution factor 25 cm3 in 250 cm3 - 1 in 10 dilution – 0.05 10 = 0.5 M

(g) Find the concentration of NaClO in the original bleach (6)

(i) in grams per litre, (3m)

– 0.5 74.5 = 37.25 g/l

(ii) as a percentage (w/v). (3m)

– 37.25 g/l = 3.725 g/100 cm3 = 3.725 % (w/v)

2017 LC Chemistry [HL]



2. A sample of soap was prepared in the school laboratory by refluxing a mixture of 6.3 g of glyceryl tristearate (obtained from animal fat), 3 g of sodium hydroxide pellets (an excess) and 25 cm3 of ethanol in a round bottomed flask.

At the end of the experiment 5 g of pure dry soap was isolated.

C17H35COOCH2

C17H35COOCH2

C17H35COOCH

HOCH2

HOCH2

HOCH3C17H35COONa+ 3NaOH +

Glyceryl tristearate Sodium stearate Glycerol

(a) What term is used to describe the reaction that takes place between the sodium hydroxide and the glyceryl tristearate? (3m) (3)

– saponification / base hydrolysis of an ester

(b) What other substance is added to the flask prior to refluxing? (3)

Any 1: (3m) – boiling chips // – anti-bumping granules // – small pieces of pumice stone // – glass beads // etc.

(c) What is the purpose of heating the reaction mixture under reflux? (6)

Any 2: (2 × 3m) – to speed up the reaction // – to boil without loss of solvent (ethanol) vapour // – to maximise the yield of soap // – to drive the reaction to completion // – to give time for a complete reaction

(d) What is the function of the ethanol? (3m) (6)

– to act as a solvent for the animal fat

Why is it desirable to remove the ethanol after reflux?

Any 1: (3m) – to make it easier to isolate the soap // – to avoid waste of ethanol // – to maximise the yield of soap as it remains dissolved in ethanol




(e) Describe with the aid of a clearly-labelled diagram how ethanol is removed after the reflux stage of the experiment. (9)

Diagram (3m)

Description (2 × 3m)

– flask with condenser in position for distillation to remove ethanol in solution – thermometer (heat with water bath, distil at 78 C) and heat source

–

** Deduct 3m if no diagram.

(f) How is the soap isolated from all the other substances left in the reaction mixture? (4 × 2m) (8)

– dissolve residue in minimum of boiling (hot) water – reaction mixture added to brine / saturated sodium chloride solution / pour onto brine – the precipitated soap is then obtained by filtration / filter – wash with more brine / wash with a little ice water

(g) Where is the co-product of the reaction located at the end of the experiment? (3m) (3)

– (in) brine / filtrate

** Do not accept ‘in the flask’ unless correct flask specified by drawing or description.

(h) Given that the sodium hydroxide was in excess, calculate the percentage yield of soap (sodium stearate). (4 × 3m) (12)

– 6.3

890 = 0.007079 mol glyceryl tristearate

– 0.007079 × 3 = 0.02123 (theoretical yield) – 0.02123 × 306 = 6.5 g soap (theoretical yield)

– 5.6

5 × 100 = 76.92%

condenser

thermometer

ethanol

heat source

water bath

distilling flask

anti-bumpinggranules




3. An experiment was carried out to measure the relative molecular mass of a volatile liquid.

(a) Name a liquid suitable for use in this experiment. (5)

Any 1: (5m) – propanone (acetone) (56 C) // – propanal (49 C) // – methanol (65 C) // – trichloromethane (chloroform) (6 1C) // – hexane (69 C) // etc.

(b) Draw a labelled diagram of an apparatus that could be used to determine the relative molecular mass of the liquid. (9)

** Diagram (0m, 3m, 6m). ** Any 3 labels: (3 × 1m)

Any 1: (9m)

Apparatus 1

– flask and tinfoil with pinhole immersed in beaker of water

– immersed so at least half is under water

Apparatus 2

– gas syringe with self-sealing cap // – surrounded by heating device / oven / steam jacket / beaker of water0

(c) Describe how (i) the mass, (ii) the volume of the vapour is determined. (15)

** Method given should correspond to the apparatus drawn above.

(i) the mass

Any 1: (3 × 3m)

Method 1

– measure mass of flask – allow liquid to completely vaporise – cool and reweigh – subtract

volatile liquid

clamp

foil top with a hole

heat source

water bath

Inject here

syringe

oven

thermometer




(c) (cont’d.)

Method 2

– measure mass of hypodermic syringe with liquid – inject liquid into gas syringe and reweigh hypodermic syringe – find difference

(ii) the volume

Any 1: (2 × 3m)

Method 1

– fill flask with water and empty into a graduated (measuring cylinder) – measure volume of water in graduated (measuring) cylinder

Method 2

– measure the volume of hot air in the gas syringe – subtract from the volume of liquid measured in the gas syringe

(d) How may the pressure be measured? (3m) (3)

– using a barometer / pressure sensor

(e) A mass of 0.12 g of the liquid was vaporised by heating it at 100 C in a suitable apparatus. The volume of the vapour was 84.5 cm3 and the pressure was 9.5 × 104 Pa.

Calculate the relative molecular mass of the volatile liquid. (15)

Moles vaporised (3 × 3m)

PV = nRT – (9.5 × 104) × (84.5 × 10–6) = n × (8.3) × (373) – 8.0275 = n (3095.9) – n = 2.59 × 10-3

Relative molecular mass (2 × 3m)

2.59 × 10-3 moles = 0.12 g

– Mr = 31059.2

12.0

– Mr = 46.3

(f) Why is this method unsuitable for non-volatile liquids? (3)

Any 1: (3m) – they do not vaporise easily // – their boiling points are too high // – their boiling points are too near (higher than) boiling point of water / 100 C (373 K) // – they do not vaporise below 100 C



SECTION B


4. Answer eight of the following (a), (b), (c), etc. Award 1 bonus mark to each of the first two fully correct answers.

(a) State Charles’ law. (6m) (6)

– at constant pressure, the volume of a fixed mass of gas is directly proportional to its temperature measured on the Kelvin scale (absolute zero)

(b) Define bond energy. (2 × 3m) (6)

– the average energy required to break a bond (1 mole of a covalent bond) – and separate the neutral atoms / into separate (single) atoms / in the gaseous state

(c) Draw a reaction profile diagram for an endothermic reaction indicating clearly on your diagram the activation energy (EA) for the reaction. (2 × 3m) (6)

– axes correctly labelled and suitable curve with energy of reactants less than energy of products

– EA (between reactant energy and peak) correctly labelled

–

(d) Distinguish between sigma (σ) and pi (π) covalent bonding. (2 × 3m) (6)

– sigma bonding: head on overlap of orbitals – pi bonding: sideways overlap of orbitals

(e) What is the oxidation number of (i) phosphorus in 34PO and (ii) sulfur in Na2S2O3? (2 × 3m) (6)

(i) phosphorus in 34PO

– +5

(ii) sulfur in Na2S2O3

– +2

Reaction progress

Reactants

Activation energyProducts

∆H (+)

Ener

gy




(f) What is the percentage by mass of iron in iron(III) oxide (Fe2O3)? (2 × 3m) (6)

– 100160

112

– = 70%

(g) Distinguish between an atomic orbital and an energy sublevel. (2 × 3m) (6)

atomic orbital

– a region of space within an atom where the probability of finding an electron is high

energy sublevel

– a group of atomic orbitals within an atom, all of which have the same energy

(h) Rutherford’s experiment which led to the discovery of the nucleus involved bombarding particles at a thin foil. (6)

What was the nature of the bombarding particles? (3m)

– alpha particles

From what material was the foil made? (3m)

– gold foil

(i) Under what circumstances can ionic compounds conduct electricity? (2 × 3m) (6)

– when molten – when dissolved in water

(j) What colour is observed in a flame test on a salt of (i) lithium and (ii) potassium? (6)

(i) lithium (3m)

– deep red

(ii) potassium (3m)

– lilac




(k) Answer part A or part B.

A List two factors that would influence the site location of a chemical factory. (6)

Any 2: (2 × 3m) – availability of raw materials // – availability of a water supply // – skilled workforce available // – good rail / road links // – near a port // – near third level / university // – local demand for product // – important for economy // etc.

or

B State two properties of transition metals. (6)

Any 2: (2 × 3m) – form coloured compounds // – act as catalysts // – have variable valency / different oxidation states




5. (a) (i) Define electronegativity. (5m) (5)

– the relative attraction of an atom for shared pairs of electrons in a covalent bond

(ii) Describe and account for the trends in electronegativity values of the elements across the second period of the periodic table. (6 )

Describe (3m)

– trend in electronegativity is to increase

Account

Any 1: (3m) – increase due to increasing nuclear charge – increase due to decreasing atomic radius

(iii) Use electronegativity values to predict the bonding in (a) ammonia (NH3), (b) aluminium oxide (Al2O3), (c) hydrogen sulfide (H2S). (3 × 3m) (9)

(a) ammonia (NH3),

– 3.04 – 2.20 = 0.84 - polar covalent

(b) aluminium oxide (Al2O3),

– 3.44 – 1.61 = 1.83 - ionic

(c) hydrogen sulfide (H2S).

– 2.58 – 2.20 = 0.38 - very slightly polar covalent

(iv) Explain why ammonia has a higher boiling point (–33 C) than hydrogen sulfide (–60 C). (2 × 3m) (6)

– hydrogen bonding present between NH3 molecules is stronger (hydrogen is bonded to the more electronegative element nitrogen / the strongest type of molecular force)

– dipole-dipole interactions in hydrogen sulfide are weaker

(b) (i) Write the s, p electron configuration of a sulfur atom in its ground state. (6m) (6)

– 1s2,2s2,2p6,3s2,3p4

(ii) How are the electrons in the 3p sublevel arranged in orbitals in an atom of sulfur? (6m) (6)

– 3px2,3py

1,3pz1

or

–

px py pz




(b) (cont’d.)

(iii) Draw the shape of the three p orbitals. (3 × 2m) (6)

– dumbell shape drawn –

(iv) Give a reason in terms of electron configuration why phosphorus has a greater stability than sulfur. (6)

Any 1: (6m) – phosphorus has a half-filled 3p3 sublevel, which has extra stability – sulfur has a 3p sublevel that is neither filled nor half-filled; losing an electron gives a

stable 3p3 configuration

z

y

x

z

y

x

z

y

x

3py3px 3pz




6. (a) (i) Define heat of combustion of a substance. (6m) (6)

– the heat change in kilojoules when one mole of substance is completely burned in excess oxygen

(ii) Name the piece of equipment used to measure the heats of combustion of substances. (3m) (3)

– bomb calorimeter

(iii) Write the balanced equation for the complete combustion of ethyne in an adequate supply of oxygen. (15 )

Any 1: (6m) – C2H2(g) + 2½O2(g) 2CO2(g) + H2O(g) – 2C2H2(g) + 5O2(g) 4CO2(g) + 2H2O(g)

** Equation correct (3m). ** Equation balanced (3m).

Calculate the heat of combustion of ethyne, given that the heats of formation of water, carbon dioxide and ethyne are –285.8, –393.5 and 227.4 kJ mol–1, respectively.

Hc = Hf(2CO2 + H2O) – Hf(C2H2 + 2½O2) – Hc = (2 (–393.5)+(–285.8)) – (227.4 + 2½O) (3m) – Hc = –787 – 285.8 – 227.4 (3m) – Hc = –1300.2 kJ mol–1 (3m)

(b) (i) Name two processes that are carried out in an oil refinery to modify the hydrocarbon structures and improve their octane number. (6)

Any 2: (2 × 3m) – dehydrocyclisation / reforming // – catalytic cracking// – isomerisation

(ii) Name two components of LPG. (2 × 3m) (6)

– butane – propane

(iii) Why are mercaptans added to natural gas? (3m) (3)

– for safety / to give natural gas an odour / to make it easier to detect a gas leak




(c) Name a fraction, obtained from crude oil, that is used to make petrol. (3)

Any 1: (3m) – naphtha // – light gasoline

(d) Name and draw the structures of two isomers of the hydrocarbon C4H8. (8)

Any 2: (2 × 4m)

Name (1m) Draw (3m)

– but-1-ene // –

– but-2-ene // –

– 2-methylpropene // –

H

H

C

H

HC

H

H

H

H

CC

H

H

C

H

HC

H

H

H

H

C C

H

H

C

H

H

C

H

H

HH

C C




7. (a) Distinguish between the terms temporary and permanent hardness in water, referring to (i) a compound which causes each type of hardness and (ii) a method of removal. (8)

(i) Compound that causes Compound that causes

Temporary hardness Permanent hardness

Any 1: (2m) Any 1: (2m) – calcium hydrogen carbonate / Ca(HCO3)2 // – calcium sulfate / CaSO4 // – magnesium hydrogen carbonate/ Mg(HCO3)2 – magnesium sulfate / MgSO4

(ii) Method of removal

Temporary hardness Permanent hardness

(2m) Any 1: (2m) – boiling – ion exchange // – distillation // – using washing soda

(b) (i) Name the reagent used to determine, by titration, the total hardness present in a water sample. (3m) (12)

– ethylenediaminetetraacetic acid / edta

(ii) Name the indicator used in this titration and state the colour change observed at the end point.

Indicator used

Any 1: (3m) – eriochrome black T // – solochrome black T

Colour change at end point (2 × 3m)

– wine red – blue

(c) Describe what takes place in the primary and secondary treatment of sewage. (12)

Primary treatment (2 × 3m)

– removal of solids (large particles, floating debris, large items, twigs, etc.) – by screening through large metal screens and settlement (sedimentation, grit channels) in a

settling tank

Secondary treatment

Any 2: (2 × 3m) – the aerobic bacterial breakdown of nutrients present in effluent – in a trickling filter or an activated sludge unit – churning of liquid in aeration tanks to maintain levels of dissolved oxygen




(d) Some sewage treatment plants carry out a tertiary treatment process, which further reduces the levels of certain substances. (6)

(i) What are these substances? (3m)

– nitrates and phosphates

(ii) Why is tertiary treatment considered necessary? (3m)

– to prevent eutrophication

(e) A sample of polluted lake water was analysed to determine the concentration of suspended and dissolved solids. A 600 cm3 sample of the water was found to contain 0.03 g of suspended solids and 0.96 g of dissolved solids. (12)

Calculate the concentration, in parts per million, of

(i) suspended solids, (2 × 3m)

– (0.03×1000)

600 = 0.05 g/l

– 0.05 × 1000 = 50 p.p.m.

(ii) dissolved solids. (2 × 3m)

– (0.96×1000)

600 = 1.6 g/l

– 1.6 × 1000 = 1600 p.p.m.




8. A mixture of 15 moles of hydrogen and 15 moles of iodine is allowed to come to equilibrium in a closed vessel at 773 K. Initially, the colour of the mixture is deep purple due to the high concentration of iodine vapour. The purple colour fades and when equilibrium is established the colour of the mixture is pale pink and there are 23 moles of hydrogen iodide present. The chemical equilibrium established is represented below.

H2(g) + I2(g) 2HI(g) H = 51.8 kJ colourless purple colourless

(a) Explain the term chemical equilibrium. (5m) (5)

– the state in which the rate of forward reaction is equal to the rate of the reverse / backward reaction

(b) When the colour of the mixture becomes pale pink, has the reaction ceased? (3m) (6)

– no

Justify your answer.

Any 1: (3m) – chemical equilibrium is a dynamic state // – concentration of reactants and products unchanged (remain the same) because rates equal // – losses in one direction are balanced by (equal to) gains in the other direction // – both forward and reverse reactions still occur

(c) (i) Write the equilibrium constant (Kc) expression for the reaction. (6m) (18)

– 2

2 2

[HI]

[H ][I ]

** Award 3m for top or bottom correct.

(ii) Calculate the value of the equilibrium constant (Kc) for the reaction at 773 K.

Equation H2 I2 2HI

Start (2m) 15 15 0

Change (3m) –11.5 –11.5 +23

At equilibrium (3m) 3.5 3.5 23

– Kc =2[23]

[3.5][3.5]

– = 43.18 (4m)




(d) State Le Châtelier’s principle. (2 × 3m) (6)

– when a system at equilibrium is subjected to a stress – the equilibrium shifts in such a way as to minimise the effect of the stress

(e) What effect, if any, would (i) a decrease in temperature and (ii) an increase in pressure have on the yield of hydrogen iodide? Use Le Châtelier’s principle to justify your answer in each case. (12)

(i) a decrease in temperature (3m)

– decreases the yield of hydrogen iodide

Why?

Any 1: (3m) – reaction shifts in the exothermic direction // – reaction shifts backwards to increase the temperature // – reaction disfavours the forward endothermic reaction

(ii) an increase in pressure (3m)

– no change

Why?

Any 1: (3m) – equal numbers of molecules (moles) on both sides of equation // – pressure does not change / has no effect on the number of molecules (moles)

for this reaction

(f) What effect would a decrease in temperature have on the colour intensity of the equilibrium mixture? (3m) (3)

– darker (deeper, more intense) pink (purple) / more purple (light purple, pinker)




9. Study the reaction scheme and answer the questions that follow.

C2H4

A BC2H5OHX Y

Z

CH3COOC2H5D

CCH3CHO

(a) Give the IUPAC names for A, B, C and D. (4 × 2m) (8)

A – ethene B – ethanol C – ethanal D – ethyl ethanoate

(b) Identify the type of reaction involved in (6)

(i) conversion X, (3m)

– addition

(ii) conversion Z. (3m)

– substitution / condensation / esterification

(c) Compound B reacts with which organic compound to form compound D? (3m) (3)

– ethanoic acid

(d) Name the homologous series to which compound D belongs. (3m) (3)

– ester




(e) Draw the structure of compound D and indicate on your diagram (i) a carbon atom that has a planar geometry and (ii) a carbon atom that has tetrahedral geometry. (12m) (12)

** Diagram (6m). ** Planar carbon marked (3m). ** Tetrahedral carbon marked (3m).

–

(f) What reagents are required to bring about conversion Y? (2 × 3m) (6)

– Ni / nickel – hydrogen gas (H2)

(g) Compound A reacts with bromine (Br2) to form C2H4Br2. (12)

(i) Describe the mechanism of this reaction.

** Accept written description or appropriate reaction equations.

Any 3: (3 × 3m) – polarisation of Br2 / Brδ+– Brδ– under influence of double bond // – heterolytic fission of the bromine molecule / splitting into ions / Br2 → Br+ +

Br– // – addition of bromonium ion (Br+) across the double bond / addition of Br+ forming

bridged intermediate (cyclic bromonium ion) // – addition of bromide (Br–) to bridged intermediate / addition of bromide ion (Br–) to

cyclic Br+ / addition of bromide ion (Br–) to carbonium ion (C+)

** Accept ‘localised carbonium ion’ attack (addition) of bromide ion (Br–) to the bridged intermediate {cyclic bromonium ion, carbonium ion (C+)}.

(ii) State one piece of experimental evidence to support this mechanism.

Any 1: (3m) – other products are formed when the reaction is carried out in the presence of other

nucleophiles (anions, negative ions), e.g. Cl– (NaCl, HCl) / OH– (H2O)

** Accept a specific example, e.g. 2-bromoethanol formed using bromine water (Br2 /H2O).

HH

H

H

CH

H

C

H

H

C

O

OCCC

C Ctetrahedral carbonplanar carbon




10. Answer any two of the parts (a), (b) and (c). (2 × 25)

(a) A certain indicator which is a weak acid dissociates in water as follows:

HIn(aq) H+

(aq) + In–

(aq)

(i) Define a weak acid according to the theory of Arrhenius. (3m) (3)

– a substance that only slightly dissociates in aqueous solution to form hydrogen ions

(ii) Define pH. (6m) (6)

– the negative logarithm to the base 10 of the hydrogen ion concentration expressed in mol/l

(iii) Calculate the pH of a 0.01 M solution of the indicator at 25 C. The Ka value for the indicator is 2 × 10–5 mol l–1 at 25 C. (3 × 3m) (9)

pH = –log10[H+]

[H+] = AK [acid]

– [H+] = 5102 (0.01)

– pH = –log10[0.00044721]

– = 3.35

(iv) The indicator changes colour in the pH range 3.7 – 5.7. State a type of acid-base titration for which it would be suitable. Justify your answer. (4m + 3m) (7)

Suitable acid-base titration

Any 1: – strong acid - weak base – strong acid - strong base

Justify answer

– the pH range of the indicator lies within the pH range of these titrations and will therefore give a sharp endpoint




(b) In 1922, Francis Aston, pictured on the right, was awarded the Nobel Prize in Chemistry for detecting the existence of isotopes using the first mass spectrometer.

(i) What is the principle of the mass spectrometer? (3 × 3m) (9)

– positive ions (charged particles) separated (deflected, spread out)

– based on (according to) relative mass(es) (charge-to-mass ratio)

– when moving in a magnetic field

** Allow ‘weight’ for ‘mass’. ** Allow ‘between electromagnets’ for ‘in a magnetic field’.

(ii) What are the fundamental processes that occur in mass spectrometry? (5 × 2m) (10)

– vaporisation – ionisation – acceleration – separation – detection

(iii) A sample of the element boron is composed of 19% boron–10 and 81% boron–11.

Calculate the relative atomic mass of boron from this information. (6)

in 100 atoms of boron there are – 81 atoms of mass 11 = 81 ×11 = 891 (2m) – 19 atoms of mass 10 = 19 × 10 = 190 (2m) total mass of 100 atoms = 1081

– relative atomic mass of boron = 1081

100 = 10.81 (2m)



Section B – Question 10 (cont’d.)

(c) (i) Name the piece of apparatus shown that can be used in the electrolysis of acidified water. (4m) (4)

– Hofmann voltameter

(ii) Suggest a suitable material for the electrodes. (6)

Any 1: (3m) – graphite // – platinum

Why is this material suitable? (3m)

– inert / does not react with the electrolyte into which it is placed

(iii) At which electrode, A or B, does oxidation occur? (3m) (3)

– B / anode / positive electrode

(iv) Which species is oxidised? (3m) (9)

– water / H2O

Write a balanced equation for the oxidation reaction that takes place.

Any 1: (6m) – H2O 2H+ + ½O2 + 2e– / 2H2O 4H+ + O2 + 4e– // – 3H2O ½O2 + 2H3O

+ + 2e– / 6H2O O2 + 4H3O+ + 4e– //

– H2O – 2e– ½O2 + 2H+ / 2H2O – 4e– O2 + 4H+ // – 3H2O –2e– ½O2 + 2H3O

+ / 6H2O – 4e– O2 + 4H3O+

** Award 3m for a balanced equation representing the oxidation of a hydroxide ion. ** Equation correct (3m). ** Balanced (3m)

(v) How would you identify the gas that is produced at the cathode? (3m) (3)

– burns with a squeaky pop (hydrogen)


+-BATTERY

BA



11. Answer any two of the parts (a), (b) and (c). (2 × 25)

(a) A sample of ethanoic acid (CH3COOH) was prepared by reacting 8.5 cm3 of ethanol (density 0.8 g cm–3) with an acidified solution containing 29.8 g of sodium dichromate(VI) (Na2Cr2O7.2H2O).

The reaction is described by the following balanced equation.

3C2H5OH + 272O2Cr + 16H

+ 3CH3COOH + 4Cr

3+ + 11H2O

After purification, it was found that 4.3 g of ethanoic acid was formed.

(i) What is meant by limiting reagent? (4)

Any 1: (4m) – the reagent that controls the amount of product formed in a reaction // – the reactant that is fully used up in the reaction // – the substance not present in excess

(ii) Determine the limiting reagent in the above reaction. (4 × 3m) (12)

– mass of ethanol = 8.5 0.8 = 6.8 g

– moles of ethanol = 46

8.6 = 0.148 moles

– moles of dichromate = 298

8.29 = 0.1 moles

– 0.148 moles of ethanol

3

2 = 0.0987 moles dichromate /

0.1 moles dichromate

2

3 = 0.15 moles ethanol,

therefore ethanol is the limiting reagent

(iii) Calculate the percentage yield of ethanoic acid. (3 × 3m) (9)

– theoretical yield of ethanoic acid = 0.148 × 60 = 8.88 g

– % yield = 10088.8

3.4

– = 48.42%




(b) (i) What colour of light is associated with the line emission spectrum of sodium? (4m) (4)

– yellow

(ii) Describe how Bohr used line emission spectra to explain the existence of energy levels in an atom. (4 × 3m) (12)

– electrons in ground state / electrons restricted to energy levels (energy values, discrete energies) / energy level diagram / energy of electron quantised

– fixed energies (photons) absorbed / jump (move, promoted) to higher level(s) (excited state) / energy only emitted (absorbed) when electrons move between fixed levels

– excited state / unstable electrons fall back to lower levels / electrons fall back, emitting energy as light (electromagnetic energy, photons) / emitting discrete packets (photons) of energy

– energy difference between levels gives specific (definite) frequency (wavelength, line) of light in spectrum / energy of photons dictated by energy gap between levels / E2 – E1 = hf

(iii) Why does each element have a unique line emission spectrum? (2 × 3m) (6)

– each element has a different distribution (set, arrangement) of energy levels / different electron configurations

– giving rise to different electron transitions

(iv) Bohr’s atomic theory was later modified. Give one reason why his atomic theory was updated. (3)

Any 1: (3m) – only worked for simple (hydrogen, hydrogen-like) species (atoms) // – wave nature of electron (wave-particle duality) // – Heisenberg uncertainty principle (or its statement) // – discovery of sublevels (subshells) // – could not explain the Zeeman effect // – it did not explain splitting of lines




(c) Answer part A or part B.

A

Write a brief note on the contribution made by (i) Dorothy Hodgkin, and (ii) Lawrence and William Bragg to the understanding of crystal structures. (4m + 3m) (7)

(i) Dorothy Hodgkin,

– used X-ray crystallography to find structure of complex molecules, e.g. penicillin, vitamin B12 (cobalamin)

(ii) Lawrence and William Bragg

– developed X-ray crystallography (X-rays to find crystal structures)

Give an example of a (i) molecular crystal, (ii) covalent macromolecular crystal. (6)

(i) molecular crystal,

Any 1: (3m) – iodine // – solid carbon dioxide // – naphthalene // – sulfur (S8) // – solid hydrogen chloride // – ice

(ii) covalent macromolecular crystal.

Any 1: (3m) – diamond // – quartz (silicon dioxide, SiO2)

Distinguish between molecular crystals and covalent macromolecular crystals using the following headings: (6 × 2m) (12)

(i) binding forces,

Molecular crystals Covalent macromolecular crystals

– Van der Waals’ forces – strong covalent bonds and dipole-dipole attractions or hydrogen bonding


(ii) melting point,

– low – high


(iii) hardness.

– soft – hard




11. (c) (cont’d.)

B

(i) Describe, with the aid of chemical equations, how an increase in the number of cars has contributed to the increased concentration of NO2 in the atmosphere in the last sixty years. (3 × 3m) (9)

– spark plug (electrical discharge) / high temperature / compression (high pressure) in the ignition system provides the energy for (allow facilitates) the first reaction

– N2 + O2 2NO / ½N2 + ½O2 NO – 2NO + O2 2NO2 / NO + ½O2 NO2 /

** Award 6m for the combined equation N2 + 2O2 2NO2 or ½N2 + O2 NO2, but 0m unless fully correct and balanced.

(ii) Give one major commercial use for nitrogen gas. (3m) (3)

Any 1: (3m) – in flushing (purging) oil tanks // – as an inert atmosphere // – in preserving food / in keeping food fresh / in packaging food (crisps) // – over gas (oil, flammables) in tankers (being transported) // – in glass production // – in semiconductor (micro-chip) production // – to dilute atmospheric oxygen // – in the production of ammonia (urea, nitric acid, fertilisers) // etc.

(iii) Explain why nitrogen gas is chemically inert. (4)

Any 1: (4m) – high energy (strong, difficult to break) bond // – it is non-polar // – it contains a triple bond

(iv) What is meant by nitrogen fixation? (3m) (9)

– the conversion of atmospheric nitrogen to nitrogen compounds that can be used (useful compounds, chemically reactive)

State two ways by which nitrogen is fixed in nature.

Any 2: (2 × 3m) – electrical storms (lightning) // – legumes (Rhizobium) (named legume) // – alder (Plasmodiophorales) // – nitrogen-fixing bacteria (Azotobacter, Clostridium, Klebsiella) // – blue-green algae (Cyanophyceae, Anabaena) // – photosynthetic bacteria



Notes:



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Chemistry · 2020. 4. 4. · Chemistry Ordinary Level Marking Scheme (400 marks) Answer eight questions in all. These must include at least two questions from Section A. All questions