1 3k State Symbols and Ionic Equations

8/10/2019 1 3k State Symbols and Ionic Equations

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Exam Questions

Q1.Most compounds of lead are insoluble, an exception being lead(II) nitrate. Therefore agood method of preparing lead(II) sulfate is

A adding dilute sulfuric acid to lead metal.

B adding concentrated sulfuric acid to lead metal.

C adding dilute sulfuric acid to lead(II) nitrate solution.

D adding dilute sulfuric acid to solid lead(II) oxide.

Q2.An important reaction which occurs in the catalytic converter of a car is

In this reaction, when 500 cm3of CO reacts with 500 cm3of NO at 650 C (theoperating temperature of the catalyst) and at 1 atm, the totalvolume of gases producedat the same temperature and pressure is

A 500 cm3

B 750 cm3

C 1000 cm3

D impossible to calculate without knowing the molar volume of gases under theseconditions.

(Total for Question = 1 mark)Q3.Sodium burns in oxygen to give a pale yellow solid X.(a) (i) 1.73 g of sodium reacts with 1.20 g of oxygen.

Calculate the empirical formula of X.(2)

(ii) The molar mass of Xis 78 g mol1. Give the molecular formula of X.(1)

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(iii) Write the equation, including state symbols, for the reaction of sodium with oxygen to produce X.

(2)

1.3.k State Symbols and ionic equations

Students will be assessed on their ability to:k. carry out and interpret the results of simple test tube reactions, such as displacements, reactions of acids,

precipitations, to relate the observations to the state symbols used in equations and to practise writing full and ionicequations.


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(iv) Calculate the volume of oxygen in dm3(at room temperature and pressure) which reactswith 1.73 g of sodium. (The molar volume of any gas at room temperature and pressure=24dm3mol1.)

(2)

(v) Calculate the number of oxygen moleculesthat react with 1.73 g of sodium.(The Avogadro constant = 6.02 1023mol1.)

(1)

>(b) If sodium is burnt in air, compound Xis not the only product. Suggest why this is so.(1)

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(Total for question = 9 marks)

Q4.

When aqueous solutions of barium chloride and potassium sulfate are mixed, a whiteprecipitate forms. The ionic equation for the reaction is

A K+(aq) + Cl(aq) KCl(s)

B K2+(aq) + 2Cl(aq) KCl2(s)

C Ba+(aq) + SO4(aq) BaSO4(s)

D Ba2+(aq) + SO42(aq) BaSO4(s)

(Total for question = 1 mark)

Q5.A 2Mg(s) + O2(g) 2MgO(s)

B Mg(NO3)2(s) MgO(s) + 2NO2(g) + O2(g)

C MgO(s) + 2HCl(aq) MgCl2(aq) + H2O(1)

D Mg(s) + CuSO4(aq) MgSO4(aq) + Cu(s)

(a) Which equation is notbalanced?(1)

A B C D

(b) Which equation can be classified as a displacement reaction?

(1)

A B C D (Total for question = 2 marks)


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Q6.This question is about the properties of ions and ionic compounds.(a) Solid calcium carbonate, CaCO3, has a giant ionic structure.(i) Draw a diagram (using dots or crosses) for a calcium ion. Show ALLthe

electrons and the charge on the ion.(2)

(ii) Complete the electronic configuration for a calcium ion.(1)

1s2...................................................................................................................................................................

(iii) Would you expect a calcium ion to be bigger, smaller or the same size as acalcium atom? Give TWO reasons to explain your answer.

(2)

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(iv) Explain why ionic compounds have relatively high melting temperatures.(2)

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(b) Changes in the concentration of ions in a solution can be estimated by measuring theelectrical conductivity of the solution.

(i) Explain why solutions of ions are able to conduct electricity.(1)

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(ii) Suggest why aqueous solutions of calcium chloride, CaCl2(aq), and bariumchloride, BaCl2(aq), of the same molar concentration, have different electricalconductivities.

(1)..............................................................................................................................................

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(iii) 1 kg of a solution contains 0.100 mol of calcium ions, Ca2+.

What is the concentration of the calcium ions by mass inparts per million (ppm)?

[Assume the relative atomic mass of calcium is 40.](2)

.....................................ppm

*(c) Some buildings are made from limestone, which is mainly calcium carbonate. Gasesin the atmosphere such as sulfur dioxide, SO2, and nitrogen dioxide, NO2, can beresponsible for damaging these buildings.

Describe how these gases come to be present in the atmosphere and explain howthey can damage a limestone building.

(3)..............................................................................................................................................

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(d) The lattice energy of calcium chloride, CaCl2, is -2258 kJ mol-1based on an

experimental Born-Haber cycle and -2223 kJ mol-1based on theoretical calculations.

Would you expect its bonding to match the ionic model? Justify your answer.(1)

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Q7.Magnesium chloride can be made by reacting solid magnesium carbonate, MgCO3, with

dilute hydrochloric acid.

(a) Write an equation for the reaction, including state symbols.

(2)

(b) Give TWO observations you would make when the reaction is taking place.

(2)..............................................................................................................................................

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(c) In an experiment to make crystals of hydrated magnesium chloride, MgCl2.6H2O,magnesium carbonate was added to 25 cm3of hydrochloric acid with concentration

2.0 mol dm-3

. The molar mass of magnesium carbonate is 84.3 g mol-1

.

(i) How many moles of acid are used in the reaction?

(1)

(ii) What mass of magnesium carbonate, in grams, reacts with this amount of acid?

(1)

(iii) Suggest why slightly more than this mass of magnesium carbonate is used in practice.

(1)..............................................................................................................................................

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(iv) How would you separate the magnesium chloride solution from the reaction mixture in (iii)?

(1)..............................................................................................................................................

(v) The magnesium chloride solution was left to crystallise. The crystals were separated and driedcarefully. A sample of 3.75g of hydrated crystals, MgCl2.6H2O, which have molar mass 203.3 g mol

-1, was

obtained. Calculate the percentage yield of this reaction.(2)

(vi) Give ONE reason why the yield of crystals is less than 100%, even when pure compounds are used inthe preparation.

(1)..............................................................................................................................................

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(d) Lattice energies can be measured using the Born-Haber cycle, or calculated fromelectrostatic theory. Lattice energies of magnesium chloride and magnesium iodideare shown below.

(i) What does this data indicate about the bonding in magnesium chloride?

(1)..............................................................................................................................................

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*(ii) Explain why there is a greater difference between the experimental (Born-Haber) and theoreticallattice energies for magnesium iodide, MgI2, compared with magnesium chloride.

(2)..............................................................................................................................................

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(e) Blood plasma typically contains 20 parts per million (ppm) of magnesium, by mass.

(i) Calculate the mass of magnesium, in grams, present in 100 g of plasma.

(1)

(ii) Magnesium chloride can be used as a supplement in the diet to treat patients with low amounts ofmagnesium in the blood. Suggest ONE property which makes it more suitable for this purpose thanmagnesium carbonate.

(1)..............................................................................................................................................

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Examiners report

None for Q1-2

Q3.(a)(i)

Many candidates calculated the empirical formula correctly as NaO.

Results Plus: Examiner Comments1:1 ratio of Na:O had not been made explicit, but the correct answer NaO was given soboth marks were awarded.

Results Plus: Examiner TipAlways show every step in your working.

(a)(ii)

A surprising number of candidates gave a molecular formula for a compound for which the molarmass was not 78 g mol1.

(a)(iii)

Many candidates were awarded both marks, but a significant number gave the state symbol forNa2O2as (aq) rather than (s).

Results Plus: Examiner CommentsThis is a correctly balanced equation, with the state symbols which are also correct, theanswer scored both marks.

Results Plus: Examiner TipAlways consider state symbols carefully when asked for them in an equation.

(a)(iv)

Most candidates gained the second scoring point which required the calculated moles of oxygen,O2, to be multiplied by 24 dm

3mol1in order to obtain the volume of gas in units of dm3. There


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was some confusion, however, between calculating moles of oxygen atoms, O(g), and moles ofoxygen molecules, O2(g).

Results Plus: Examiner CommentsThis is a well laid-out answer which scored both marks.

Results Plus: Examiner Tip

Set out your answers to calculation questions in an orderly way as illustrated in theabove response.

(a)(v)

This question was generally well answered, although sometimes candidates chose to calculatethe number of oxygen atoms instead of molecules.

Results Plus: Examiner CommentsThis was the correct response, which scored the mark available.

Results Plus: Examiner TipAlways check which type of particle (atom, ion or molecule) is being referred to in thequestion.

(b )

This proved difficult for the majority of candidates. A significant number of responses includedincorrect references to the presence of hydrogen gas, H2, in air.


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Results Plus: Examiner CommentsThis response scored a mark as it acknowledged that other oxides of sodium may havebeen formed in the reaction described.

Results Plus: Examiner TipBe aware of the gases present in air.

Q4.

No Examiner's Report available for this question

Q5.No Examiner's Report available for this question

Q6.(a) (i-ii)

Although for many this question was an easy start to the structured questions, a significantminority dropped at least one mark.

Surprisingly, this occurred in part (i) as well as part (ii), with an incorrect number of electronshells being the most common way to fail to score.

It's worth reminding candidates that transfer of key skills from GCSE, such as 'dot and cross'diagrams, is a feature of AS and they should not just focus their revision on new concepts, suchas s, pand dorbitals.


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Results Plus: Examiner CommentsThis is a clear, well presented answer to part (i), scoring both marks.However, it looks like the candidate has attempted (incorrectly) togive the electronic configuration for a calcium atom in part (ii).

Results Plus: Examiner Tip

Pay careful attention to anything written in bold on the paper. It's donedeliberately to help guide you towards answering the question. In this case itshows the question requires the electron configuration of the ion, not the atom.

(a) (iii-iv )

It was quite rare to award two marks in part (iii) and this item did help discriminate at the highergrades.

Whilst the vast majority recognised the ion was smaller, justifi cations were not always clear. Forinstance, many described simply loss of electrons, rather than loss of an electron shell, whichwas not quite creditworthy. In addition, the idea of increasing effective nuclear charge, whilstacceptable in this context, did lead to a number of responses that suggested some candidatesbelieved the actual nuclear charge was increasing, rather than the increase of its net effect onthe remaining electrons. The best answers using this approach made it clear the number ofprotons remained the same, but now outweighed the number of electrons.

Part (iv) showed that most candidates understood the clear link between the strength of themany ionic bonds in a lattice and related this clearly to the input of energy needed to break upthe structure. A few failed to score in this question either because they answered in terms ofintermolecular forces or metallic bonds.

Results Plus: Examiner CommentsIn part (iii) 'stronger positive charge' without further clarificationseems to imply more protons in the nucleus, so this answer wasonly awarded one mark, for loss of an electron shell.


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Results Plus: Examiner TipThis answer would have been improved if the candidate had clarified why theybelieved they were subject to 'greater positive charge' for example by stating thatthe ion has more protons than electrons.

(b) (i)The majority of candidates believed that delocalised electrons caused conductivity in ionicsolutions. Of those who recognised the charged particles were ions, most went on to score themark by emphasizing their freedom to move.

Results Plus: Examiner CommentsThis answer didn't score as, although it recognises themovement of charged particles is needed for a substance to

conduct, it states, incorrectly, that in the case of a solutionthose particles are electrons.

(b) (ii)

The misconception regarding electrons in b (i) further hindered candidates in this item. A verysimple answer regarding the size of the ions affecting the conductivity would have sufficed. Itwas acceptable for such an argument to be based on the ions alone, or in their hydrated form,that they were different sizes, without any further justification.

However, many candidates tried to justify the difference in terms of number of electrons, as they

seemed to believe, as shown in part (i), that compounds release electrons on dissolving to allowconduction.

(b) (iii)

This calculation seemed unfamiliar to some candidates who, despite often scoring marks oncalculations in questions 16 and 17, found this very difficult.

Although essentially a scaling exercise, many did not appreciate this and left their answer asparts per thousand. Many others divided through by a million rather than scaling up to partsper million. As this is a relatively straightforward task, the poor responses from some centressuggests its relative novelty to the specification means it isn't as well practised as some other

numerical tasks.


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Results Plus: Examiner CommentsAlthough this candidate is unsure how to scale

to parts per million, they have made some effortto show what calculations they are attemptingand label their answers accordingly. Hencewe were able to award this candidate 1 markfor correctly determining the mass of calciumpresent in the solution.

Results Plus: Examiner TipTry to always label any values you have calculatedso it is clear to the examiner what you areattempting to do.

(c )

Many responses showed a good understanding of the processes involved in this question and

examiners were pleased to see generally sound chemical principles used to explain a 'reallife' problem. When candidates failed to score it was generally due to omissions rather thanincorrect chemistry. For instance, some ignored the first part of the question and didn't discussthe source of the gases. Others gave vague generic answers for the source, such as 'pollutionfrom factories or industry'.

However, some excellent explanations of the formation of nitrogen oxides in car engines wereseen from more able candidates. Some responses for the third mark did not score as theyassumed the process was just physical, rather than chemical, tending to just describe erosion

andnot any preceding chemical reaction.


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there to help candidates. Always read it carefullyto see if it could be useful in your written answer.

Q7.The question was linked to practical work on preparation of a salt. The formula of magnesiumchlorideappeared early in the question, but some candidates did not use it in the first equation.The question alsoreferred to the magnesium chloride solution, but many candidates used thestate symbol (s) for magnesiumchloride in the equation. This was not penalised unless there

were other errors.

In (b) observations were required, and though most candidates realised that bubbles oreffervescence wouldbe seen, they found it harder to suggest a second observation.

Results Plus: Examiner TipThis question asks for TWO observations, so they must be distinctly different.

Results Plus: Examiner CommentsThis answer has described one observation in two ways (fizzing and bubbling). Sayingthat a gas is given off is not an observation, as most gases are invisible.

Results Plus: Examiner CommentsThis answer has described two different observations. As candidates are unlikely toknow whether a reaction is exothermic or endothermic they could just say that a

temperature change occurs.

Many of the calculations in (c)(i) were well done, but failure to appreciate the 1:2 ratio whencalculating thenumber of moles of magnesium carbonate in c(ii) was common. However,allowance was made for answersbased on incorrect reacting ratios in the original equation.

Answers to (c)(iii) were sometimes imprecise, and it was not always clear that an excess ofmagnesiumcarbonate would use up all the acid.

Results Plus: Examiner TipIf an excess of magnesium carbonate is used, all the acid will be used up and the

excess solid is easily filtered off to leave a pure solution of magnesium chloride.


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Results Plus: Examiner CommentsThis does not say what reacts fully. The magnesium carbonate will not react fully as it isin excess.The answer should say that all the acid reacts fully.

Answers to (c)(iv) often included heating or distillation, and were not based on removing the solidmagnesium carbonate.

Calculating the percentage yield in (c)(v) caused difficulty, and some candidates simply dividedtheactual yield by the mass of magnesium carbonate they had calculated previously, without

calculating thetheoretical yield.

In (c)(vi) answers had to be related to the experimental procedure, not mistakes by theexperimenter. Onlya few candidates had a clear idea of the experimental limitations on yield, andthere were many generalanswers. In questions like this comments should be specific about howthe loss in yield occurs and it is not enough just to refer to human error.

Results Plus: Examiner TipThere are several possible answers as well as the one below. The answer could havereferred to filtering. Solutions soak into filter paper, so the solute will be lost and theyield of crystals reduced. If hydrated crystals are heated too strongly when they arebeing dried, they would lose some of their water of crystallisation.

Results Plus: Examiner CommentsThis answer gets the mark for the idea that material is always lost when transferringchemicals from one container to another.

Answers to (d) required careful wording. A statement in (d)(i) that magnesium chloride was moreionicthan covalent was not accepted. The data shows that the bonding is very close to the ionicmodel, but doesnot exactly match it. There are very many ways of expressing this but it had tobe reflected in the answer.In (d)(ii) candidates often referred to the size of iodine molecules andatoms, but to gain credit it had tobe clear that the ionic radius was what mattered. Again, therewere many ways of expressing the idea ofincreased covalent character or polarisation of theiodide ion, when explaining the difference in theoretical

and experimental lattice energies.Results Plus: Examiner Tip

Answers to this question need careful wording. In (d)(i) the data shows that the bondingis almost completely, but not 100% ionic. The different values for magnesium iodide


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occur because the iodide ion is larger than the chloride ion and the iodide ion becomespolarised. If magnesium chloride was described as having some covalent character in(i), the answer in (ii) had to make clear that this is increased in magnesium iodide.

Results Plus: Examiner CommentsThis is a good answer scoring full marks.

Results Plus: Examiner CommentsThis doesn't state the type of bonding in magnesium chloride. In (ii), it does not refer toions or state clearly that the iodide ion is larger than the chloride ion, though it would geta mark for the idea that there is more covalent character in magnesium iodide than inmagnesium chloride.

The calculation on parts per million in (e)(i) was well done, and there were many sensible

suggestions aboutuse of magnesium chloride in (e)(ii).


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1.3 k Mark scheme

Q1.C Q2.B

Q3.


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Q4 D Q5.(a) B (b) DQ6.


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Q7.


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1 3k State Symbols and Ionic Equations