A2 – Unit 5

CHEMISTRY

RevisionRevisionBookletBooklet

Redox Equilibria

Transition Metals

Organic Chemistry Arenes, Nitrogen compounds

and Synthesis

Revised March 2014

Command words used in Chemistry Exams

State Recall and write a fact.

Define All the words in bold in the question must be defined, with the exception in definitions of enthalpy changes, the term enthalpy need not be defined.

Identify Either the name or the formula is acceptable.

Calculate Use the numbers given. Show your working, unless it is a simple one-step calculation.

Explain The answer must contain the theoretical basis for the answer which must be presented in a logical order.

Justify your You must include the theoretical basis for the answer.answer

Discuss This term will be used in more open-ended questions. There will probably be more marking points than marks, so there will be a variety of ways in which full marks can be obtained.

Analyse Use the data given.

Compare You must comment on both substances being compared.

Evaluate Discuss the pro’s and con’s in an experiment / process.

Deduce The answer must be worked out based either on data supplied in the question or in one or more previous answers in the question.

Suggest You are not expected to know the answer, but to be able to work it out based on their knowledge and understanding of material you have been taught.

Comment critically onThis term will most likely be used when you are being stretched and challenged. The answer must be detailed to achieve full marks. There will also be some easier marks, accessible for the average candidate.

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5.1 – Redox Chemistry

What is meant by the term redox?

Use the following half equations to write a balanced equation for the reaction between MnO4- and Fe2+.

MnO4- + 8H+ + 5e- Mn2+ + 4H2O

Fe2+ Fe3+ + e-

Describe the titration used to find the concentration of Iodine in a solution. Include a balanced equation.

Using redox reactions

Mg2+(aq) + 2e- Mg(s) Eø = -2.37VZn2+(aq) + 2e- Zn(s) Eø = -0.76VCu2+(aq) + 2e- Cu(s) Eø = +0.34VFe2+(aq) + 2e- Fe(s) Eø = -0.44VSn2+(aq) + 2e- Sn(s) Eø = -0.14VO2(g) + 2H2O(l) + 4e- 4 OH-(aq) Eø = +0.40V

Br2(aq) + 2e- 2Br-(aq) Eø = +1.09VCl2(aq) + 2e- 2Cl-(aq) Eø = +1.36V

MnO4-(aq) + 5e- + 8H+(aq) Mn2+(aq) + 4H2O(l) Eø = +1.51V

‘The standard electrode potential for Cu2+(aq)/Cu(s) is +0.34V’. What do you understand by this?

What is a standard electrode and why is one needed in order to measure electrode potentials?

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Calculate Eøcell for cells made from the following standard half cells:

i) Mg2+(aq)/Mg(s) with Zn2+(aq)/Zn(s)

ii)Zn2+(aq)/Zn(s) with Cu2+(aq)/Cu(s)

iii) Br2(aq)/2Br-(aq) with Cl2(aq)/2Cl-(aq)

The equation for the first stage of rusting is: 2Fe(s) + O2(g) + 2H2O(l) → 2Fe(OH)2(s). Calculate the E ø and show the reaction is feasible.

Use the E ø values to explain why zinc is used in preference to tin for preventing corrosion of steel car bodies.

The Eøcell value for Mg2+(aq)/Mg(s) with Ag+(aq)/Ag(s) is 3.18V (Mg electrode -ve).

What is Eø for Ag+(aq)/Ag(s)?

You can make Cl2 by the oxidation of Cl-. What oxidising agent, from the list above, is capable of doing this? Write an equation for the reaction.

How can Eø values be used to predict the likely spontaneity of redox reactions?

What two limitations are there to the predictions made using Eø values?

How is E øcell related to Stot?

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How is E øcell related to the equilibrium constant?

Briefly describe two useful applications of electrode potentials.

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5.2 – Transition Metals

What is a transition metal?

Give three characteristics transition metals.

Why do transition metals share similar properties?

Draw out ‘electrons in boxes’ and use the s p d notation to represent the electron configuration of the following atoms and ions.

(a) Sc

(b) Sc3+

(c) Fe2+

(d) Fe3+

Suggest why Fe2+ ions are readily oxidised to Fe3+ ions but Mn2+ ions are not readily oxidised to Mn3+ ions.

Why is Zn2+ colourless?

Describe the bonding a complex ion.

Give an example of: a monodentate, a bidentate and a polydentate ligand.

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Draw, give the shape and name the following complexes:

(a) [Fe(H2O)6]3-

(b) [Cu(NH3)4(H2O)2]2+

(c) [CrCl4]-

(d) [CuCl2]-

(e) [Pt(NH3)2Cl2]

Explain why the complexes above, apart from [CuCl2]- , are coloured.

Explain why the complexes above have different colours.

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Give an example of a ligand exchange reaction involving [CuCl4]2-. What observations that would be made druing the reaction.

Give an example of a deprotonation reaction involving Cr(H2O)63+.

What observations that would be made druing the reaction.

Why is Cr(H2O)63+ more acidic than Cr(H2O)6

2+?

Describe the reaction between Cu(H2O)62+ and NH3.

Use the following half equations to explain why copper(I) compounds are very unstable.Cu2+ + e- Cu+ Eo = +0.15VCu+ + e- Cu Eo = +0.52V

What colour are the following species?

Cr(OH)3(s) Cr(H2O)67+ Ni(OH)2(s)

Fe(OH)2(s) Fe(OH)2(s) Mn(OH)2(s)

Zn(OH)2(s)

Describe a use for [Pt(NH3)2Cl2]

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1. Ammonium vanadate(V), NH4VO3 reacts with dilute sulphuric acid to form a solution containing yellow VO2

+ ions.a. Write an ionic equation for the reaction of the anion in NH4VO3 with dilute sulphuric acid.

b. Is the reaction in (a) a redox reaction? Justify your answer.

c. Addition of zinc to the solution containing VO2+ ions causes the colour to change from

yellow to green then to blue, followed by green again and finally violet. State the formulae of the ions responsible for each of these colour changes.First green colour: Second green colour: Violet colour:

2. A few drops of NH3 is added to [Ni(H2O)6]2+ and a green precipitate forms. More NH3 is added, the precipitate redissolves and a blue solution forms.

a. What type of bonds are present in the [Ni(H2O)6]2+ ion?b. Write an equation to show the formation of the green precipitate.c. Explain why the first reaction is deprotonation.d. Name the type of reaction occurring in the second step.e. Give an equation for the reaction in the second step.f. Explain why to [Ni(H2O)6]2+ is coloured.

3. Brass is a widely used alloy that contains copper and zinc. There are many varieties of brass with different compositions. In the volumetric analysis of the composition of brass, the first step is to react a weighed sample of the alloy with nitric acid. This gives a greenish-blue solution. Use the following standard electrode potentials for the question:

Zn2+ + 2e- Zn Eθ = -0.76VCu2+ + 2e- Cu Eθ = +0.34V

NO3- + 2H+ + e- NO2 + H2OEθ = +0.81V

a. Use the information above to calculate the standard electrode potential for the reaction between zinc and nitric acid derive the equation.

b. Suggest why zinc does not produce hydrogen with nitric acid.c. If the greenish-blue solution is diluted with water it turns light blue and contains

hydrated copper (II) ions. Name the lights blue complex ion and name its shape.d. If concentrated hydrochloric acid is added to a portion of the light blue solution

it turns green. State the type of reaction that occurs and give an equation for the reaction type.

e. The light blue solution is then neutralized and reacted with potassium iodide solution. Use the electrode potentials below to explain why you would not expect a reaction to occur.

Cu2+ + 2e- Cu+ Eθ = +0.15VI2 + 2e- 2I- Eθ = +0.54V

f. Explain why, in practice, the reaction does occur and iodine is liberated. g. When the precipitate formed in the reaction in (e) is filtered off and then

dissolved in concentrated aqueous ammonia, a colourless solution is formed. Suggest the formula for the cation in this solution.

h. If the colourless solution is left to stand in air for some time, it turns blue. State why this is so, naming the reactant responsible for the change.

i. In determination of the composition of a sample of brass, 1.50 g of the alloy was treated to give 250 cm3 of a neutral solution of copper (II) nitrate and zinc

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nitrate. Excess potassium iodide solution was added to 25.0 cm3 portions of this solution and the liberated iodine titrated with 0.100 moldm-3 sodium thiosulphate solution. The mean titration was 16.55 cm3.

a. State which indicator you would use for the titration and the colour change seen at the end point.

b. Explain why the indicator is not added until the reaction is nearly complete.

c. Calculate the percentage of copper by mass in the brass.

4. a. Complete the electronic configuration of: Cr and Cr3+.

b. State and explain the shape of [Cr(H2O)6]3+.c. State what you would see when dilute sodium hydroxide is added to a solution containing [Cr(H2O)6]3+ until it is in excess.d. Give equations for the reactions taking place in (c). Identify the role of the hydroxide ion in the reactions.e. Give the structural formula of an organic compound that can be oxidized by potassium dichromate (VI) in dilute sulphuric acid and of an organic product of the reaction.f. Both dichromate (VI) ions and manganate (VII) ions need hydrogen ions in order to act as oxidizing agents in titration experiments. Explain using electrode potentials whether hydrochloric acid could be used to provide the H+ ions for these oxidations.

Cr2O72- + 14H+ + 6e- 2Cr3+ + 7H2O Eθ = +1.33V

Cl2 + 2e- 2Cl- Eθ = +1.36VMnO4

- + 8H+ + 5e- Mn2+ + 4H2O Eθ = +1.51Vg. When aqueous alkali is added to an aqueous solution containing dichromate (VI) ions, the following change takes place:

Cr2O72- + 2OH- 2CrO4

2- + H2OExplain in terms of oxidation numbers why this is not a redox reaction.

5. Storage cells and fuel cells are types of electrochemical cells used as sources of energy. Information about five redox systems that could be used in electrochemical cells is shown below.Redox System 1: Fe2+ + 2e- Fe(s) Eθ = -0.44VRedox System 2: 2H2O(l) + 2e- 2OH-(aq) + H2(g) Eθ = -0.83VRedox System 3: 2H+ + 2e- H2(g) Eθ = 0.00VRedox System 4: O2(g) + 2H2O(l) + 4e- 4OH-(aq) Eθ = +0.40VRedox System 5: O2(g) + 4H+(aq) + 4e- 2H2O(l) Eθ = +1.23V

a. The standard electrode potential of redox system 1 can be measured by constructing an electrochemical cell. Draw a diagram to show how the standard electrode potential could be measured for redox system 1. State the conditions needed to measure this standard electrode potential.

b. When an alkaline hydrogen-oxygen fuel cell is being used to produce electrical energy, chemical changes take place within the cell. Write half equations for the changes that take place at each electrode, write the overall equation for the cell reaction and calculate the standard cell potential of this fuel cell.

c. State one important difference between a fuel cell and a conventional storage cell.d. People often assume that hydrogen-oxygen fuel cells are a source of energy that is

carbon neutral. State one reason why this assumption may not be correct.

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e. A student constructs a cell as follows: A half cell is made from a strip of chromium metal and a solution of chromium

(III) sulphate. A second half cell is made from a strip of metal X and a solution of XSO4(aq). The two half cells are connected together and a current is allowed to pass for a

length of time.

The chromium electrode gains 1.456 g in mass.The electrode made of metal X loses 1.021 g in mass.Determine the identity of metal X.

6. a. Construct a balanced equation for the reaction between acidified dichromate (VI) ions and methanoic acid, HCOOH.

HCOOH → HCHOCr2O7

2- → Cr3+

b. A student added some chromium metals to an acidified solution containing copper (II) ions. A reaction took place. The student confirmed that “chromium is more reactive than copper”.i) Explain in terms of their electrode potentials why chromium is more reactive than copper.ii) When the experiment is carried out, the student observed some bubbles of gas. Suggest an explanation for this observation.c. Methanoic acid, HCOOH, has the common name of “formic acid”. Direct-Formic acid fuel cells are being developed for use in small, portable electronics such as phones and laptops. Suggest two advantages of using methanoic acid as the fuel rather than hydrogen.

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Organic Chemistry – Arenes

Describe the bonding in benzene.

Describe the evidence that supports your description above.

Explain why arenes undergo electrophilic substitution reactions rather than addition reactions.

Why are phenols often described as ‘activated arenes’?

Give an example reaction that supports this description.

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Give the reagents and conditions for the reactions below.

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+N N

Cl

SO2OH

Br

NO2

R CR

O

X

N N X

NH2

Draw an example mechanism showing Electrophilic Substitution

Nitrogen Compounds

Why are amines classed as bases?

Why are short chained amines soluble in water?

What class of compounds are formed when amines react with acyl chlorides?

Describe how Nylon (6,6) is made.

Describe how a mixture of amino acids can be separated and identified.

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What are the products from the following reactions;

R-C ≡N  + LiAlH4  à

CH3-CN + HCl + 2H2O à

CH3CH2CH(NH2)COOH (aq) + H+ (aq) à

CH3CH2CH(NH2)COOH (aq) + OH- (aq) à

CH3CH2CH(NH2)COOH(aq) + CH2(NH2)COOH(aq) à

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Synthesis

Design a synthetic scheme for the following conversion.Include the reagents and conditions needed in each step.

Design a synthetic scheme for the following conversion.Include the reagents and conditions needed in each step.

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Synthesis in action

Explain why organic reactions are often heated under reflux.

Draw the apparatus for heating under reflux.

Why are organic solvents added to organic reaction mixtures during purification?

Why are organic reaction mixtures washed with water during purification?

Why are organic reaction mixtures washed with sodium carbonate solution during purification?

Name a drying agent that can be used during the purification of organic reaction mixtures.

Describe the key points involved in the purification of an organic solid.

Describe the key points involved in the purification of an organic liquid.

Describe how sensitive organic compounds can be extracted from plant materials.

What is the major advantage of using combinatorial chemistry in drug research?

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