NT Exampro 1

1. (i) Give an equation for the dissociation of propanoic acid and hence an expression for its dissociation constant, Ka.

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(ii) At 25°C Ka for propanoic acid is 1.30 × 10–5 mol dm–3. Find the pH of a solution of propanoic acid of concentration 0.0100 mol dm–3. State any assumptions you make.

(iii) Increasing the temperature of the propanoic acid solution causes the pH to decrease. What does this tell you about the enthalpy of dissociation? Justify your answer.

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2. Using the following data, construct a Born-Haber cycle for potassium chloride and use it to find the electron affinity of chlorine.

∆H/kJ mol–1

First ionisation energy of potassium +419 Enthalpy of atomisation of potassium +89.2 Enthalpy of atomisation of chlorine +121.7 Enthalpy of formation of potassium chloride –436.7 Lattice enthalpy of potassium chloride –711

(6)

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NT Exampro 2

3. (a) What name is given to the enthalpy change of the following reaction?

K+(g) + Br–(g) → KBr(s)

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

(b) Use the following data to construct a fully-labelled energy diagram and use it to calculate the enthalpy change in (a).

∆H / kJ mol–1

Br2(1) → 2Br(g) +224

Br(g) + e– → Br– –348

K(s) + ½Br2(1) → KBr(s) –392

K(s) → K(g) +90

K(g) → K+(g) + e– +424 (6)

(Total 7 marks)

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NT Exampro 3

4. A Born-Haber cycle for the formation of calcium oxide is shown below.

Ca (g) + O(g) + 2e

Ca (g) + ½ O (g) + 2e

Ca (g) + ½ O (g) + e

Ca(g) + ½ O (g)

Ca(s) + ½ O (g)

H

H

H

H

H

H

5

4

3

2

1

7

H6

CaO(s)

2+

2+

+

–

–

–

Ca (g) + O (g)2–2+

2

2

2

2

Data ∆H/kJ mol–1:

∆H1 = +193; ∆H2 = 590; ∆H3 = +1150; ∆H4 = +248; ∆H6 = –3513; ∆H1 = –635.

(i) Identify the change which represents the lattice enthalpy of CaO.

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

(ii) Use the data above to calculate ∆H5.

(3)

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NT Exampro 4

(iii) Use this value of ∆H5 to calculate the first electron affinity of oxygen, given that the

second electron affinity of oxygen is +844 kJ mol–1.

(2)

(Total 6 marks)

5. Benzene reacts with concentrated nitric acid in the presence of concentrated sulphuric acid at about 50 ºC in an electrophilic substitution reaction to give nitrobenzene.

(a) (i) Give the equation representing the overall reaction. (1)

(ii) Give the equation representing the formation of the electrophile.

………………………………………………………………………………… (1)

(iii) Give the mechanism for the reaction of the electrophile with benzene. (3)

(b) In an experiment to determine the kinetics of this reaction, it is found that the attack of the electrophile on the benzene ring is the rate determining step. The following data shows the effect of changing the concentrations on the rate:

[benzene](relative)

[electrophile](relative)

Rate(relative)

1 1 11 2 22 1 22 2 4

(i) What is the order with respect to

benzene …………………………………..……………………………………

the electrophile? ……………………………………………………………… (2)

(ii) Write a rate equation for the reaction.

………………………………………………………………………………… (1)

(iii) In some electrophilic substitution reactions of aromatic compounds, the rate determining step is the production of the electrophile. In such cases what would be the order of the reaction with respect to the aromatic compound?

………………………………………………………………………………… (1)

(c) The equation representing the hydrogenation of ethene is

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NT Exampro 5

H2C CH2+H2 → H3C–CH3 ∆H = –120 kJ mol–1

(i) Assuming that benzene consists of a ring with three double bonds, predict the enthalpy change for the reaction

+ 3H2

………………………………………………………………………………… (1)

(ii) The enthalpy of hydrogenation of benzene is actually –205 kJ mol–1. What can you deduce from this and your answer to part (i) about the stability of the benzene ring? Use an enthalpy level diagram to illustrate your answer.

(2)

(iii) The compound cyclo–octatetraene

has an enthalpy of hydrogenation of –480 kJ mol–1; the molecule, unlike that of benzene, is not flat. Suggest in terms of the possibility of orbital overlap why cyclo–octatetraene does not show the same type of stability as benzene, despite superficial similarities in their structures.

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………………………………………………………………………………… (2)

(Total 14 marks)

6. (a) Define the term lattice enthalpy.

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

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NT Exampro 6

(b) Using the following data, construct a Born-Haber cycle for sodium fluoride and from it

determine the lattice enthalpy of sodium fluoride.

Process The value of the energy change/kJ mol–1

Na(g) → Na+(g) + e– +494

F2(g) → 2F(g) +158

F(g) + e– → F– (g) –348

Na(s) + ½F2(g) → NaF(s) –569

Na(s) → Na(g) +109 (5)

(c) The table below gives some information about the hydroxides of the Group 2 elements.

salt lattice enthalpy/kJ mol–l

hydration enthalpy/kJ mol–l

solubility in water/g per

100 g of water

magnesium hydroxide

–2383 –2380 0.9 × 10–4

calcium hydroxide

–2094 –2110 156 × 10–4

strontium hydroxide

–1894 –1940 800 × 10–4

barium hydroxide

–1768 –1820 3900 × 10–4

(i) Explain why energy is required to break up an ionic lattice.

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

(ii) Suggest why the lattice enthalpies of the hydroxides of Group 2 metals become more exothermic from Ba(OH)2 to Mg(OH)2.

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

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NT Exampro 7

(iii) Suggest why the lattice enthalpy of beryllium hydroxide, Be(OH)2, cannot be

predicted from the data in the table.

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(iv) Explain why energy is released when ions are hydrated.

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(v) Hence, account for the trend in solubilities from Ba(OH)2 to Mg(OH)2

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(Total 16 marks)

7. (a) (i) Draw a Born-Haber cycle for the formation of magnesium chloride, MgCl2. Use the values below to calculate the lattice enthalpy of magnesium chloride.

∆H /kJ mol–1

1st electron affinity of chlorine –364

1st ionisation energy of magnesium +736

2nd ionisation energy of magnesium +1450

Enthalpy of atomisation of chlorine +121

Enthalpy of atomisation of magnesium +150

Enthalpy of formation of MgCl2(s) –642

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NT Exampro 8

(5)

(ii) The value of the lattice enthalpy of magnesium chloride calculated from a purely ionic model is –2326 kJ mol–1. Explain why this differs from the value determined from a Born-Haber cycle.

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(b) Use the following data to answer the questions in this section.

∆H /kJ mol–1

∆Hhydration of Sr2+ –1480

∆Hhydration of Ba2+ –1360

∆Hhydration of OH– –460

Lattice enthalpy of Sr(OH)2 –1894

Lattice enthalpy of Ba(OH)2 –1768

(i) Explain why the lattice enthalpy of strontium hydroxide is different from that of barium hydroxide.

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

(ii) Explain why the hydration enthalpy of a cation is exothermic.

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

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NT Exampro 9

(iii) Use the lattice enthalpy and hydration enthalpy values to explain why barium

hydroxide is more soluble in water than strontium hydroxide.

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(Total 15 marks)

8. (a) The Born-Haber cycle for the formation of sodium chloride is shown below.

Na (g) + Cl(g) + e

Na (g) + Cl (g) + e

Na(g) + Cl (g)

Na(s) + Cl (g)

Na (g) + Cl (g)

Na Cl (s)

–

–

–

–

+

+

+

+

1

1

1

2

2

2

2

2

2

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NT Exampro 10

Use the data below to calculate the lattice enthalpy of sodium chloride.

Enthalpy change

Value of the enthalpy change

/kJ mol–1

Enthalpy of atomisation of sodium +109

1st ionisation energy of sodium +494

Enthalpy of formation of sodium chloride –411

Enthalpy of atomisation of chlorine +121

Electron affinity of chlorine –364

(2)

(b) Sodium chloride and magnesium oxide have very similar crystal lattices. Suggest why the lattice enthalpy of magnesium oxide is very much larger than that of sodium chloride.

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

(c) The lattice enthalpy of silver iodide can be calculated but the experimental value does not match the calculated value as well as those for sodium chloride match each other.

Explain why the calculated and experimental values for silver iodide are different.

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

(Total 6 marks)

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NT Exampro 11

9. You are required to carry out a quantitative exercise to find the enthalpy change when a

Group 1 compound, S, dissolves in water. (13 marks]

Quantitative exercise to find the enthalpy change when a Group 1 compound dissolves in water

You are provided with:

• a sample of a Group 1 compound, S, in a specimen tube.

You are required to find the molar enthalpy change when S dissolves in water.

S(s) + aq → S(aq)

PROCEDURE

(1) Rinse out a 50 cm3 measuring cylinder with distilled water then use the measuring cylinder to measure 50 cm3 of distilled water into a dry plastic cup held firmly in a beaker. Place the thermometer in the distilled water in the cup.

(2) Weigh the specimen tube containing S. Record the mass of the tube and S in the first table below.

(3) Measure the temperature of the distilled water in the cup to the nearest 0.2 °C. Record the temperature in the second table below.

(4) Empty the sample of S from the specimen tube into the plastic cup, stirring gently with the thermometer. (The accuracy of your results will not be affected if a small amount of S is left in the tube.) Continue stirring until all of the solid has dissolved then record the steady temperature of the solution to the nearest 0.2 °C. Record the temperature in the second table below.

(5) Weigh the emptied specimen tube and record the mass in the first table below.

Mass of specimen tube + S g

Mass of emptied specimen tube g

Mass of S used g

(2) - Temperature of distilled water before adding S T1 °C

Steady temperature of solution, T2 °C

Calculate the change in temperature, ∆T, by subtracting T1 from T2.

∆T = T2-T1 = = .........................°C (6)

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NT Exampro 12

Calculations and questions

(a) Calculate the molar enthalpy change when S dissolves in water using the formula

∆H = – usedofMass8.17

ST∆×

kJ mol–1

Include a sign and units with your value of ∆H which should be expressed to an appropriate number of significant figures.

∆H = (3)

(b) Suggest a change to the procedure, using the same apparatus, that may lead to a more accurate value of the molar enthalpy change. Explain the reason for your suggestion.

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

(Total 13 marks)

10. (a) Using the following data, construct a Born-Haber cycle for potassium chloride and use it to calculate the electron affinity of chlorine.

∆H / kJ mol–1 1st ionisation energy of potassium + 419 Enthalpy of atomisation of potassium + 89.2 Enthalpy of atomisation of chlorine + 121.7 Enthalpy of formation of KCl(s) – 436.7 Lattice enthalpy of potassium chloride – 711

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NT Exampro 13

(5)

(b) Calcium is in the same period in the periodic table as potassium. The lattice enthalpy of calcium chloride is –2258 kJ mol–1. Explain why this is so different from the value for potassium chloride given in (a).

……...………………………………………………………………………………..

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

(c) Lattice enthalpies may be calculated based on an assumption about the structure of the solid or found experimentally using data in the Born-Haber cycle. The experimental lattice enthalpy of potassium chloride is 9 kJ mol–1 more exothermic than that calculated; for calcium chloride the experimental value is 35 kJ mol–1 more exothermic than that calculated. Suggest why the calculated and experimental values are different in both compounds.

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

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NT Exampro 14

(d) The solubility of calcium sulphate in water at room temperature is much greater than that

of barium sulphate. Suggest reasons for this difference.

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

(Total 12 marks)

11. The formation of magnesium chloride from magnesium and chlorine may be represented by the following Born-Haber cycle:

Mg (g) + 2Cl(g) + 2e

MgCl (s)

Mg (g) + 2Cl (g)

2+

2+

Mg(g) + Cl (g)2

Mg(s) + Cl (g)2

2

–

Mg (g) + Cl (g) + 2e2+2

– –

(a) Define the terms:

Lattice enthalpy.

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

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NT Exampro 15

Enthalpy of atomisation.

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....................................................................................................................................

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

(b) (i) Identify on the diagram the chance representing the enthalpy of atomisation of magnesium.

(1)

(ii) Use the data below to calculate the first electron affinity of chlorine.

Enthalpy change Value of the enthalpy

change / kJ mol–1

Enthalpy of atomisation of magnesium

1st Ionisation energy of magnesium

2nd Ionisation energy of magnesium

Enthalpy of formation of magnesium chloride

Enthalpy of atomisation of chlorine

Lattice enthalpy of magnesium chloride

+150

+736

+1450

–642

+121

–2493

(2)

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NT Exampro 16

(c) Hydrogen gas reacts with sodium metal to form an ionic solid, NaH, which contains

sodium cations.

Draw a Born-Haber cycle which could be used to determine the electron affinity of hydrogen.

(3)

(Total 11 marks)

12. (a) Write an equation which represents the change when the second electron affinity of oxygen is measured.

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

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NT Exampro 17

(b) Construct a Born-Haber cycle and use it and the data below to calculate the second

electron affinity of oxygen.

∆H / kJ mol–1

Enthalpy of atomisation of magnesium +150

Bond energy of O == O in oxygen +496

1st ionisation energy of magnesium +736

2nd ionisation energy of magnesium +1450

Ist electron affinity of oxygen –142

Lattice enthalpy of magnesium oxide –3889

Enthalpy of formation of magnesium oxide –602

(4)

(c) (i) MgO(s) has the same crystal structure as NaCl(s). The lattice enthalpy of NaCl(s) is –771 kJ mol–1 whilst that of MgO(s) is –3889 kJ mol–1.

Explain the difference in lattice enthalpies.

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(ii) Despite its high lattice enthalpy sodium chloride is soluble in water.

What other factor is important in enabling compounds such as sodium chloride to be soluble in polar solvents such as water?

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

(iii) Explain why magnesium oxide is insoluble in water.

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NT Exampro 18

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

(Total 13 marks)

13. (a) The bombardier beetle Metrius contractus persuades potential predators to disappear by firing a boiling mixture of irritants at them. The reaction producing this ammunition is a redox reaction, H2O2 being the oxidising agent.

The two half-reactions involved are:

OH O

OH O

+ 2H + 2e + 0.70+ –

+ –

E /V

+1.77H O + 2H + 2e 2H O2 2 2

(i) Write the overall equation for the reaction and show that the reaction is feasible. (3)

(ii) The beetle makes use of an enzyme catalyst in the reaction. Explain in general terms how catalysts increase the rate of a chemical reaction using a graph of the Maxwell-Boltzmann distribution of molecular energies.

(5)

(iii) The reaction is highly exothermic; in principle its enthalpy of reaction could be found by using average bond enthalpies. By a consideration of the structure and bonding in the compounds involved, suggest why the use of the average bond enthalpies for C==O, CC, C==C and OH would give a highly inaccurate answer for the enthalpy of reaction.

(2)

(b) On heating hydrogen peroxide decomposes according to the equation

2H2O2 → 2H2O + O2

Hydrogen peroxide is marketed as an aqueous solution of a given ‘volume strength’. The common 20-volume solution gives 20 dm3 of oxygen from 1 dm3 of solution. What is the concentration in g dm–3 of such a solution? (Molar volume of any gas at the temperature and pressure of the experiment is 24 dm3.)

(3)

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NT Exampro 19

(c) Hydrogen peroxide, H2O2, can also act as a reducing agent.

The rapid oxidation of hydrogen peroxide was used in World War II to generate steam to launch the V1 ‘flying bomb’. H2O2 (100 volume) was reacted with acidified potassium manganate(VII) solution.

(i) Write the half-equation for the oxidation of hydrogen peroxide to oxygen, O2. (1)

(ii) The MnO –4 ions are reduced to Mn2+ during the reaction. Derive the overall

equation for the reaction between H2O2 and acidified KMnO4. (2)

(iii) Suggest in terms of the collision theory of chemical kinetics why 100-volume hydrogen peroxide (this gives l00 dm3 of oxygen from 1 dm3 of hydrogen peroxide when it decomposes to water and oxygen) was used rather than the more common 20-volume solution.

(2) (Total 18 marks)

14. (a) Define the term lattice enthalpy.

………………...……………….……………………………………………………..

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

(b) (i) Construct a Born-Haber cycle for the formation of calcium chloride, CaCl2. Use the cycle and the data below to calculate the lattice enthalpy of calcium chloride.

∆H / kJ mol–1

Enthalpy of atomisation of calcium +193

1st ionisation energy of calcium +590

2nd ionisation energy of calcium +1150

Enthalpy of formation of calcium chloride –795

Enthalpy of atomisation of chlorine +121

Electron affinity of chlorine –370 www.

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NT Exampro 20

Cycle:

Calculation:

(5)

(c) Explain why the lattice enthalpy of magnesium fluoride, MgF2, is more exothermic than that of calcium chloride.

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

(d) The theoretical and actual values of the lattice enthalpy of magnesium fluoride are very similar because magnesium fluoride is almost completely ionic. Explain why magnesium fluoride is almost completely ionic.

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

(Total 13 marks)

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NT Exampro 21

15. (a) Define the terms

(i) lattice energy;

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

(ii) enthalpy of hydration.

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

(b) Lattice energy and enthalpies of hydration can sometimes be used to rationalise the solubility trend in a series of related salts.

(i) Draw a labelled Hess’s Law cycle that would enable the enthalpy of solution, ∆Hsol, of an ionic solid M+X– to be calculated from the lattice energy and enthalpies of hydration of the ions.

(3)

(ii) Express the enthalpy of solution ∆Hsol in terms of the other quantities.

......................................................................................................................

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

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NT Exampro 22

(iii) The approximate relative solubility (relative to magnesium sulphate) of the

sulphates of the elements of group 2 are given below.

Salt Relative solubility

MgSO4 1

CaSO4 10–2

SrSO4 10–4

BaSO4 l0–6

Explain the reasons for this trend in solubility in terms of changes of lattice energies and enthalpies of hydration.

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...................................................................................................................... (4)

(Total 13 marks)

16. (a) Consider the following equilibrium, which illustrates one industrial method used to produce hydrogen:

CH4(g) + 2H2O(g) CO2(g) + 4H2(g)

In a certain experiment, 10 g of methane, CH4, and 54 g of water, H2O, were heated in a container of volume 4 dm3. At equilibrium, 2.0 moles of hydrogen, H2, had formed. Write an expression for the equilibrium constant, Kc, for the system, and use the data to calculate a value for Kc, with units.

(8)

(b) The following table shows some data for enthalpies of formation, ∆Hf.

Substance ∆H f /kJ mol–1

CH4(g) –76

H2O(g) –242

CO2(g) –394

Use these data to calculate the enthalpy change for the reaction in (a). (3)

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NT Exampro 23

(c) In practice, the industrial production of hydrogen by this method is conducted at the

moderately high pressure of 30 atm, and the high temperature of 750 °C, in the presence of a nickel catalyst. Suggest why these conditions are used, considering the factors of rate and yield.

(7) (Total 18 marks)

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