moulsham high school 1 · when dinitrogen tetroxide, n 2o4, dissociates, the following equilibrium...

moulsham high school 1


a) Interpret the natural direction of change (spontaneous change) as the direction of increasing Interpret the natural direction of change (spontaneous change) as the direction of increasing Interpret the natural direction of change (spontaneous change) as the direction of increasing Interpret the natural direction of change (spontaneous change) as the direction of increasing number of ways of sharing energy and therefore of increasing entropy (positive entropy change) number of ways of sharing energy and therefore of increasing entropy (positive entropy change) number of ways of sharing energy and therefore of increasing entropy (positive entropy change) number of ways of sharing energy and therefore of increasing entropy (positive entropy change) –––– Entropy is a measure of the disorder or randomness of a system. The greater the degree of the disorder, the greater the stability of a system. Changes that tend to go naturally are caled spontaneous change.

b) Recall that the entropy change in any reaction is made up of the entropy change in the system Recall that the entropy change in any reaction is made up of the entropy change in the system Recall that the entropy change in any reaction is made up of the entropy change in the system Recall that the entropy change in any reaction is made up of the entropy change in the system addedaddedaddedadded to the entropy change in the surroundings, summarised by the expression to the entropy change in the surroundings, summarised by the expression to the entropy change in the surroundings, summarised by the expression to the entropy change in the surroundings, summarised by the expression

∆∆∆∆SSSStotaltotaltotaltotal = = = = ∆∆∆∆SSSSsystemsystemsystemsystem + + + + ∆∆∆∆SSSSsurroundingssurroundingssurroundingssurroundings For spontaneous change ∆S is positive.

c) Recall the factors affecting the standard entropy of a substance, in particular its physical stateRecall the factors affecting the standard entropy of a substance, in particular its physical stateRecall the factors affecting the standard entropy of a substance, in particular its physical stateRecall the factors affecting the standard entropy of a substance, in particular its physical state, , , , and predict the relative entropies of different substances (qualitative only).and predict the relative entropies of different substances (qualitative only).and predict the relative entropies of different substances (qualitative only).and predict the relative entropies of different substances (qualitative only).

d) Calculate the standard entropy change in the system for a stated shemical reaction using Calculate the standard entropy change in the system for a stated shemical reaction using Calculate the standard entropy change in the system for a stated shemical reaction using Calculate the standard entropy change in the system for a stated shemical reaction using standard entropy data.standard entropy data.standard entropy data.standard entropy data.

e) Recall the expression Recall the expression Recall the expression Recall the expression ∆∆∆∆SSSSsurroundingssurroundingssurroundingssurroundings = = = = ----∆∆∆∆H/ TH/ TH/ TH/ T AAAAnd use it to calculate entropy changes in the surroundings and, hence, caculate nd use it to calculate entropy changes in the surroundings and, hence, caculate nd use it to calculate entropy changes in the surroundings and, hence, caculate nd use it to calculate entropy changes in the surroundings and, hence, caculate ∆∆∆∆SSSStotaltotaltotaltotal

f) Recall that the feasibility of a reaction depends on the balance between Recall that the feasibility of a reaction depends on the balance between Recall that the feasibility of a reaction depends on the balance between Recall that the feasibility of a reaction depends on the balance between ∆∆∆∆SSSSsystemsystemsystemsystem and and and and ∆∆∆∆SSSSsurroundingssurroundingssurroundingssurroundings


Topic 13 Entropy

1. (a) The fermentation of glucose is an exothermic reaction and is catalysed by enzymes in yeast.

C6H12O6(aq) → 2C2Η5ΟΗ(aq) + 2CO2(g)

The reaction is slow at room temperature.

(i) Would you expect ∆Ssystem to be positive or negative for this reaction? Justify your answer with TWO pieces of evidence.

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(ii) Deduce the sign of ∆Ssurroundings. Show your reasoning.

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3. In the first stage of an industrial process for purifying nickel, carbon monoxide is passed over impure nickel at 323 K. Gaseous nickel tetracarbonyl, Ni(CO)4, is formed.

Ni(s) + 4CO(g) Ni(CO)4(g) ∆Hο = –191 kJ mol–1

(a) (i) Calculate ∆Sοsystem for this reaction given the following standard entropy values.

Substance Sο

/J mol–1 K–1

Ni(s) +29.9

CO(g) +197.6

Ni(CO)4(g) +313.4

Include a sign and units in your answer.


(2)

(ii) Refer to the equation above and comment on the sign of your answer.

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(iii) Calculate ∆Sοsurroundings at 323 K. Include a sign and units in your answer.

(2)

(iv) Deduce the direction of this reaction at 323 K. Justify your answer.

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4. The reaction between nitrogen and hydrogen can be used to produce ammonia.

N2(g) + 3H2(g) 2NH3(g) ∆Hο = – 92.2 kJ mol–1

Standard entropies are given below

Sο [N2(g)] = +191.6 J mol–1 K–1

Sο [H2(g)] = +130.6 J mol–1 K–1

Sο [NH3(g)] = +192.3 J mol–1 K–1

(a) Calculate the entropy change of the system, ∆Sοsystem, for this reaction. Include a sign and units in your answer.


(2)

(b) Calculate the entropy change of the surroundings, ∆Sοsurroundings, at 298 K. Include a sign and units in your answer.

(2)

(c) (i) Calculate the total entropy change, ∆Sοtotal, at 298 K. Include a sign and units in your answer.

(1)

(ii) Is this reaction feasible at 298 K? Justify your answer.

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6. When dinitrogen tetroxide, N2O4, dissociates, the following equilibrium is established.

N2O4(g) 2NO2(g)

(f) Predict the sign of ∆Ssystem for the reaction, giving a reason for your answer.

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(g) Write the equation for the relationship between ∆Ssurroundings and ∆H for the reaction.


(1)

(h) The magnitude of ∆Ssystem for the reaction is greater than the magnitude of ∆Ssurroundings. Explain why this must be the case.

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8. The equation below shows a possible reaction for producing methanol.

CO(g) + 2H2(g) → CH3OH(l) ∆Hο = −129 kJ mol–1

(a) The entropy of one mole of each substance in the equation, measured at 298 K, is shown below.

Substance

Sο

/J mol−1 K−1

CO(g) 197.6

H2(g) 130.6

CH3OH(l) 239.7

(i) Suggest why methanol has the highest entropy value of the three substances.

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(ii) Calculate the entropy change of the system, ∆Sοsystem, for this reaction.

(2)

(iii) Is the sign of ∆Sοsystem as expected? Give a reason for your answer.

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(iv) Calculate the entropy change of the surroundings ∆Sοsurroundings, at 298 K.


(2)

(v) Show, by calculation, whether it is possible for this reaction to occur spontaneously at 298 K.

(2)

10. The reaction between solid barium hydroxide and solid ammonium chloride can be represented by the equation below.

Ba(OH)2(s) + 2NH4Cl(s) → BaCl2(s) + 2NH3(g) + 2H2O(l) ∆Hο = +51.1 kJ mol–1

The standard entropies, at 298 K, for the reactants and products are:

Sο[Ba(OH)2(s)] = + 99.7 J mol–1K–1

Sο[NH4Cl(s)] = + 94.6 J mol–1K–1

Sο[BaCl2(s)] = + 123.7 J mol–1K–1

Sο[NH3(g)] = + 192.3 J mol–1K–1

Sο[H2O(l)] = + 69.9 J mol–1K–1 (a) Why is the standard entropy of ammonia more positive than the standard entropy of

barium chloride?

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(b) Use the values given to calculate the standard entropy change, ∆Sοsystem, for this reaction. Include the sign and units in your answer.


(2)

(c) Calculate the standard entropy change of the surroundings, ∆Sοsurroundings, at 298 K for this reaction.

(2)

(d) Use your answers to (b) and (c) to show that this reaction is feasible at 298 K.

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(e) Calculate the minimum temperature, in kelvin, at which the reaction is spontaneous.

(2)

12. Thermochemical data, at 298 K, for the equilibrium between zinc carbonate, zinc oxide and carbon dioxide is shown below.

ZnCO3(s) ZnO(s) + CO2(g) ∆Hο = +71.0 kJ mol–1

Sο[ZnO(s)] = +43.6 J mol–1 K–1


Sο[ZnCO3(s)] = +82.4 J mol–1 K–1

Sο[CO2(g)] = +213.6 J mol–1 K–1 (a) (i) Suggest reasons for the differences between the three standard entropies.

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(ii) Calculate the entropy change for the system, ∆Sοsystem

, for this reaction. Include the

sign and units in your answer.

(2)

(b) Calculate the entropy change for the surroundings, ∆Sοsurroundings, at 298 K, showing your method clearly.

(2)

(c) (i) Calculate the total entropy change for this reaction, ∆Sοtotal, at 298 K.

(1)


14.. Zinc is extracted commercially by heating its oxide with coke in a furnace. The equation for the reaction is

ZnO(s) + C(s) → Zn(s) + CO(g)

Some data for this reaction at 298 K are given below: ∆H t = +237.5 kJ mol–1

∆Stsystem = +190 J mol–1 K–1

(a) (i) Explain why ∆Ssystem has a positive value.

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(ii) The reaction is spontaneous at temperatures greater than 1250 K.

Show that this statement is correct by appropriate calculation of ∆Ssurroundings and

∆Stotal. (Assume ∆Stsystem is unaffected by temperature change.)

(3)

(iii) The boiling point of zinc is 1180 K. What difference is this likely to make to the entropy change of the system at 1250 K?

Explain your reasoning.

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(iv) How will the difference predicted in (iii) affect the temperature required for spontaneous reaction?

Explain your reasoning.

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a) Demonstrate understanding of the term equilibrium as applied to physical and chemical systems, Demonstrate understanding of the term equilibrium as applied to physical and chemical systems, Demonstrate understanding of the term equilibrium as applied to physical and chemical systems, Demonstrate understanding of the term equilibrium as applied to physical and chemical systems, inclinclinclincluding links to topic 7 uding links to topic 7 uding links to topic 7 uding links to topic 7 ----

b) Apply the equilibrium Law to a chemical reaction in order to deduce the expression for the equilibrium Apply the equilibrium Law to a chemical reaction in order to deduce the expression for the equilibrium Apply the equilibrium Law to a chemical reaction in order to deduce the expression for the equilibrium Apply the equilibrium Law to a chemical reaction in order to deduce the expression for the equilibrium constants, Kconstants, Kconstants, Kconstants, Kcccc and K and K and K and Kpppp, and their units., and their units., and their units., and their units.

(i) Equilibrium constant Kc in terms of concentrations of reactants and products A2 + 3B2 ⇒ 2AB3

Kc = [AB3]2

[A2][B2]3

(ii) Equilibrium constant Kp in terms of partial pressures for the formation of products from reactants. Kp = ρ(AB3)2

ρ(A2)ρ(B2)3

c) Perform simple calculations related to the Equilibrium Law.Perform simple calculations related to the Equilibrium Law.Perform simple calculations related to the Equilibrium Law.Perform simple calculations related to the Equilibrium Law.

d) Predict whethePredict whethePredict whethePredict whether a system is capable of spontaneous change, using r a system is capable of spontaneous change, using r a system is capable of spontaneous change, using r a system is capable of spontaneous change, using ∆∆∆∆S and KS and KS and KS and Kcccc as indicators of as indicators of as indicators of as indicators of thermodynamic feasibility, and the position of equilibrium (qualitative only)thermodynamic feasibility, and the position of equilibrium (qualitative only)thermodynamic feasibility, and the position of equilibrium (qualitative only)thermodynamic feasibility, and the position of equilibrium (qualitative only)

e) Understand and use the term: acid, base, neutral, pH, indicator, buffer.Understand and use the term: acid, base, neutral, pH, indicator, buffer.Understand and use the term: acid, base, neutral, pH, indicator, buffer.Understand and use the term: acid, base, neutral, pH, indicator, buffer.

f) Demonstrate understandiDemonstrate understandiDemonstrate understandiDemonstrate understanding of the term equilibrium as applied to acidng of the term equilibrium as applied to acidng of the term equilibrium as applied to acidng of the term equilibrium as applied to acid----base systemsbase systemsbase systemsbase systems

g) Apply the equilirium law to an acidApply the equilirium law to an acidApply the equilirium law to an acidApply the equilirium law to an acid----base system in order to deduce the expression for the equilibrium base system in order to deduce the expression for the equilibrium base system in order to deduce the expression for the equilibrium base system in order to deduce the expression for the equilibrium constant Kconstant Kconstant Kconstant Kaaaa. . . .

h) Demonstrate understanding of practical methods of determining acid and alkaliDemonstrate understanding of practical methods of determining acid and alkaliDemonstrate understanding of practical methods of determining acid and alkaliDemonstrate understanding of practical methods of determining acid and alkali concentrations by concentrations by concentrations by concentrations by titration including the interpretation of pH changes to select indicators.titration including the interpretation of pH changes to select indicators.titration including the interpretation of pH changes to select indicators.titration including the interpretation of pH changes to select indicators.

i) Concentrations by titration including the interpretation of pH changes to select indicator.Concentrations by titration including the interpretation of pH changes to select indicator.Concentrations by titration including the interpretation of pH changes to select indicator.Concentrations by titration including the interpretation of pH changes to select indicator.

j) Plan an investigation using acidPlan an investigation using acidPlan an investigation using acidPlan an investigation using acid----alkali titrations, and justify the alkali titrations, and justify the alkali titrations, and justify the alkali titrations, and justify the procedures involved.procedures involved.procedures involved.procedures involved.

k) Perform calculations from acidPerform calculations from acidPerform calculations from acidPerform calculations from acid----alkali titration data.alkali titration data.alkali titration data.alkali titration data.

l) Demostrate understanding of the BrønstedDemostrate understanding of the BrønstedDemostrate understanding of the BrønstedDemostrate understanding of the Brønsted----Lowery theory of acidLowery theory of acidLowery theory of acidLowery theory of acid----base behaviour, and use it to base behaviour, and use it to base behaviour, and use it to base behaviour, and use it to interpret the behaviour of strong acids/ bases, weak acids/ bases, conjugate acid/ basinterpret the behaviour of strong acids/ bases, weak acids/ bases, conjugate acid/ basinterpret the behaviour of strong acids/ bases, weak acids/ bases, conjugate acid/ basinterpret the behaviour of strong acids/ bases, weak acids/ bases, conjugate acid/ base pairs, and e pairs, and e pairs, and e pairs, and buffer solutions.buffer solutions.buffer solutions.buffer solutions.


m) Recall the terms pH, KRecall the terms pH, KRecall the terms pH, KRecall the terms pH, Kaaaa and K and K and K and Kwwww and perform simple calculations using them, including calculating and perform simple calculations using them, including calculating and perform simple calculations using them, including calculating and perform simple calculations using them, including calculating the pH of buffer solutions.the pH of buffer solutions.the pH of buffer solutions.the pH of buffer solutions.

n) Deduce and interpret qualitatively the effect of changes in temperature and pressure on systems atDeduce and interpret qualitatively the effect of changes in temperature and pressure on systems atDeduce and interpret qualitatively the effect of changes in temperature and pressure on systems atDeduce and interpret qualitatively the effect of changes in temperature and pressure on systems at equilibrium in terms of entropy changes; and the value of equilibrium constants.equilibrium in terms of entropy changes; and the value of equilibrium constants.equilibrium in terms of entropy changes; and the value of equilibrium constants.equilibrium in terms of entropy changes; and the value of equilibrium constants.


Topic 14 How Far? Reversible Reactions

2. Maleic acid has the structure shown below.

It is a dioic acid and is one of a pair of geometric isomers.

(b) Write the equation for the reaction of sodium hydroxide with maleic acid, HO2CCH=CHCO2H, to form disodium maleate (state symbols are not required).

(2)

(c) Maleic acid has two acid dissociation constants with the following values

Ka1 = 1.48 × 10–2 mol dm–3 Ka2 = 8.51 × 10–7 mol dm–3

(i) Explain why maleic acid has two different Ka values.

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(ii) Complete an equation for the first dissociation of maleic acid.

HO2CCH=CHCO2H (1)

(iii) Maleic acid is a weak acid. Calculate the pH of a solution of maleic acid of concentration 0.0100 mol dm–3 using the value of Ka1 but not Ka2.

(2)


(iv) Both carboxylic acid groups in maleic acid do dissociate. Suggest how your answer in (c)(iii) might be affected if you consider Ka2 in addition to Ka1?

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3. In the first stage of an industrial process for purifying nickel, carbon monoxide is passed over impure nickel at 323 K. Gaseous nickel tetracarbonyl, Ni(CO)4, is formed.

Ni(s) + 4CO(g) Ni(CO)4(g) ∆Hο = –191 kJ mol–1

(b) (i) Write the expression for the equilibrium constant, Kp, for this reaction.

(1)


(ii) 100 moles of gaseous carbon monoxide is mixed with excess solid nickel at 323 K

in a vessel kept at 1.00 atmosphere pressure. At equilibrium, 1.00 mole of the carbon monoxide has reacted.

Complete the table below and then calculate the value of Kp at this temperature. Include the units of Kp in your answer.

Substance Moles at start Moles at

equilibrium Partial pressure, peq

/atm

Ni(CO)4 0

CO 100 99.0

(4)

(iii) As Kp has such a small value, suggest THREE ways in which this industrial process could be improved to increase profitability. Justify each of your suggestions.

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(c) The second stage of this process is to recover the nickel from the nickel tetracarbonyl,

Ni(CO)4. By considering your calculations of the entropy changes, suggest how this could be done. Justify your suggestion.

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4. The reaction between nitrogen and hydrogen can be used to produce ammonia.

N2(g) + 3H2(g) 2NH3(g) ∆Hο = – 92.2 kJ mol–1

(d) In industry the reaction is carried out at about 700 K using an iron catalyst and high pressures.

(i) The yield of ammonia produced at equilibrium is less at 700 K than at 298 K, if the pressure remains constant. In terms of entropy, explain why this happens.

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(ii) Higher pressures increase the yield of ammonia at equilibrium. Suggest a reason why pressures greater than 300 atmospheres are not routinely used.

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(iii) Iron is a heterogeneous catalyst. Explain what is meant by heterogeneous.

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5. This question is about butanoic acid, CH3CH2CH2CO2H.

(a) Butanoic acid can be described as a weak acid. Explain what is meant by weak as used in this context.


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(b) The acid dissociation constant, Ka, for butanoic acid is 1.50 × 10–5 mol dm–3.

(i) Write the expression for Ka.

(1)

(ii) Calculate the pH of a 0.0100 mol dm–3 solution of the acid.

(2)

(c) A buffer solution can be made by mixing aqueous solutions of butanoic acid and sodium butanoate.

Explain what is meant by a buffer solution.

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(d) The following equilibrium reactions are present in the buffer solution.

A CH3CH2CH2CO2H(l) CH3CH2CH2CO2–(aq) + H+(aq)

B H+(aq) +OH–(aq) H2O(l)

(i) Which TWO of the substances behave as Brønsted–Lowry bases?

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(ii) Use the two equations, A and B, to explain how the buffer solution responds to the addition of a solution containing hydroxide ions.


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(iii) The pH of a buffer solution is given by the equation

pH = −1g Ka − 1g [base][acid]

Using this equation, calculate the pH of a buffer solution made by mixing 100 cm3 of 0.0100 mol dm–3 butanoic acid solution with 300 cm3 of 0.00500 mol dm–3 sodium butanoate solution. [The Ka for butanoic acid is 1.50 × 10–5 mol dm–3.]

(2)

6. When dinitrogen tetroxide, N2O4, dissociates, the following equilibrium is established.

N2O4(g) 2NO2(g)

(a) State a property which could be measured to follow the progress of this reversible reaction.

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(b) Write an expression for the equilibrium constant, Kc, for this reaction.

(1)

(c) When a sample of 0.0370 moles of gaseous dinitrogen tetroxide is allowed to dissociate at 25 °C in a container of volume 1 dm3, 0.0310 moles of N2O4(g) remain in the equilibrium mixture.

Complete the table below, and use the data to calculate Kc for the reaction. Include a unit in your answer.

N2O4 NO2

Number of moles at start 0.0370 0

Number of moles in 1 dm3 at equilibrium

0.0310

Kc calculation:

(3)

(d) The reaction was repeated at a higher pressure, maintaining the temperature at 25 °C.

(i) How does this increase in pressure affect the amount of nitrogen dioxide, NO2(g), in the equilibrium mixture?

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(ii) How does this increase in pressure affect the value of Kc?

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(e) The reaction was repeated at the original pressure, but the temperature was increased to 75

°C. The value of Kc was approximately twenty times greater.

How does this information show that the reaction is endothermic?

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7. (a) Benzoic acid is a weak acid. What is meant by a weak acid?

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(b) Write an expression for the acid dissociation constant, Ka, for benzoic acid.

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(c) Ka for benzoic acid has value 6.3 × 10–5 mol dm–3. Use this data to calculate the pH of a

solution of benzoic acid of concentration 0.050 mol dm–3.

(3)

(d) In a titration, 40.0 cm3 of 0.050 mol dm–3 sodium hydroxide was added from a burette to 20.0 cm3 of 0.050 mol dm–3 benzoic acid. Sketch a curve on the grid below to show how the pH of the solution would change as the sodium hydroxide was added. Detailed calculations are not required.

(3)

(e) Name a suitable indicator to detect the end-point of this titration.

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(f) A mixture of benzoic acid and sodium benzoate can act as a buffer solution.

In what proportion must solutions of benzoic acid and sodium benzoate, of the same concentration, be mixed to produce a buffer solution of pH 4.5?

You may use the equation pH = –log Ka –log [base][acid]

.

Ka for benzoic acid has value 6.3 × 10–5 mol dm–3.


(2)

(d) (i) Write an equation for the dissociation of benzoic acid in aqueous solution.

(1)

(ii) Write an expression for the dissociation constant of benzoic acid in terms of the appropriate equilibrium concentrations.

Ka = (1)

(iii) Calculate the pH of a 0.010 mol dm–3 solution of benzoic acid in water.

You should make the usual two simplifying assumptions.

(3)

(iv) State ONE of the assumptions that you made in your calculation in (d)(iii).

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(g) Suggest a salt which would make a buffer solution with pH greater than 7 when mixed with aqueous ammonia.

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8. The equation below shows a possible reaction for producing methanol.

CO(g) + 2H2(g) → CH3OH(l) ∆Hο = −129 kJ mol–1

(b) When methanol is produced in industry, this reaction is carried out at 400 ºC and 200 atmospheres pressure, in the presence of a catalyst of chromium oxide mixed with zinc oxide. Under these conditions methanol vapour forms and the reaction reaches equilibrium. Assume that the reaction is still exothermic under these conditions.


CO(g) + 2H2(g) CH3OH(g)

(i) Suggest reasons for the choice of temperature and pressure.

Temperature ........................................................................................................

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Pressure ...............................................................................................................

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(ii) The catalyst used in this reaction is heterogeneous. Explain this term.

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(iii) Write an expression for the equilibrium constant in terms of pressure, Kp, for this reaction.

CO(g) + 2H2(g) CH3OH(g)

(1)

(iv) In the equilibrium mixture at 200 atmospheres pressure, the partial pressure of carbon monoxide is 55 atmospheres and the partial pressure of hydrogen is 20 atmospheres.

Calculate the partial pressure of methanol in the mixture and hence the value of the equilibrium constant, Kp. Include a unit in your answer.

(2)


9. (a) Calculate the pH of a solution of sodium hydroxide of concentration 0.600 mol dm–3, assuming it is a strong base.

The ionisation constant for water, Kw, is 1.00 × 10–14 mol2 dm–6.

(2)

(b) Calculate the pH of a solution of ethanoic acid of concentration 0.600 mol dm–3.

The acid dissociation constant, Ka, for ethanoic acid is 1.70 × 10–5 mol dm–3.

(3)

(c) A 100 cm3 sample of sodium hydroxide of concentration 0.600 mol dm–3 is mixed with 200 cm3 of ethanoic acid of concentration 0.600 mol dm–3.

(i) Write an equation for the reaction which occurs.

(1)

(ii) Calculate the number of moles of unreacted ethanoic acid remaining in the resulting mixture.

(2)

(iii) Calculate the concentration, in mol dm−3, of the unreacted ethanoic acid in the resulting mixture.


(1)

(iv) The mixture which forms is a buffer. Why does the pH of the mixture remain constant when small quantities of solutions containing H+ or OH– ions are added?

You may find it helpful to use equations in your explanation.

Addition of solution containing H+ ions. ...........................................................

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Addition of solution containing OH– ions. .........................................................

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(v) The concentration of sodium ethanoate in the mixture is 0.200 mol dm−3. Calculate

the pH of the mixture, using your answer to (iii).

You may use the equation

pH = – log Ka – log [base][acid]

(2)

11. A saturated solution of calcium hydroxide, Ca(OH)2(aq), has a pH of 9.6.

(a) Write an expression linking hydrogen ion concentration and pH. Use this to calculate the concentration of hydrogen ions in this solution.

(3)

(b) (i) The ionisation constant for water, Kw = 1.0 × 10–14 mol2 dm–6.

Write the expression for Kw.

Kw =

(1)

(ii) Calculate the concentration of hydroxide ions in the saturated solution of calcium hydroxide.

(1)

(iii) Calculate the concentration of calcium hydroxide in the saturated solution.


(1)

(iv) Calculate the solubility of calcium hydroxide in g dm–3.

Give your answer to three significant figures.

(1)

(v) Suggest why your calculated value may differ significantly from the value in chemistry reference books.

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(c) An alternative method for finding the solubility of calcium hydroxide is to titrate 100 cm3 of the saturated solution with hydrochloric acid of concentration 0.00100 mol dm–3.

Ca(OH)2 (aq) + 2HCl (aq) → CaCl2 (aq) + 2H2O (l)

(i) Calculate the pH of the hydrochloric acid.

(1)

(ii) Use your answer to (b)(iii) and the information above to calculate the volume of hydrochloric acid needed to neutralise 100 cm3 of the saturated calcium hydroxide solution.

(3)

(iii) Sketch the titration curve for this reaction.


(2)

(iv) Suggest why phenolphthalein is not a suitable indicator for this reaction.

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13.. (a) Some data concerning three carboxylic acids are shown in the following table.

Name of acid Formula Acid dissociation constant, Ka/mol dm–3

Chloroethanoic CH2C1CO2H 1.3 × 10–3

Methanoic HCO2H 1.6 × 10–4

Propanoic CH3CH2CO2H 1.3 × 10–5

(i) Complete the expression for the acid dissociation constant of chloroethanoic acid, including all appropriate symbols.

Ka =

(2)

(ii) Use the values of Ka to decide which is the strongest of the three acids. Explain how you reached your decision.

Strongest acid.........................................................................................

Explanation.............................................................................................

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

(b) (i) Calculate the pH of a 0.050 M solution of propanoic acid. Make the usual approximations in your calculation.

(2)

(ii) Calculate the pH of a 0.050 M solution of sodium hydroxide. (The ionization constant for water, Kw = 1.0 × 10–14

Mol² dm–6)

(2)

(iii) Draw a graph, taking into account your values from(i) and(ii), to show how the pH changes when 0.050 M sodium hydroxide is added from a burette to 25.0 cm³ of 0.050 M propanoic acid.

Assume a total of 40.0 cm³ of alkali is added.

14

12

10

8

6

4

2

0

pH

10 20 30 40Volume of NaOH/cm3

(3)

(iv) The names and pH ranges of three acid/alkali indicators are shown in the following table:

Name of indicator pH range

Bromophenol blue 2.8 – 4.6

Bromothymol blue 6.0 – 7.6

Phenolphthalein 8.2 – 10.0

Which indicator would you use for the titration described in (iii)? Justify your decision.

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

(c) (i) Methanoic acid reacts with sodium carbonate, liberating carbon dioxide gas. Write a balanced equation for this reaction.

(1)

(ii) Draw the displayed formula of the organic product of this reaction.

(1)

(d) Carboxylic acids and some of their derivatives react with alcohols, forming esters.

(i) Draw the displayed formula of the ester produced when chloroethanoic acid reacts with methanol.

(2)

(ii) Why is a small quantity of concentrated sulphuric acid added to the mixture of chloroethanoic acid and methanol when carrying out this reaction?

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

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

(iii) Esters often contribute to the characteristic flavours and fragrances of fruit. They can be made synthetically for use in the food industry.

Suggest ONE advantage and ONE disadvantage of using synthetically made esters in place of natural fruit flavourings.

Advantage

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

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

Disadvantage

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

(Total 20 marks) 14.. Zinc is extracted commercially by heating its oxide with coke in a furnace. The equation for the

reaction is


ZnO(s) + C(s) → Zn(s) + CO(g)

Some data for this reaction at 298 K are given below: ∆H t = +237.5 kJ mol–1

∆Stsystem = +190 J mol–1 K–1

(b) (i) The carbon monoxide produced during the extraction of zinc is toxic and must not be allowed to escape into the atmosphere. One way of removing it is to convert it into carbon dioxide:

2CO(g) + O2(g) → 2CO2(g)

Write an expression for the equilibrium constant, Kc, for the above reaction. (2)

(ii) By considering the entropy change of the system or by using Le Chatelier’s principle, explain why the formation of carbon dioxide is favoured by increasing the pressure.

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…………………..……………………………………………………………. (2)

(iii) Suggest a reason why it may be undesirable to allow the carbon dioxide to escape into the atmosphere.

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…………………..……………………………………………………………. (1)

(c) Zinc ores often contain small but significant quantities of other so–called ‘heavy’ metals which can cause problems if released into the local environment. Suggest one metal which might cause such problems. You may find it helpful to refer to the Periodic Table.

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

(Total 12 marks)

15. This question is concerned with boric acid, H3BO3, which is a weak acid.

When it is dissolved in water several equilibria are established, one of them being

H3BO3(aq) H+(aq) + H2BO3–(aq)

(a) (i) Write the expression for the acid dissociation constant, Ka, for the equilibrium shown above.

(1)


(ii) Using a pH meter, a student finds the pH of a 0.1 M solution of boric acid to be 5.1.

Assuming that this equilibrium makes the only significant contribution to the hydrogen ion concentration, calculate the value of Ka for this equilibrium.

(3)

(iii) Write equations for TWO other equilibria likely to be established in an aqueous solution of boric acid.

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(2) (iv) Unlike many acids, boric acid does not react with sodium carbonate to produce

carbon dioxide gas. Suggest the reason for this.

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…………………..……………………………………………………………. (2)

(v) Suggest the formula of a compound which would make a buffer solution when added to an aqueous solution of boric acid.

…………………..……………………………………………………………. (1)

moulsham high school 1 · when dinitrogen tetroxide, n 2o4, dissociates, the following equilibrium...

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