Unit 4 - Entropy
Unit 4 - Jan 2010
Q1.
 The oxidation of iron metal in the presence of oxygen is spontaneous.

(a)  Explain the meaning of spontaneous in a thermodynamic context.
(1)
 ............................................................................................................................................. 
 ............................................................................................................................................. 
(b)  (i)  Find the values of the standard molar entropies of iron and of iron(III) oxide
             from your data booklet.
(1)
 ............................................................................................................................................. 
 ............................................................................................................................................. 
(ii)  The standard molar entropy at 298 K for oxygen molecules O2 is +205 J mol−1 K−1.
        Calculate the standard entropy change of the system for the reaction between
        iron and oxygen. Include a sign and units in your answer.
(2)

(iii)  The standard enthalpy change for the reaction at 25 °C is −1648 kJ mol−1.
        Calculate ΔSsurroundings.
(1)

(iv)  Use your answers to (b)(ii) and (iii) to calculate the total standard entropy
        change for the reaction. Include a sign and units in your answer.
(2)

*(v)  The reaction is thermodynamically spontaneous.
          Use your answers to (b)(ii), (iii) and (iv) to explain, in terms of the physical
          states of the substances in the reaction and the movement of the molecules in
          the surroundings, why this is so.
(3)
 ............................................................................................................................................. 
............................................................................................................................................. 
............................................................................................................................................. 
(Total for question = 10 marks)
Q2.
 When ammonium nitrate crystals dissolve in water, the entropy of the system
   A     remains the same.
   B     falls, because the hydrated ions are more ordered than the solid.
   C     rises, because the ions in the crystal become hydrated in the solution.
   D     rises, because the ions are arranged more randomly in the solution than in the
                crystal.
(Total for question = 1 mark)

Unit 4 - Jun 2010
Q3.
 (a)  Crystals of hydrated cobalt(II) chloride, CoCl2.6H2O, lose water when they are heated, forming anhydrous cobalt(II) chloride, CoCl2.

(i)  Calculate the entropy change of the system, , at 298 K. Include a sign and units in your answer. You will need to refer to your data booklet.
(2)

(ii)  Explain whether the sign of your answer to (a)(i) is as expected from the equation for the reaction.
(1)
 ............................................................................................................................................. 
 ............................................................................................................................................. 
 ............................................................................................................................................. 
(iii)  The standard enthalpy change for the reaction, , is +88.1kJ mol−1.
        Calculate the entropy change in the surroundings, , at 298 K for this reaction. Include a sign and units in your answer.
(2)

(iv)  Calculate the total entropy change, , at 298 K for the reaction
(1)

(v)  Does your answer to (a)(iv) indicate whether hydrated cobalt(II) chloride can be stored at 298 K without decomposition?  Explain your answer.
 ............................................................................................................................................. 
 ............................................................................................................................................. 
 ............................................................................................................................................. 
(b)  A student attempted to measure the enthalpy change of solution of anhydrous cobalt(II) chloride by adding 2.00 g of cobalt(II) chloride to 50.0 cm3 of water in a well-insulated container.  A temperature rise of 1.5 °C was recorded.
The student used a balance which reads to 0.01g, a 50.0 cm3 pipette, and a thermometer which can be read to 0.25 °C.
(i)  Which measuring instrument should be changed to give a result which is closer to the accepted value?  Justify your answer.
(2)
 ............................................................................................................................................. 
............................................................................................................................................. 
 ............................................................................................................................................. 
 ............................................................................................................................................. 
(ii)  Suggest ONE other change the student could make to give a result which is closer to the accepted value.  Justify your suggestion.
(2)
 ............................................................................................................................................. 
 ............................................................................................................................................. 
 ............................................................................................................................................. 

*(c)  The lattice energies of magnesium chloride, MgCl2, calcium chloride, CaCl2, and strontium chloride, SrCl2 are shown in the table below.

(i)  Use data on ionic radii, from your data booklet, to explain the trend in these values.  Estimate a value for the lattice energy of cobalt(II) chloride, giving ONE piece of data to justify your estimate.
(4)
 ............................................................................................................................................. 
............................................................................................................................................. 
 ............................................................................................................................................. 
 ............................................................................................................................................. 
 (ii)  Explain how lattice energy values, together with other data, can be used to predict the solubility of ionic compounds.
(3)
 ............................................................................................................................................. 
 ............................................................................................................................................. 
............................................................................................................................................. 
*(d)  Cobalt forms another chloride, CoCl3, but scientists predict that MgCl3 cannot be made. Suggest a reason for this.

          You should consider the enthalpy changes in the Born-Haber cycle, which provide evidence about why cobalt(III) chloride is known but magnesium(III) chloride is not.
(2)
 ............................................................................................................................................. 
............................................................................................................................................. 
 ............................................................................................................................................. 
 ............................................................................................................................................. 
(Total for question = 20 marks)

Q4.
Which of the following is true for the exothermic reaction shown below?

(Total for question = 1 mark)

Unit 4 - Jan 2011
Q5.
Hydrogen can be manufactured by reacting methane with steam, as shown in the equation below.

Use these values:
     the standard entropy of 1 mol of H2(g) is (2 × 65.3) = 130.6 J mol−1K−1 the standard entropy of 1 mol of H2O(g) is 188.7 J mol−1K−1
You will also need to refer to the data booklet in the calculations which follow.
(a) Calculate the standard entropy change of the system,, for this reaction at 298 K.
(2)

(b) Calculate the standard entropy change of the surroundings, , for this reaction at 298 K. Include a sign and units in your answer.
(2)

(c) Calculate the total entropy change, , for this reaction at 298 K.
            Explain why this value shows that the reaction is not spontaneous at this temperature.
(2)

 ............................................................................................................................................. 
 ............................................................................................................................................. 
(d) The composition of an equilibrium mixture produced at 2.0 atmospheres pressure and at a much higher temperature is shown below.

    Amount in equilibrium                            0.80          0.80         1.20        3.60
           mixture / mol
*(i) Write the expression for the equilibrium constant, Kp, of the reaction and calculate its value. Include units in your answer.
(6)

(ii) The total entropy change in J mol−1K−1 is related to the equilibrium constant by the equation

Calculate the total entropy change at the temperature of the reaction.
[R = 8.31 J mol−1 K−1]
(1)

(iii) Calculate the temperature at which this equilibrium is reached using your answer to (ii) for  .  Assume that ΔH is still +206.1 kJ mol−1 and that  = +225 J K−1 mol−1. (This is not the same as the value for  calculated in (a) which is at 298 K.)
(2)

*(e) Use the magnitude and signs of the entropy changes to explain the effect of a temperature increase on the equilibrium constant of this endothermic reaction.
(2)
 ............................................................................................................................................. 
............................................................................................................................................. 
 ............................................................................................................................................. 
 ............................................................................................................................................. 
(Total for question = 17 marks)

Q6.
Which reaction has the most positive entropy change for the system, ΔSsystem?
   A  NaOH(aq) + HCl(aq) → NaCl(aq) + H2O(l)
   B  AgNO3(aq) + NaCl(aq) → AgCl(s) + NaNO3(aq)
   C  C2H4(g) + HCl(g) → C2H5Cl(l)
   D  C4H10(g) → C2H4(g) + C2H6(g)

(Total for question = 1 mark)

Unit 4 - Jun 2011
Q7.
A decrease in the entropy of the system, ΔSsystem, occurs when
   A     water freezes.
   B     water boils.
   C     water reacts with sodium.
   D     water reacts with ethanoyl chloride.
(Total for question = 1 marks)

Unit 4 - Jan 2012
Q8.
The hydrocarbon butane can be cracked to form propene and methane by passing it over a heated aluminium oxide catalyst at a temperature of 700 K. The equation for the reaction is

(a) (i) Use page 20 of the data booklet to complete the table below.
(1)

(ii) Calculate the standard entropy change of the system, ΔS , for this reaction.
Include a sign in your answer.
(2)

 
(iii) Was the sign for your answer as you expected? Fully justify your answer.
(2)
 ............................................................................................................................................. 
 ............................................................................................................................................. 
 ............................................................................................................................................. 
 ............................................................................................................................................. 
(iv) Calculate the entropy change of the surroundings, ΔSsurroundings, at 700 K.
Include a sign and units in your answer.
Use this value and your answer to (ii) to explain why butane cracks into propene and methane at this temperature.
(3)

 
 ............................................................................................................................................. 
............................................................................................................................................. 
(v) Calculate the minimum temperature needed for this reaction to be thermodynamically feasible.
(3)

 

(b) The aluminium oxide behaves as a heterogeneous catalyst. Explain both what is
meant by the term heterogeneous and how, in terms of activation energy, the catalyst
is able to speed up the reaction.
(3)
 ............................................................................................................................................. 
 ............................................................................................................................................. 
 ............................................................................................................................................. 
 ............................................................................................................................................. 
 ............................................................................................................................................. 
 ............................................................................................................................................. 
 ............................................................................................................................................. 
(Total for question = 14 marks)

Unit 4 - Jun 2012
Q9.
 The reaction below is carried out at 25 °C. Use the equation and the data to answer the questions that follow.

 (a) The standard entropy change of the system, in J mol−1 K−1, is
(1)
   A      −186
   B      +186
   C      −233
   D      +233
 (b) The standard entropy change of the surroundings, in J mol−1 K−1, is
(1)
   A      107.4 × 1000 / 25
   B      −107.4 × 1000 / 25
   C      107.4 × 1000 / 298
   D      −107.4 × 1000 / 298
(Total for question = 2 marks)

Q10.
This question is about the solubilty of some Group 1 halides.
(a)  Potassium fluoride is a soluble, white, crystalline solid used in etching glass. A Hess cycle can be used to calculate its enthalpy of solution, using data including enthalpies of hydration of ions.

       Define the term enthalpy of hydration of an ion.
(2)
 ............................................................................................................................................. 
............................................................................................................................................. 
 ............................................................................................................................................. 
(b)  Consider the Hess cycle below.

(i)  Complete the cycle by filling in the empty box.
(1)

(ii)  Apply Hess's Law to obtain an expression for ΔHsol in terms of ΔH1 and ΔH2.
(1)
ΔHsol =
(iii)  Give the name of the energy change ΔH1
(1)
 ............................................................................................................................................. 

(iv)  Referring to page 12 of the data booklet and your answer to (ii), calculate the standard enthalpy of solution of potassium fluoride.
(2)

(c)  The standard enthalpy of solution of sodium chloride is + 3 kJ mol−1.
(i)  1 g of sodium chloride was added to 250 cm3 of water in a beaker and stirred with a thermometer graduated in intervals of 1 °C. Describe and explain what would happen to the reading on the thermometer as the sodium chloride dissolves. No calculation is required.
(3)
 ............................................................................................................................................. 
............................................................................................................................................. 
 ............................................................................................................................................. 
*(ii)  Explain, in terms of entropy changes, why sodium chloride dissolves in water under standard conditions. No calculation is required.
(4)
 ............................................................................................................................................. 
............................................................................................................................................. 
 ............................................................................................................................................. 
 ............................................................................................................................................. 
*(d)  Lithium iodide is generally much more soluble in organic solvents than lithium chloride. Explain this observation using values of lattice energies from your data booklet and your knowledge of the trend in ionic radii down Group 7.
(4)
 ............................................................................................................................................. 
 ............................................................................................................................................. 
 ............................................................................................................................................. 
 ............................................................................................................................................. 
 ............................................................................................................................................. 
 ............................................................................................................................................. 
 ............................................................................................................................................. 
 ............................................................................................................................................. 
 ............................................................................................................................................. 
(Total for question = 18 marks)
Unit 4 - Jan 2013
Q11.
Ethanoic acid and ethanol react together to form the ester ethyl ethanoate, CH3COOC2H5, and water.

(a)  (i)   Give the expression for Kc.
(1)

(ii)   An equilibrium was reached when the amounts of substances shown in the table below were used.

        Complete the table to show the amounts of each substance present at equilibrium.
(2)

(iii)   Explain why Kc for this reaction has no units.
(1)
 ............................................................................................................................................. 
 ............................................................................................................................................. 
(iv)   Calculate the numerical value of Kc.
(1)

(b)  The esterification reaction above was carried out in the presence of hydrochloric acid as the catalyst.
        State the effect on the equilibrium position and the rate of attainment of equilibrium if the concentration of the acid catalyst were to be increased.
(2)
 ............................................................................................................................................. 
  ............................................................................................................................................. 
 ............................................................................................................................................. 
(c)  (i)   Identify which bonds are broken and which bonds are made in the esterification reaction.
(2)
Bonds broken:

Bonds made:

(ii)   Explain why ΔH for this reaction is not exactly zero.
        (A calculation is not required.)
(1)
 ............................................................................................................................................. 
 ............................................................................................................................................. 
(d)  (i)   State the relationship between ΔStotal and the equilibrium constant, K, of a reaction.
(1)

*(ii)   Use entropy considerations and your answer to (d)(i) to predict any effect of an increase in temperature on the value of the equilibrium constant of a reaction for which ΔH is zero.  Assume that ΔSsystem does not change with temperature.
(3)
 ............................................................................................................................................. 
............................................................................................................................................. 
 ............................................................................................................................................. 
 ............................................................................................................................................. 
(e)  An alternative method for preparing ethyl ethanoate is to react ethanoyl chloride with ethanol.
(i)   Give the equation for the reaction.
(1)

(ii)   Draw the skeletal formula of ethyl ethanoate.
(1)

(iii)   Ethanoyl chloride also reacts with concentrated ammonia.  Draw the displayed formula of the organic product of this reaction.
(1)

(f)  (i)   Complete the equation below for the alkaline hydrolysis of ethyl ethanoate using sodium hydroxide.  State symbols are not required.
(1)

(ii)   Explain why the reaction in (f )(i) gives a better yield of the alcohol compared with acid hydrolysis of the ethyl ethanoate.
(1)
 ............................................................................................................................................. 
............................................................................................................................................. 
 ............................................................................................................................................. 
(Total for question = 19 marks)
Unit 4 - Jun 2013
Q12.
This question is about the equilibrium reaction below.

Fe2+(aq) + Ag+(aq)  Fe3+(aq) + Ag(s)

The equilibrium is reached slowly.

*(a)  Describe the changes you would see if aqueous solutions of iron(II) sulfate and silver nitrate were mixed and allowed to stand for a few hours.
(2)
 ............................................................................................................................................. 
............................................................................................................................................. 

(b)  The concentration of silver ions in the equilibrium mixture can be found by titration with potassium thiocyanate. Silver thiocyanate precipitates.

Ag+(aq) + CNS−(aq)  AgCNS(s)

When all the silver ions have reacted, a deep red complex ion of iron(III) thiocyanate forms.

In an experiment, 25.0 cm3 of 0.100 mol dm−3 silver nitrate solution was added to 25.0 cm3 of 0.100 mol dm−3 of iron(II) sulfate solution, mixed thoroughly, and allowed to stand overnight in an air-tight container.

10.0 cm3 samples of the reaction mixture were then titrated with 0.0200 mol dm−3 potassium thiocyanate solution. The average titre was 5.60 cm3.

(i)  The initial concentrations of silver ions and iron(II) ions in the reaction mixture are the same.

Calculate this initial concentration in mol dm−3.
(1)

(ii)  Calculate the number of moles of silver ions in the 10.0 cm3 sample at equilibrium and hence calculate the equilibrium concentration of silver ions in the mixture.
(2)

(iii)  Deduce the equilibrium concentration of iron(II) ions.
(1)

(iv)  Hence calculate the equilibrium concentration of iron(III) ions.
(1)

(v)  Write the expression for the equilibrium constant, Kc, for the reaction

Fe2+(aq) + Ag+(aq)  Fe3+(aq) + Ag(s)
Calculate its value and give your answer, with appropriate units, to three significant figures.
(4)

(c)  (i)  The relationship between the total entropy change for a reaction and the equilibrium constant is

Calculate the total entropy change for this reaction, giving a sign and appropriate units.
[R = 8.31 J mol−1 K−1]
(2)

(ii)  Fe2+(aq) + Ag+(aq)  Fe3+(aq) + Ag(s)
 for this reaction is −208.3 J mol−1 K−1
Use ideas about entropy to explain why this value is negative.
(2)
 ............................................................................................................................................. 
............................................................................................................................................. 
 ............................................................................................................................................. 
(iii)  Calculate the entropy change of the surroundings, .
(1)

(iv)  Use your answer to (c)(iii) to calculate the standard enthalpy change, ΔH, for this reaction at 298 K.

Hence state and explain the effect of increasing temperature on the value of 
(3)

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

*(d)  After the samples from the original mixture for the titration are taken, the remainder was filtered and then allowed to stand overnight, in an air-tight container at the same temperature. Another 10.0 cm3 sample was taken and titrated. How, if at all, would you expect the titre to change?

Justify your answer. A calculation is not required.
(2)
............................................................................................................................................. 
(Total for Question = 21 marks)

Q13.
Calcium carbonate decomposes at high temperature to form calcium oxide and carbon dioxide:

CaCO3(s) → CaO(s) + CO2(g)
Calcium carbonate is thermodynamically stable at room temperature because for this reaction
   A    the activation energy is high.
   B    the enthalpy change, ΔH, is positive.
   C    entropy change of the system (ΔSsystem) is positive.
   D    entropy change of the system (ΔSsystem) is negative.

(Total for Question = 1 mark)

Q14.
The overall equation for a reaction between two chemicals, M and N, is

M + 2N → P + Q
(a)  This reaction occurs spontaneously at room temperature. Which of the following must be true?
(1)
   A    ΔH is positive.
   B    ΔH is negative.
   C    ΔS is positive.
   D    ΔS is negative.

(b)  The reaction above occurs in two stages via an intermediate, T.

M + N → T            slow

N + T → P + Q            fast
From this it can be deduced that the rate equation for the reaction between M and N is
(1)
   A    rate = k[M][N]
   B    rate = k[M][N]2
   C    rate = k[M][T]
   D    rate = k[N][T]

(Total for Question = 2 marks)

Unit 4 - Jun 2014
Q15.
 This question is about calcium chloride, CaCl2.
It can be formed by burning calcium in chlorine.

You must include a sign and units in your answers to the calculations in this question.
(a)  (i)  The standard molar entropy at 298 K for 1 mole of chlorine molecules, Cl2, is +165 J mol−1 K−1. Use this, and appropriate values from your Data Booklet, to calculate the standard entropy change, , for this reaction.
(2)
*(ii)  Explain fully why the sign for the standard entropy change of the system,  is as you would expect.
(2)
 ............................................................................................................................................. 

(b)  Calculate the total entropy change, , in J mol−1 K−1, for this reaction, giving your answer to three significant figures.
(2)

(c)  Use the standard entropy change of the surroundings, , to calculate the standard enthalpy change, , in kJ mol−1, for the reaction at 298 K. 
(2)

(d)  0.0500 mol of calcium chloride, prepared by burning calcium in chlorine, is added to 51.8 cm3 of water.
50.0 cm3 of a 1.00 mol dm−3 solution is formed, and the temperature rise, ΔT, is 15.0°C.
(i)  Calculate the energy transferred, in joules, for this process using:
Energy transferred in joules = volume of solution formed × 4.2 × ΔT
(1)

(ii)  Calculate the enthalpy change of solution, , of calcium chloride in kJ mol−1.
(2)

*(iii)  The enthalpy change of hydration of Ca2+(g) is −1560 kJ mol−1.
Use this, your value from (d)(ii) and the experimental lattice energy from your Data Booklet, to calculate the standard enthalpy change of hydration of Cl−(g).
(3)

Answer ........................................................... kJ mol−1
(iv)  Draw diagrams to represent hydrated calcium ions and hydrated chloride ions.
(2)

(v)  Suggest why the addition of anhydrous calcium chloride to water results in an increase in temperature and a decrease in volume.
(2)
Temperature increases
  ............................................................................................................................................. 
 ............................................................................................................................................. 

Volume decreases
............................................................................................................................................. 
 ............................................................................................................................................. 
  
(Total for question = 18 marks)

Q16.
This question is about magnesium chloride, MgCl2.
It can be formed by burning magnesium in chlorine.
Mg(s) + Cl2(g) → MgCl2(s)      ΔS = +2152 J mol−1 K−1
Remember to include a sign and units in your answers to the calculations in this question.
(a)  (i)  The standard molar entropy at 298 K for 1 mol chlorine molecules, Cl2, is +165 J mol−1 K−1. Use this, and appropriate values from your Data Booklet, to calculate the standard entropy change of the system, ΔS, for this reaction.
(2)

*(ii)  Explain fully why the sign for the standard entropy change of the system, ΔS, is as you would expect.
(2)
 ............................................................................................................................................. 
............................................................................................................................................. 

(b)  Calculate the total entropy change, ΔS, in J mol−1K−1, for this reaction, giving your answer to three significant figures.
(2)

(c)  Use the standard entropy change of the surroundings, ΔS, to calculate the standard enthalpy change, ΔH, in kJ mol−1, for the reaction at 298 K.
(2)

(d)  0.0300 mol of magnesium chloride, prepared by burning magnesium in chlorine, is added to 51.5 cm3 of water. 
50.0 cm3 of 1.00 mol dm−3 solution is formed, and the temperature rise, ΔT, is 22.5°C. 

(i)  Calculate the energy transferred in joules for this process using:
Energy transferred in joules = volume of solution × 4.2 × ΔT
(1)

(ii)  Calculate the enthalpy change of solution, ΔHsolution, of magnesium chloride in kJ mol−1.
(2)

*(iii)  The enthalpy change of hydration of Mg2+(g) is −1920 kJ mol−1.
Use this, your value from (d)(ii), and the experimental lattice energy from your Data Booklet, to calculate the enthalpy change of hydration of Cl−(g).
(3)

Answer ........................................................... kJ mol−

(iv)  Draw a diagram to represent a hydrated chloride ion.
(1)

(v)  Suggest why the addition of anhydrous magnesium chloride to water results in an increase in temperature and a decrease in volume.
(2)
Temperature increases
 ............................................................................................................................................. 
 ............................................................................................................................................. 
 ............................................................................................................................................. 

Volume decreases
 ............................................................................................................................................. 
 ............................................................................................................................................. 
 ............................................................................................................................................. 

(Total for question = 17 marks)