CIE A Level Chemistry

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A LevelChemistryCIETopic Questions5. Physical Chemistry (A Level Only)5.2 Entropy (A Level Only)

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First teaching 2020

Last exams 2024

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5.2 Entropy (A Level Only)

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1a1 mark

The feasibility of a reaction is determined by the relationship between enthalpy change, and entropy change.

Write the equation that shows the relationship between ΔG, ΔH and ΔS.

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1b1 mark

What is the requirement for a reaction to be feasible?

 

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1c
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Chlorine can be formed in the following reversible reaction

4HCl(g)  +  O2(g)  rightwards harpoon over leftwards harpoon  2Cl2(g)  +  2H2O(g)

Use the following data to calculate the standard entropy change, ΔS for this reaction. 

  HCl (g) O2 (g) Cl2 (g) H2O (g)
S/ J K–1 mol–1 187 205 223 189

The equation to calculate standard entropy change is:   


ΔS = ΣSproducts – ΣSreactants 

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1d
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The standard enthalpy change for the reaction in (c) is -116 kJ mol-1

Calculate the minimum temperature at which this reaction is feasible.

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2a3 marks

Ammonium chloride, NH4Cl, can dissociate to form ammonia, NH3, and hydrogen chloride, HCl. 

NH4Cl (s)    →   NH3(g)  +   HCl(g)

Explain whether entropy increases or decreases in this reaction. 

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2b
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At 298K, ΔH = +176 kJ mol–1 and ΔS = +285 J mol–1 K-1.

 

Calculate ΔG for this reaction at 298K.

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2c2 marks

Explain whether the reaction is feasible under standard conditions.

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2d2 marks

State and explain how the feasibility of the reaction will change with increasing temperature. 

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3a1 mark

The following equation represents the melting of ice.

H2O (s)   →    H2O (l)            ∆Hϴ = +6.03 kJ mol–1    ∆Sϴ = +22.1 J K–1 mol–1

State the meaning of the symbol ϴ in ∆Hϴ.

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3b3 marks

Explain whether the entropy of the system increases or decreases. 

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3c
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Calculate the temperature at which ∆Gϴ = 0 for this process.

Show your working.

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1a
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Carbon dioxide and hydrogen can react to form methanol. 

CO2 (g) + 3H2 (g) ⇌ CH3OH (g) + H2O (g)

Data for enthalpy of formation and entropy is shown in Table 1.1.

Table 1.1

  CO2 (g) H2 (g) CH3OH (g) H2O (g)
ΔHf / kJ mol–1 -394 0 -201 -242
S / J K–1 mol–1 214 131 238 189

Calculate the Gibbs free-energy change (ΔG), in kJ mol–1, for this reaction at 790 K.

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1b1 mark

Explain whether this reaction is feasible at 790 K.

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1c
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Use your values of ΔH and ΔS from part (a) to calculate the temperature above which this reaction is feasible.

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1d1 mark

Explain why the reaction in part (a) has a negative entropy change. 

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2a
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This question is about the feasibility of a reaction. 

Calcium carbonate can be thermally decomposed to make calcium oxide. 

CaCO3 (s)  →   CaO (s)   +   CO2 (g)        ΔH = + 178 kJ mol-1

Table 2.1 shows the standard entropies of each substance. 

Table 2.1 

Substance CaCO3 (s) CaO (s) CO2 (g)
S / J K–1mol–1 89 40 214

i)
Use the information in Table 2.1 to show that calcium carbonate is stable at room temperature (25 °C).
[4]

ii)
Calculate the minimum temperature needed to decompose calcium carbonate.
[2]

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2b3 marks

Explain how the conditions can be changed for the reaction in (a) to make the reaction feasible.

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2c4 marks

The evaporation of water is an endothermic reaction but also a spontaneous reaction. 


Explain why.

Refer to the entropy change that occurs and the change in the arrangement of water molecules. 

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2d
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Use the information in Table 2.2 to calculate the temperature above which the reaction between hydrogen and oxygen to form gaseous water is not feasible.

Table 2.2

 

Sϴ / J K−1 mol−1

ΔH fϴ/ kJ mol−1

H2 (g)

131

0

O2 (g)

205

0

H2O (g)

189

−242

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2e2 marks

Explain what would happen to the sample of gaseous water if it was heated to a higher temperature than that calculated in part (d). 

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3a2 marks

Free energy changes can be used to predict the feasibility of chemical reactions and processes.

Table 3.1 shows the equations of five different chemical reactions or processes.

Complete the table by predicting the sign of ΔS for each chemical reaction and process

Table 3.1

chemical reaction or process sign of ΔS
 MgCl2 (s) + (aq) → MgCl2 (aq)  
 Ca (s) + H2SO4 (aq) → CaSO4 (aq) + H2 (g)  
 Na2SO4 (s) + 10H2O (l) → Na2SO4•10H2O (s)  
 2CO (g) + O2 (g) → 2CO2 (g)  
 C2H5OH (g) → C2H5OH (l)   

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3b2 marks

The enthalpy and entropy changes of a reaction are both negative.

Explain how the feasibility of this reaction will change as the temperature decreases.

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3c
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The equation a reversible reaction is shown below:

 2SO2 (g) + O2 (g) ⇌ 2SO3 (g)

Use the information in Table 3.2 to determine the temperature at which the reaction becomes feasible. Show your working.

Table 3.2

  SO2 O2 SO3
ΔHӨf  / kJ mol-1 -296.8 0 -395.7
SӨ / J K-1 mol-1 248.2 205.1 256.8
 

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1a3 marks

The spontaneity of a chemical reaction depends on the standard Gibbs free energy change, ΔGθ. 

State and explain whether the following processes will lead to an increase or decrease in entropy.

i)
Steam condensing to water

entropy change ................................................................................

explanation .......................................................................................
[1]
ii)
Zinc reacting with nitric acid 


entropy change .................................................................................

explanation ........................................................................................

[1]

iii)
Sodium chloride dissolving in water

entropy change ...................................................................................

explanation ..........................................................................................
[1]

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1b
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Calcium carbonate thermally decomposes to form calcium oxide and carbon dioxide, as shown below:

CaCO3 (s) → CaO (s) + CO2 (g)

The enthalpy change of the above reaction is ΔHθ = +178 kJ mol-1

Standard entropies are shown in Table 1.1

Table 1.1

substance CaCO3 (s)  CaO (s) CO2 (g)
Sθ / J K-1 mol-1 +92.9 39.8 +214

Calculate the free energy change, ΔGθ, at 298 K for this process. Include a relevant sign and give your answer to three significant figures. Show your working. 




ΔGθ = .......................... kJ mol-1

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1c1 mark

Explain, with reference to ΔGθ , why this reaction becomes more feasible at higher temperatures.

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1d
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On heating, sodium hydrogencarbonate decomposes into sodium carbonate as shown.

2NaHCO3 (s) → Na2CO3 (s) + CO2 (g) + H2O (g) 

ΔHθ = +130 kJ mol-1

ΔSθ = +316 J mol K-1

Calculate the minimum temperature at which the reaction becomes spontaneous. Show your working.




Temperature = ....................... K

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2a
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Ammonia, NH3, is produced by the Haber process and is an important chemical in the manufacture of fertilisers and clearing products. 

Ammonia gas can react with oxygen to produce nitrogen monoxide and steam, and is the first step in the Ostwald process which produces nitric acid. 

i)
Write an equation for the reaction of ammonia with oxygen to produce nitrogen monoxide and steam. 
[2]
 
ii)
Table 2.1 shows the entropy values for the species involved in the reaction.

Table 2.1 

Substance

Entropy values (J K-1 mol-1)

NH3 (g)

192.8

O2 (g)

205.2

H2O (g)

188.8

NO (g)

210.8

 

Use the data in Table 2.1 to determine the entropy for change for this reaction at 298 K.



ΔSθ = ................... J K-1 mol-1
[3]

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2b1 mark

Explain why the standard entropy change for the reaction in part (a) is positive.

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2c
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The second step in the Ostwald process produces nitrogen dioxide as shown in the equation 

2NO (g) + O2 (g) → 2NO2 (g)             ΔHθ = -112 kJ mol-1   

The standard entropy for NO2 (g) is 240.0 J K-1 mol-1

Use Table 2.1 to determine the value for the free energy change, ΔGθ , for this reaction at 298 K. Show your working.








ΔGθ = ......................... kJ mol-1

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2d2 marks

Explain whether changing the temperature for the production of nitrogen dioxide will alter the spontaneity of the reaction.

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3a1 mark

Potassium nitrate is a soluble salt. When it dissolves in water the value of the enthalpy change ΔHθ = +34.9 kJ mol–1 and the value of the entropy change ΔSθ = +117 J K–1 mol–1 .

Write an equation, including state symbols, for the process that occurs when potassium nitrate dissolves in water. 

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3b1 mark

Explain why ΔSθ is positive for this process.

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3c
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Calculate the minimum temperature at which the reaction will become feasible. Show your working.





T = .................. K

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3d2 marks
i)
Deduce what happens to the value of ΔGθ when potassium nitrate dissolves in water at a temperature lower than your answer to part (c).
[1]

ii)
What does this new value of ΔGθ suggest about the dissolving of potassium nitrate at this lower temperature? 
[1]

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4a
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Ethanol is used in large quantities in the production of alcoholic beverages and as a fuel.

The combustion of ethanol is represented by the equation         

 
  CH3CH2OH (l) + 3O2 (g) → 2CO2 (g) + 3H2O (g)    ΔHθ = -1367 kJ mol-1


Using Table 4.1, determine the entropy change, ΔSθ, for the combustion of  ethanol at 298K. Show your working.

Table 4.1

substance CH3CH2OH (l) O2 (g) CO2 (g) H2O (g) 
Sθ / J K-1 mol-1 +161 +205.2 +213.8 +188.8



ΔSθ = ........................... (J K-1 mol-1)

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4b
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Calculate the free energy change, ΔGθ , for the combustion of ethanol using ΔSϴ determined in part (a).

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4c3 marks

Explain whether changing the temperature for the combustion of ethanol will alter the spontaneity of the reaction. 

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