Edexcel A Level Chemistry

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A LevelChemistryEdexcelTopic Questions5. Advanced Physical Chemistry (A Level Only)5.4 Energetics II: Entropy

5.4 Energetics II: Entropy

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16 marks

A student wrote:

Whether or not a reaction occurs depends only on the thermodynamic feasibility calculated using

∆G = ∆H — T∆Ssystem


Discuss this statement.
Include reference to:

  • changes in the values and signs of the terms in the equation for both endothermic and exothermic reactions.
  • circumstances where a reaction that is predicted to be thermodynamically feasible may not occur in practice.

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1a4 marks

This question is about entropy.

Some hydrated barium hydroxide is added to ammonium thiocyanate in a flask which is placed on a few drops of water on a wooden block. After the addition, the contents are stirred and then the flask can be lifted up with the wooden block attached, as shown.

q6a-9cho-al-1-oct-2021-edexcel-a-level-chem


The equation for the reaction is

Ba(OH)2.8H2O (s) + 2NH4SCN (s) → Ba(SCN)2 (aq) + 2NH3 (g) + 10H2O (l)

i)
Give two reasons why you would expect ΔS subscript system superscript ⦵ to be positive.

(2)

ii)

Explain why the wooden block is lifted up by the flask.

(2)

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1b1 mark

What is the standard molar entropy change, Δbegin mathsize 14px style S subscript system superscript ⦵ end style, in JK–1mol–1, for the reaction shown?

2Mg (s) + O2 (g) → 2MgO (s)

Substance Standard molar entropy, bold italic S to the power of bold ⦵/ JK–1mol–1
Mg(s) 32.7
O2(g) 205.0
MgO(s) 26.9

  ☐ A +210.8
  ☐ B –210.8
  ☐ C +216.6
  ☐ D –216.6

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2a
Sme Calculator7 marks

Prop-2-en-1-ol is an unsaturated alcohol with the structure shown.

q6-9cho-al-1-nov-2020-qp-edexcel-a-level-chem

A student planned to use bond enthalpy data to calculate a value for the enthalpy change of combustion of prop-2-en-1-ol.

i)
When researching the bond enthalpy data, the student claimed that it was not necessary to find the value for the C=C bond as they could use the value for a C–C bond and multiply it by two.
Explain why the student is incorrect.

(2)

ii)
Calculate a value for the enthalpy of combustion of prop-2-en-1-ol using the data shown.

C3H6O (g) + 4O2 (g) → 3CO2 (g) + 3H2O (g)

Bond C−C C=C C−O C=O O−H C−H O=O
Bond enthalpy /
kJ mol−1		 347 612 358 805 464 413 498

(3)

iii)
Explain, in terms of entropy, why the combustion of prop-2-en-1-ol is always feasible in the gaseous state.

(2)

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2b
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Chemists are researching a process to make ethanol and ethene directly from carbon dioxide and water.


4CO2 (g) + 5H2O (l) → CH3CH2OH (l) + C2H4(g) + 6O2 (g)       ∆H ⦵ = +2778 kJ mol−1


  CO2 (g) H2O (l) CH3CH2OH (l) C2H4 (g) O2 (g)
S⦵ / J K−1 mol−1 213.6 69.9 160.7 219.5 205.0

Calculate ∆S⦵ total for the reaction and hence determine whether the reaction is feasible under standard conditions.

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3a
Sme Calculator3 marks

Sodium hydride, NaH, can be used to generate hydrogen for fuel cells.

In order to calculate the first electron affinity of hydrogen, a student was asked to draw a Born-Haber cycle for sodium hydride.

The cycle had two errors but the numerical data were correct.

q8a-9cho-al-1-june-2019-qp-edexcel-a-level-chem
i)
Identify and correct the two errors in this Born-Haber cycle.
(2)
ii)
Calculate the first electron affinity, in kJ mol–1, of hydrogen, using the values given in the cycle.
(1)

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3b
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The equation for the formation of sodium hydride is

Na (s) + ½H2 (g) → NaH (s) 


The standard entropy change of the system, increment S to the power of ⦵ subscript space system end subscript, for this reaction is  –76.5 J K–1 mol–1

i)
Deduce the feasibility of this reaction at 298 K by calculating the free energy change, increment G .

(2)

ii)
Calculate the temperature at which  increment G = 0.

(1)

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3c1 mark

The sodium hydride is crushed in the presence of water to release the hydrogen gas for a fuel cell.

The overall equation for the reaction occurring in the fuel cell is

H2 (g) + ½O2 (g) → H2O (l)


In an alkaline fuel cell the oxidation half-equation is

H2 (g) + 2OH– (aq) → 2H2O (l) + 2e– 


Deduce the reduction half-equation for the alkaline fuel cell.

State symbols are not required.

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3d1 mark

Lattice energies provide an indication of ionic bond strength.

Which are the lattice energies of the hydrides NaH, KH and MgH2?

      Lattice energy / kJ mol–1
      Sodium hydride, NaH Potassium hydride, KH Magnesium hydride, MgH2
  ☐ A –804 –711 –1018
  ☐ B –804 –711 –2718
  ☐ C –804 –911 –1018
  ☐ D –804 –911 –2718

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4a2 marks

This question is about entropy and free energy.

Complete the table by giving the sign of the entropy change of the system, ∆Ssystem , for each reaction.

Reaction Sign of ∆Ssystem
CO2 (s) → CO2 (g)  
NaCl (s) + aq → NaCl (aq)  
N2 (g) + 3H2 (g) → 2NH3 (g)  

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4b
Sme Calculator3 marks

Calculate the total entropy change, ∆Stotal, for the thermal decomposition of calcium carbonate at 298K.

CaCO3 (s) → CaO (s) + CO2 (g)

[Data: ∆rH = +178 kJ mol−1 ∆Ssystem = +160J K−1 mol−1]

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4c
Sme Calculator10 marks

Sulfur dioxide reacts with oxygen to form sulfur trioxide.


2SO2(g) + O2 (g) ⇌ 2SO3 (g)   ΔrH = -288.4 kJ mol−1


The standard molar entropy values at 298 K are given in the table.

  SO2 (g) O2 (g) SO3 (g)
S⦵{"language":"en","fontFamily":"Times New Roman","fontSize":"18","autoformat":true}/ J K−1mol−1 +248.1 +205.0 +95.6

i)
Calculate the entropy change of the system, ΔSsystem, for the forward reaction.
Include a sign and units in your answer.

2SO2 (g) + O2( g) → 2SO3 (g)

(2)

ii)
Calculate the free energy change, ΔG, at 298 K and hence deduce whether the reaction is feasible.

(3)

iii)

In industry, the reaction is carried out at about 700 K using a vanadium(V) oxide catalyst.

Calculate the value of the equilibrium constant, K, at 700 K.

ΔG at 700K is -60 kJ mol−1

(3)

iv)
The equilibrium constant has a larger value at 298 K than at 700K.
Explain why the reaction is carried out at 700 K and not at 298 K.

(2)

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