Entropy and spontaneous changeEdexcel International A Level Chemistry: Subtopic test

1. 1
   A gas jar containing brown nitrogen dioxide is placed beneath a gas jar of colourless air, and the lid between them is removed. After a few minutes both jars contain a uniform pale brown mixture. There is no measurable energy change during mixing.

   (a)

   Why do the two gases mix spontaneously?

   [1 mark]

   • AMixing lowers the enthalpy of the system
   • BMixing releases energy to the surroundings
   • CMixing requires the molecules to attract each other
   • DMixing increases the total entropy

   (b)

   The mixed gases are never seen to separate spontaneously into brown and colourless layers. Which statement explains this?

   [1 mark]

   • AIt would decrease the total entropy, which is not the natural direction of change
   • BIt would be exothermic
   • CIt would need a catalyst
   • DThe molecules have no kinetic energy once mixed

   (c)

   Explain, in terms of molecules and energy quanta, why the entropy of the system increases when the gases mix.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   A student predicts the sign of the entropy change of the system, ΔS, for several changes at room temperature and pressure.

   (a)

   Which change has a negative entropy change for the system?

   [1 mark]

   • ACaCO₃(s) → CaO(s) + CO₂(g)
   • BN₂O₄(g) → 2NO₂(g)
   • C2NO(g) + O₂(g) → 2NO₂(g)
   • D2H₂O₂(l) → 2H₂O(l) + O₂(g)

   (b)

   Which statement about the entropy of a perfect crystal is correct?

   [1 mark]

   • AIt is zero at all temperatures
   • BIt is zero at 0 K
   • CIt is zero at 273 K
   • DIt is always greater than the entropy of the liquid

   (c)

   Explain why the entropy of water increases when liquid water at 100 °C changes to steam at 100 °C.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   A student dissolves citric acid and sodium hydrogencarbonate in water. The mixture fizzes and its temperature falls from 21 °C to 12 °C. The equation for the reaction is C₆H₈O₇(aq) + 3NaHCO₃(aq) → Na₃C₆H₅O₇(aq) + 3H₂O(l) + 3CO₂(g).

   (a)

   Explain why this reaction occurs spontaneously even though it is endothermic.

   [3 marks]

   (b)

   State the sign of the entropy change of the system for this reaction and give two reasons for your answer.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   A teacher uses water and common salt to teach entropy. A sample of pure water is heated from 0 K to 400 K at atmospheric pressure. The water melts at 273 K and boils at 373 K. In a separate demonstration, solid sodium chloride dissolves in water at room temperature with a small endothermic enthalpy change of solution (+3.9 kJ mol⁻¹).

   (a)

   Describe and explain how the entropy of the water sample changes as it is heated from 0 K to 400 K.

   [6 marks]

   (b)

   Explain why sodium chloride dissolves spontaneously in water even though the enthalpy change of solution is endothermic.

   [6 marks]

   Total for question 4: 12 marks

End of questions