Feasibility, spontaneity and endothermic processes

Overview

Spontaneous reaction in terms of Gibbs free energy, how an endothermic reaction can be spontaneous and spontaneous endothermic process.

What you need to know

Answer each one before you reveal it.

Questions from this subtopic

Each one with the model answer underneath.

1. 1

   What is meant by a spontaneous reaction in terms of Gibbs free energy?

   Model answer

   It is a reaction with a negative Gibbs free energy change, so it can proceed without a continuous input of energy from outside.

2. 2

   Explain how an endothermic reaction can be spontaneous.

   Model answer

   If the entropy change is large and positive, the −TΔS term can outweigh the positive enthalpy change and make ΔG negative.

3. 3

   Give an example of a spontaneous endothermic process.

   Model answer

   Ammonium chloride dissolving in water, which cools the solution noticeably yet happens readily.

4. 4

   Calculate ΔG at 298 K for the dissolving of ammonium chloride, given ΔH = +15 kJ mol⁻¹ and ΔS = +75 J K⁻¹ mol⁻¹.

   Model answer

   ΔG = 15 − 298 × 0.075 = 15 − 22.4 = −7.4 kJ mol⁻¹, so the process is spontaneous.

5. 5

   Which term in the Gibbs equation drives the dissolving of ammonium chloride?

   Model answer

   The −TΔS term, because the increase in entropy is large enough to overcome the endothermic enthalpy change.

6. 6

   Explain why the thermal decomposition of a metal carbonate is feasible only at a high temperature.

   Model answer

   Both ΔH and ΔS are positive, so ΔG only becomes negative once the temperature is high enough for TΔS to exceed ΔH.

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