Entropy and spontaneityEdexcel A-Level Chemistry: Flashcards

What these 13 flashcards ask

What is entropy?
What are the units of entropy?
State the natural direction of change.
Give an endothermic reaction that occurs at room temperature.
Why can endothermic reactions occur spontaneously?
Put in order of increasing entropy: gas, solid, liquid.
Why does entropy increase when an ionic solid dissolves?
When does ΔS system increase in a reaction?
Write the equation for ΔS total.
How is ΔS system calculated?
How is ΔS surroundings calculated?
Why is ΔS surroundings positive for an exothermic reaction?
Condition for a feasible reaction in terms of entropy?

Exam questions on Entropy and spontaneity

In a demonstration, solid barium hydroxide octahydrate is mixed with solid ammonium chloride in a flask. The mixture becomes a liquid, ammonia gas is given off and the flask becomes so cold that it freezes a wet wooden block to the bench. The reaction is Ba(OH)₂·8H₂O(s) + 2NH₄Cl(s) → BaCl₂(s) + 2NH₃(g) + 10H₂O(l). It happens spontaneously at room temperature even though it is endothermic.
Explain, in terms of disorder, why the entropy of the system increases in this reaction.2 marks
The Haber process for making ammonia, N₂(g) + 3H₂(g) → 2NH₃(g), is exothermic with ΔH = −92 kJ mol⁻¹. A student uses it to practise calculating the entropy change of the surroundings, and assumes that ΔH does not change with temperature.
Explain why the entropy change of the surroundings is positive for an exothermic reaction.2 marks
Calcium carbonate decomposes on heating: CaCO₃(s) → CaO(s) + CO₂(g). The enthalpy change is ΔH = +178 kJ mol⁻¹. Standard entropies in J K⁻¹ mol⁻¹: CaCO₃(s) 92.9; CaO(s) 39.7; CO₂(g) 213.6.
Calculate the entropy change of the system, ΔS system, for the decomposition.3 marks

Written by the Exaim team, led by Shaun Daswani (Head of Upper Secondary, Improve ME Institute; MSc Financial Mathematics, Imperial College London; BSc, UCL) and Jason Daswani (operational lead, Improve ME Institute; LSE).