Edexcel International A Level Chemistry
Edexcel International A Level Chemistry Entropy Exam Questions
Free Edexcel International A Level Chemistry exam questions for Entropy, with a mark scheme for every pack. Covers entropy of systems, standard entropy values, entropy changes of system and surroundings, total entropy change, feasibility and Gibbs energy.
Entropy exam questions
1 question pack of real past exam questions, each with its mark scheme. Do not stop after a few: answer as many as you can without notes, then mark every answer with the mark scheme. The more questions you do, the more quickly you will spot how examiners ask about each idea.
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About Edexcel International A Level Chemistry Entropy
Entropy in Unit 4 of Edexcel International A Level Chemistry explains why some reactions happen and others do not. You learn that entropy is a measure of the number of ways particles and energy can be arranged, predict entropy changes from changes of state and numbers of gas molecules, and calculate entropy changes of the system from standard entropies. The entropy change of the surroundings depends on the enthalpy change and temperature, and the total entropy change decides feasibility. The topic draws on Energetics and Lattice energy and links to Chemical equilibria. Examiners set calculations of total entropy change and the temperature at which a reaction becomes feasible.
What you need to know
- What entropy measures Entropy, S, is a measure of the dispersal of energy and the number of ways particles and their energy can be arranged. Gases have much higher entropy than liquids, which are higher than solids.
- Entropy of the system Delta S system = sum of S of products minus sum of S of reactants, in J K-1 mol-1. Increases in the number of gas molecules usually make it positive.
- Entropy of the surroundings Delta S surroundings = -delta H / T, with delta H in J mol-1 and T in kelvin. Exothermic reactions increase the entropy of the surroundings.
- Total entropy and feasibility Delta S total = delta S system + delta S surroundings. A reaction is thermodynamically feasible when delta S total is positive.
- Gibbs energy Delta G = delta H - T delta S system. A reaction is feasible when delta G is negative. Setting delta G = 0 gives the temperature at which a reaction just becomes feasible.
- Kinetic stability A feasible reaction may not happen at a noticeable rate because of a high activation energy. Such systems are kinetically stable but thermodynamically unstable.
Common mistakes in Entropy
- Forgetting to convert delta H from kJ mol-1 to J mol-1 when combining with entropy values in J K-1 mol-1.
- Using T in Celsius. Temperature must be in kelvin.
- Assuming elements have zero entropy. Unlike enthalpy of formation, standard entropies of elements are not zero.
- Concluding a feasible reaction must happen quickly. Feasibility says nothing about rate.
- Getting the sign of delta S surroundings wrong. For an endothermic reaction it is negative.
Try three Entropy exam-style questions
1. CaCO3(s) gives CaO(s) + CO2(g). Delta H = +178 kJ mol-1 and delta S system = +161 J K-1 mol-1. Calculate delta S total at 298 K and comment on feasibility. (3 marks)
Show the mark scheme answer
Delta S surroundings = -178000 / 298 = -597 J K-1 mol-1; delta S total = 161 + (-597) = -436 J K-1 mol-1; negative so not feasible at 298 K.
2. Using the same data, calculate the minimum temperature at which the decomposition becomes feasible. (2 marks)
Show the mark scheme answer
Feasible when T delta S system is at least delta H, so T = 178000 / 161; = 1106 K (about 1110 K).
3. Predict and explain the sign of delta S system for 2H2(g) + O2(g) gives 2H2O(l). (2 marks)
Show the mark scheme answer
Negative; three moles of gas form two moles of liquid, so there are fewer ways to arrange the particles and energy.
Entropy FAQs
What does Edexcel International A Level Chemistry Entropy cover?
It covers the meaning of entropy, predicting and calculating entropy changes of the system and surroundings, total entropy change, feasibility, Gibbs energy and kinetic stability.
Which unit is Entropy examined in?
Entropy is part of Unit 4, and it is often combined with lattice energy and enthalpy of solution data in longer questions.
What equations do I need for Entropy?
You need delta S system = sum S products - sum S reactants, delta S surroundings = -delta H / T, delta S total = delta S system + delta S surroundings, and delta G = delta H - T delta S.
