CCEA A Level Chemistry
CCEA A Level Chemistry 4.2 Enthalpy, Entropy and Free Energy Exam Questions
Free CCEA A Level Chemistry revision for 4.2 Enthalpy, Entropy and Free Energy: what you need to know, common mistakes and exam-style questions with answers. Covers feasibility, entropy and entropy change calculations, Gibbs free energy and the temperature at which a reaction becomes feasible.
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About CCEA A Level Chemistry 4.2 Enthalpy, Entropy and Free Energy
Section 4.2 of Unit A2 1 explains why some endothermic reactions happen spontaneously. CCEA uses the reaction of ammonium carbonate with ethanoic acid as an example of an endothermic but feasible reaction. You need to understand entropy as a measure of disorder, calculate standard entropy changes from tabulated values, and use delta G = delta H - T delta S to decide whether a reaction is feasible. When delta H and delta S have the same sign, you must calculate the temperature at which a reaction becomes, or stops being, feasible. This section builds on energetics in AS 2 and lattice enthalpy in 4.1.
What you need to know
- Feasibility A feasible reaction can occur without continuous outside input. Enthalpy alone cannot predict feasibility, since some endothermic reactions, such as ammonium carbonate with ethanoic acid, are feasible.
- Entropy Entropy, S, is a measure of disorder. Gases have much higher entropy than liquids, which are higher than solids. Entropy increases when a solid dissolves, when more moles of gas form and when temperature rises.
- Entropy change Delta S = sum of S of products - sum of S of reactants, in J K-1 mol-1, using the stoichiometric coefficients.
- Gibbs free energy Delta G = delta H - T delta S. A reaction is feasible when delta G is zero or negative. Convert delta S to kJ K-1 mol-1 by dividing by 1000 before combining with delta H in kJ mol-1.
- Temperature of feasibility Set delta G = 0, so T = delta H / delta S. If both are positive, the reaction is feasible above this temperature; if both are negative, it is feasible below it.
- Limitations A negative delta G does not mean the reaction is fast; a high activation energy can make a feasible reaction very slow.
Common mistakes in 4.2 Enthalpy, Entropy and Free Energy
- Mixing J and kJ in delta G calculations. Convert delta S from J to kJ before substituting.
- Using temperature in Celsius. T must be in kelvin.
- Saying a reaction with negative delta G will happen quickly. Feasibility says nothing about rate.
- Forgetting to multiply standard entropies by coefficients. Each value must be multiplied by the number of moles in the equation.
- Saying elements have zero entropy. Standard entropies of elements are not zero (unlike enthalpies of formation).
Try three 4.2 Enthalpy, Entropy and Free Energy exam-style questions
1. For CaCO3(s) gives CaO(s) + CO2(g), delta H = +178 kJ mol-1 and delta S = +161 J K-1 mol-1. Calculate the minimum temperature at which the reaction is feasible. (2 marks)
Show the mark scheme answer
T = delta H / delta S = 178 / 0.161; = 1106 K.
2. Using the data for the reaction above, calculate delta G at 298 K and comment on feasibility. (2 marks)
Show the mark scheme answer
Delta G = 178 - 298 x 0.161 = 178 - 48.0 = +130 kJ mol-1; positive so not feasible at 298 K.
3. Explain why the entropy change is positive when ammonium carbonate reacts with ethanoic acid. (2 marks)
Show the mark scheme answer
Solid and liquid reactants produce a gas (carbon dioxide) and more particles; increase in disorder.
4.2 Enthalpy, Entropy and Free Energy FAQs
What does CCEA A Level Chemistry 4.2 Enthalpy, Entropy and Free Energy cover?
It covers feasibility, entropy and entropy change calculations, Gibbs free energy, and the temperature at which reactions become feasible.
Which unit is 4.2 examined in?
It is examined in Unit A2 1: Further Physical and Organic Chemistry, a 2 hour paper with a multiple-choice section and structured questions. Related practical skills can also be assessed in Unit A2 3.
What equations do I need for 4.2?
Delta S = sum of S(products) - sum of S(reactants), delta G = delta H - T delta S, and T = delta H / delta S when delta G = 0. Watch units carefully.
