A-LevelChemistryChemical energetics9701/41 · Oct/Nov 2019 · Q37 Marks

Explain what is meant by the term entropy of a system. [1]

(a)Explain what is meant by the term entropy of a system. [1][1]
(b)(i)• liquid water at 80°C is cooled to 60°C The entropy change is .................................................... because ....................................................[1]
(b)(ii)• solid calcium chloride is added to water and the mixture is stirred The entropy change is .................................................... because ....................................................[1]
(b)(iii)• the change corresponding to the lattice energy of calcium chloride, ∆Hlatt CaCl₂(s), takes place The entropy change is .................................................... because ....................................................[1]
(c)(i)Give the full name for the term ∆G°. [1][1]
(c)(ii)Describe how the temperature at which the reaction becomes spontaneous can be calculated. Include an equation in your answer. equation .......................................................................................................................................................... [2][2]

Mark scheme

Marking points from the official mark scheme (Oct/Nov 2019). The mark or mark type is shown in brackets.

[1] a measure / degree of disorder / randomness of a system
[1] M1: negative – molecules have less energy in the system
[1] M2: positive – solid being converted into an aqueous solution
[1] M3: negative – gaseous ions being converted into a solid
[1] (standard) Gibbs free energy change
[2] M1: (Δ)G = ΔΗ-TAS M2: description of calculating the minimum value of T for which ΔG is zero / becomes negative OR T = ΔΗ / ΔS [1]

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