9701 · 23.3
Entropy change, ΔS flashcards
Revision flashcards for Cambridge 9701 Entropy change, ΔS (syllabus 23.3). Flip, recall, then mark a real past-paper question.
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What is entropy (S)?
A measure of the degree of disorder or randomness in a system. More formally, it's a measure of the dispersal of energy and matter among the available microstates.
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What are the standard units for entropy (S°) and entropy change (ΔS°)?
Joules per Kelvin per mole (J K⁻¹ mol⁻¹). This is a common trap in calculations!
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What does a positive ΔS value signify?
The system has become more disordered. The products are more random than the reactants.
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What does a negative ΔS value signify?
The system has become more ordered. The products are less random than the reactants.
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Predict the sign of ΔS for H₂O(l) → H₂O(g).
Positive (ΔS > 0). A gas is significantly more disordered than a liquid.
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Predict the sign of ΔS for N₂(g) + 3H₂(g) → 2NH₃(g).
Negative (ΔS < 0). The reaction goes from 4 moles of gas to 2 moles of gas, a significant decrease in disorder.
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What is the formula for calculating the standard entropy change of a reaction?
ΔS°_reaction = ΣS°(products) - ΣS°(reactants). (The sum of the standard entropies of the products minus the sum of the standard entropies of the reactants).
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Why is the standard entropy of an element in its standard state NOT zero?
Unlike standard enthalpy of formation, standard entropy (S°) is an absolute value based on the Third Law of Thermodynamics. Only a perfect crystal at 0 K has zero entropy.
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What is the most common calculation error when using the Gibbs free energy equation, ΔG = ΔH - TΔS?
A units mismatch. ΔH is usually given in kJ mol⁻¹, while ΔS is in J K⁻¹ mol⁻¹. You must convert one to match the other, typically by multiplying ΔH by 1000.
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What must be done to the temperature in °C before using it in TΔS?
It must be converted to Kelvin (K) by adding 273. T(K) = T(°C) + 273.
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How does dissolving an ionic solid, like NaCl, in water affect entropy?
It generally leads to a positive entropy change (ΔS > 0). The highly ordered crystal lattice is broken down, and the ions become free to move in the solution, which is a large increase in disorder.