Entropy and Gibbs Free Energy: Quiz

Entropy and Gibbs Free Energy: Quiz

7 questions

1. Question 1EasyWhich process has a negative ΔS?

   Aice meltingBwater boilingC2SO₂(g) + O₂(g) → 2SO₃(g)DCaCO₃(s) → CaO(s) + CO₂(g)
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   Answer: 2SO₂(g) + O₂(g) → 2SO₃(g)

   3 mol of gas become 2 mol, so the entropy decreases. The others increase disorder or the moles of gas.

2. Question 2MediumΔH = +50.0 kJ and ΔS = +0.200 kJ/K. Above what temperature is the reaction spontaneous?

   A0.250 KB250. KC10.0 KD4.00 × 10³ K
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   Answer: 250. K

   T = ΔH ÷ ΔS = 50.0 kJ ÷ 0.200 kJ/K = 250. K. Above this, TΔS outweighs ΔH.

3. Question 3EasyA reaction has ΔH negative and ΔS positive. It is spontaneous:

   Aat all temperaturesBonly at high temperaturesConly at low temperaturesDnever
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   Answer: at all temperatures

   ΔG = ΔH − TΔS = (negative) − (positive) is negative for every T.

4. Question 4MediumCalculate ΔG at 300. K for ΔH = −100. kJ and ΔS = −200. J/K.

   A−40.0 kJB−160. kJC+59 900 kJD−100. kJ
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   Answer: −40.0 kJ

   ΔS = −0.200 kJ/K; ΔG = −100. kJ − (300. K)(−0.200 kJ/K) = −100. kJ + 60.0 kJ = −40.0 kJ. Forgetting to convert J to kJ gives the large wrong answer.

5. Question 5MediumIf ΔG° for a reaction is positive, the equilibrium constant K is:

   Agreater than 1Bequal to 1Cless than 1Dnegative
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   Answer: less than 1

   ΔG° = −RT ln K: positive ΔG° means ln K is negative, so K is less than 1 (reactants favoured).

6. Question 6MediumWhy must limestone be heated strongly to make lime?

   AΔH and ΔS are both negativeBΔH and ΔS are both positive, so ΔG is only negative at high TCthe reaction is exothermicDheating lowers the entropy
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   Answer: ΔH and ΔS are both positive, so ΔG is only negative at high T

   For CaCO₃ → CaO + CO₂, ΔH = +179.2 kJ and ΔS = +160.2 J/K, so ΔG becomes negative above about 1119 K.

7. Question 7HardWhich statement about a spontaneous reaction is correct?

   Ait must be fastBit must be exothermicCit increases the total entropy of the universeDit has a positive ΔG
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   Answer: it increases the total entropy of the universe

   That is the second law. Spontaneous reactions can be slow and can be endothermic (e.g. ice melting at 25 °C).