Midterm Question 6B

[Abigail Sanders 1E]

Can anyone fully explain why question 6B on the midterm has answer A? I guessed that it is A because it is this reaction has the smallest change in entropy but I am not sure if that is correct.

[Hiba Alnajjar_2C]

Your logic is correct. Using the equation delta G = delta H - (T)(delta S), it is apparent that the value of delta G will be close to that of delta H when you have a value of delta S close to or equal to 0. Therefore, the answer with the smallest change in entropy, as you mentioned, would be the correct answer to this question.

[Joseph Saba]

Your logic is correct. Using the equation delta G = delta H - (T)(delta S), it is apparent that the value of delta G will be close to that of delta H when you have a value of delta S close to or equal to 0. Therefore, the answer with the smallest change in entropy, as you mentioned, would be the correct answer to this question.

Why is answer b incorrect? (the reaction of sodium with water to make sodium hydroxide and H2 gas) I thought it would be spontaneous and exothermic (thus both g and h being negative)

[Hiba Alnajjar_2C]

Your logic is correct. Using the equation delta G = delta H - (T)(delta S), it is apparent that the value of delta G will be close to that of delta H when you have a value of delta S close to or equal to 0. Therefore, the answer with the smallest change in entropy, as you mentioned, would be the correct answer to this question.

Why is answer b incorrect? (the reaction of sodium with water to make sodium hydroxide and H2 gas) I thought it would be spontaneous and exothermic (thus both g and h being negative)

In answer b, you're going from a solid reactant and a liquid reactant to an aqueous product and a gas product. In answer a, you're going from solid reactants to solid products. Because the phase of the reactants is equivalent to that of the products in a, there is a very small change in entropy. This change would be smaller than that in b since the phases of the reactants and products in b are different; the overall entropy would increase in b whereas it wouldn't change very much in a. delta G and delta H being negative would not be enough for it to be considered as similar as they would be in answer a since the value of the term "-(T)(delta S)" would cause the values of delta G and delta H to be different in b since delta S is not zero.