1. For each of the following statements, indicate whether it is true or false, a
ID: 1003069 • Letter: 1
Question
1. For each of the following statements, indicate whether it is true or false, and briefly explain your reasoning.
a) A spontaneous reaction always releases heat.
b) A spontaneous reaction is always a fast reaction.
c) The entropy of a system always increases for a spontaneous change.
d) The entropy of a system and its surroundings always increases for a spontaneous change.
e) The energy of a system always increases for a spontaneous change.
2. The reaction shown below is spontaneous at room temperature, but becomes nonspontaneous at a much higher temperature:
N2(g) + 3H2(g) --> 2 NH3(g)
From this fact alone, determine the signs of H° and S°, assuming that these do not change much with temperature. Explain your reasoning.
Explanation / Answer
1. For each of the following statements, indicate whether it is true or false, and briefly explain your reasoning.
a) A spontaneous reaction always releases heat. FALSE, it only implies entropy increases
b) A spontaneous reaction is always a fast reaction. FALSE, they may take lots of years
c) The entropy of a system always increases for a spontaneous change. FALSE, by 2nd law of thermodynamics, it must be the whole universe
d) The entropy of a system and its surroundings always increases for a spontaneous change. FALSE, the total addition must be the one increasing in entropy
e) The energy of a system always increases for a spontaneous change. FALSE, energy is constant in the universe
2. The reaction shown below is spontaneous at room temperature, but becomes nonspontaneous at a much higher temperature:
N2(g) + 3H2(g) --> 2 NH3(g)
From this fact alone, determine the signs of H° and S°, assuming that these do not change much with temperature. Explain your reasoning.
dH --> negative, it will release energy due to N=N bonds and H-H breaking bonds...
dS --> it decreases, sicne 1+3 = 4 mol of reactants will form only 2
dG = dH - T*dS
as T increases, this becomes positive and dH becomes smaller, so dG becomes positive, i.e. not spontaneous
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