5 Steps to a 5 AP Chemistry

a. Is the value of ΔS°, for the above reaction, positive or negative? Justify your conclusion.

b. The above reaction is spontaneous under standard conditions. Predict what will happen

to ΔGfor this reaction as the temperature is increased. Justify your prediction.

c. Will the value of Kremain the same, increase, or decrease as the temperature increases?

Justify your prediction.

d. Show how the temperature at which the reaction changes from spontaneous to nonspon-

taneous can be predicted. What additional information is needed?

 Answers and Explanations

a. The value is negative. Give yourself 1 point if you predicted this.

The decrease in the number of moles of gas, during the reaction, means there is a

decrease in entropy. Give yourself 1 point for discussing the number of moles of gas.

You may get this point even if you did not get the first point.

b. Recalling: ΔG=ΔH−TΔS

The value of ΔGwill increase (become less negative). Give yourself 1 point for this

answer if it is clearly stated that increasing means less negative.

In general, both ΔHand ΔSare relatively constant. As the temperature increases, the

value of the entropy term, TΔS, becomes more negative. The negative sign in front or this

term leads to a positive contribution. The value of ΔGwill first become less negative (more

positive), and eventually the value will be positive (no longer spontaneous). Give yourself

1 point for the ΔG=ΔH−TΔSargument even if you did not get the first point.

c. Recalling: ΔG=−RTln K

The value of Kwill decrease. You get 1 point for this answer.

As the value of ΔGincreases (see part b) the value of Kwill decrease. You get

1 point for using ΔG=−RTln Kin your discussion. If you got the justification for part

bwrong, and you used the same argument here, you will not be penalized twice. You

still get your point.

d. Recalling: ΔG=ΔH−TΔS

Rearranging this equation to: T=(ΔG−ΔH)/ΔSwill allow the temperature to be esti-

mated. This is worth 1 point. To do the calculation, the value of ΔSis needed. Give

yourself 1 point for this.

There is total of 8 points possible. All of the mathematical relations presented in

the answers are provided in the exam booklet.

Note that many students lose points on a question like this because their answers

are too long. Keep your answers short and to the point, even if it appears that you have

multiple pages available.

 Rapid Review

 Thermodynamics is the study of heat and its transformations.

 Kinetic energy is energy of motion, while potential energy is stored energy.

Thermodynamics  135