136 ❯ STEP 4. Review the Knowledge You Need to Score High
❯ Answer and Explanation
(a) The value is negative. The decrease in the number of moles of gas during the reaction
means there is a decrease in entropy.
Give yourself 1 point if you predicted negative. Give yourself 1 point for discussing
the decreasing number of moles of gas. You may get this point even if you did not get
the first point.
(b) Recall that DG = ΔH - TDS. (This equation is given on the equation page of the AP exam.)
The value of DG will increase (become less negative) as the temperature is increased.
Give yourself 1 point for this answer if it is obvious that increasing means less
negative. In general, both DH and DS are relatively constant with respect to small
temperature changes. As the temperature increases, the value of the entropy term,
TDS, becomes more negative. The negative sign in front of this term leads to a posi-
tive contribution. The value of DG will first become less negative (more positive) and
eventually become positive (no longer spontaneous).
Give yourself 1 point for the DG = DH – TDS argument even if you did not get
the first point.
(c) Recall that DG = - RT ln K. (This equation is given on the equation page of the
AP exam.) The value of K will decrease as the temperature increases.
You get 1 point for this answer.
As the value of DG increases (see part b), the value of K will decrease. You get
1 point for using DG = - RT ln K in your discussion.
If you got the justification for part b wrong, and you used the same argument here,
you will not be penalized twice. You still get your point.
(d) The reaction changes from spontaneous to nonspontaneous when DG = 0.
Recall that DG = DH - TDS.
Rearranging this equation to T = - DH/DS allows the temperature to be estimated.
This rearranged equation is worth 1 point. To do the calculation, the value of ΔS
is the additional information needed. Give yourself 1 point for this.
(e) The Lewis structure isXe
FF FF FFThe complete Lewis structure, with all fluorine atoms attached to the xenon atom, is
worth 2 points. If all the fluorine atoms do not have an octet of electrons, the structure is
worth only 1 point. This illustrates the diversity of the free-response questions on the AP exam.
Total your points. There are 10 points possible. All the mathematical relations
presented in the answers are 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 you have multiple pages available.