340 STEP 5. Build Your Test-Taking Confidence
Answers and Explanations for Exam 2—Free Response
Question 1.
(a) Kp=PCH 4 /PH 2
2
=0.262
You get 1 point for the correct equation. The “=0.262” is optional.
(b) Kc=Kp/(RT)Δn(This is from one of the equations given in the AP Exam booklet.)
You get 1 point for the correct answer, and 1 point for choosing the correct equation.
(c) Knew=1/Kp=1/0.262 =3.82
When the equilibrium is reversed, you take the reciprocal of the equilibrium constant.
You get 1 point for the answer, and 1 point for the explanation.
(d)
PH 2 PCH 4
initial 0.000 0.200
change + 2 x –x
equilibrium 2 x 0.200 – x
Substituting:
You get 1 point for the correct answer, you get 1 point for the table, and you get 1 point for the setup.
You could correctly use the equation and answer from part (c).
(e) The reaction is exothermic. The change in the amounts of CH 4 and H 2 indicate the reaction has shifted
to the left. Exothermic reactions shift to the left as the temperature increases.
You get 1 point for answering exothermic, and 1 point for the explanation. You may get the explanation
point even if you answered endothermic.
(f ) There would be no change. Solids do not shift equilibria.
You get 1 point for “no change,” and 1 point for the explanation.
Total your points for each part. There are 12 possible points. Subtract one point if you do not have the cor-
rect number of significant figures in all cases.
Question 2.
(a) The orders for I
- and ClO–are 1. The order for OH
- is –1. If you get all three of these correct, you
get 1 point.
- is –1. If you get all three of these correct, you
Comparing Experiments 1 and 2: The concentration of I
- is doubled, and the rate is doubled. This
leads to I
having an order of 1. You get 1 point for this reasoning.
Kxx
x
Px
p
H
/
atm
=− =
=
==
(. )( ).
.
.
0 200 2 0 262
0 170
2 0 340
2
2
KPPpCHH== 42 /^2 0 262.
Δ=− =−
==−
n
K
12 1
c 0 262./[(. )( )] .0 0821 1270^1 27 3