5 Steps to a 5 AP Chemistry

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.
  
  
  
  

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