AP Preview - Equilibrium Answers

AP Preview Questions - Equilibrium
These questions offer an insight to the types of problems that occur on the AP test and how they
will, at times, use information from our Honors Chemistry course as well.
Multiple Choice – No Calculators Allowed
1. In the reaction 3W + X ↔ 2Y + Z, all substances are gases. The reaction is initiated by adding an equal
number of moles of W and X. When equilibrium is reached,
Correct Answer: D
Since every time a mole of X reacts, 3 moles of W must react, so the amount of W remaining
must be less than the amount of X.
2. The reaction 3 H2 (g) + N2 (g) ↔ 2 NH3 (g) has an enthalpy of change of –92 kJ. Increasing the
temperature of this equilibrium system causes.
Correct Answer: B
Increasing the temperature favors the endothermic side of an equilibrium reaction.
3. Consider N2 (g) + O2 (g) ↔ 2 NO (g). The reaction was initiated by adding 15.0 moles of NO to a 1.0 L
flask. At equilibrium, 3.0 moles of oxygen are present in the 1.0 L flask. The value of K must be:
Correct Answer: D
If 3 moles of oxygen are formed, 6 moles of NO must have reacted, leaving 9 moles of NO at
equilibrium. Use this to write the Ka equation:
Ka =
[NO] 2
[9.0] 2
= 9.0
[N2 ][O 2 ] [3.0][3.0]
4. At a constant temperature, the synthesis of ammonia gas is shown by 3 H2 (g) + N2 (g) ↔ 2 NH3 (g). If
this reaction has a value for K = 3.0 x 10 and [H2] = [N2] = 0.10 M, and [NH3] = 0.20 M,
Correct Answer: B
Q =
[NH3 ] 2
[H2 ] 3 [N2 ]
[.20] 2
[.10] 3 [.10]
= 400
Since Q is greater than K, it will shift towards the reactants.
5. Which of the following systems at equilibrium are not affected by a change in pressure caused by
changing the volume and keeping the temperature constant?
Correct Answer: A
This is the only system with equal numbers of moles of gas as products and reactants.
6. The position of equilibrium for the reaction:
ZnO (s) + H2 (g) ↔ Zn (s) + H2O (g)
does not depend on which of the following:
Concentration of ZnO (s)
Concentration of H2 (g)
Concentration of Zn (s)
Concentration of H2O (g)
The Value of K
Correct Answer: C
Pure solids will not affect the position of the equilibrium.
Free-Response Questions – Calculators Allowed
1. Consider the reaction 2 HI (g) ↔ H2 (g) + I2 (s)
a) How will adding more hydrogen gas at a constant volume affect the equilibrium?
This will shift the system towards the reactants forming more HI.
b) Some of the hydrogen gas is removed, how will this affect the equilibrium?
This will shift the system towards the products, forming more H2 and I2.
c) The pressure is increased by decreasing the volume. Explain how this will affect the
There are 2 moles of gas reactants forming only 1 mole of products, so it will shift towards the
products because there are fewer moles.
2. The reaction below is carried out in a 15.0 L cylinder, and 0.500 moles of PCl5 are added and allowed
to reach equilibrium. The concentration of PCl3 is found to be 0.0220 mol/L. Assume a reaction
temperature of 375 K.
PCl3 (g) + Cl2 (g) ↔ PCl5 (g)
a) How many moles of PCl5 remain at equilibrium?
molarity PCl 3 =
0.0220 M =
→ 0.330 mol PCl 3 reacted
Since it was a 1:1 ratio, that means there are only0.170 mol PCl5 remaining
b) What is the equilibrium constant equation for the reaction above?
Ka =
[PCl 5 ]
[PCl 3 ][Cl 2 ]
c) Determine the value of K.
Ka =
= 23.4
[.0220][.0 220]