Le Châtelier’s Principle (College Board AP® Chemistry)

Exam Questions

3 mins3 questions
11 mark

2HI (g)  ⇌ H2 (g) + I2 (g)

Kp = 83 at 723 K

A mixture of HI (g), H2 (g), and I2 (g) is placed in a sealed, rigid container at 723 K.

After equilibrium is established, the pressure inside the container is halved while the temperature is held constant at 723 K.

Which of the following will most likely occur?

  • The partial pressure of HI (g) will remain constant.

  • The forward reaction is favored.

  • Kp will increase.

  • The reverse reaction is favored.

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21 mark

Fe3+ (aq) + SCN- (aq) ⇋ [FeSCN]2+ (aq)

Kc = 140 at 298 K

The blood-red complex iron(III) thiocynanate, [FeSCN]2+ is formed when yellow iron(III) ions react with thiocyanate ions.

Which of the following will occur if Fe(NO3)3 (aq) ions are added?

  • The concentration of Fe3+ (aq) ions will increase.

  • The solution will turn darker.

  • The solution will turn paler.

  • The equilibrium constant, Kc, will decrease.

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31 mark

2CO2 (g) ⇌ 2CO (g) + O2 (g) Kc = 31.2 at 298 K

Which of the following is true for the above reaction when the concentration of the reactants and products are:

[CO2] = 2.0 x 10-3 M
[CO] = 0.015 M
[O2] = 0.010 M

  • The reaction will move to the right to achieve equilibrium.

  • The reaction will move to the left to achieve equilibrium.

  • Q < Kc

  • Q = Kc

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