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AP®ChemistryCollege BoardExam QuestionsUnit 7: EquilibriumLe Châtelier’s Principle

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

Exam Questions

8 mins8 questions
Medium
Hard
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

Which of the following conclusions can be made about this reaction?

H2O (g)   ⇋   H2 (g)   + 1 halfO2 (g)            Kc = 8 x 10-41 at 25 degreeC

  • The reaction does not proceed.

  • The reaction goes almost to completion.

  • The products have a higher concentration than the reactants.

  • The concentrations of reactants and products are the same.

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11 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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21 mark

The blood-red complex iron (III) thiocynanate, [FeSCN]2+, is formed when yellow iron (III) ions react with colorless thiocyanate ions in the following equilibrium reaction:

 Fe3+ + SCN- ⇋ [FeSCN]2+ ΔHᶿ = +12 kJ

Which of the following changes would shift the equilibrium to make the solution lighter?

  • Raising the temperature of the solution.

  • Lowering the temperature of the solution.

  • Adding iron(III) chloride solution.

  • Adding thiocyanate ions.

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31 mark

Nitrogen dioxide can react with itself to produce a dimer molecule called dinitrogen tetroxide in the following equilibrium reaction:

2NO2 (g)    ⇋  N2O4 (g)      Kc  = 0.01 at 25 degreeC

In an experiment, 100 cm³ of nitrogen dioxide is placed in a gas syringe, and the barrel is pushed in, meaning the volume is reduced to 50 cm³ at constant temperature. How will the equilibrium respond?

  • More N2​O4​ will form.

  • More NO2​ will form.

  • The value of Kc​ will increase.

  • The equilibrium will remain unchanged.

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41 mark

The Haber process is a key step in the manufacture of fertilizers:

N2 (g) + 3H2 (g) rightwards harpoon over leftwards harpoon 2NH3 (g)         ΔH = -ve

How does increasing the temperature affect the equilibrium and the equilibrium constant (Kc​)?

  • Shifts equilibrium to the left; Kc​ decreases.

  • Shifts equilibrium to the right; Kc​ increases.

  • Shifts equilibrium to the left; Kc​ remains unchanged.

  • Shifts equilibrium to the right; Kc remains unchanged.

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51 mark

Consider the following equilibrium reaction:

H2 (g)  + I2 (g)   ⇋   2HI (g)              ΔHᶿ = -126 kJ

Which of the following changes will increase the concentration of hydrogen iodide (HI)?

  • Increase temperature (exothermic reaction), increase pressure (two moles of gaseous product but only one mole of each gaseous reactant).

  • Increase temperature (endothermic reaction), no change in pressure (equal numbers of moles of gases).

  • Decrease temperature (exothermic reaction), decrease pressure (two moles of gaseous product but only one mole of each gaseous reactant).

  • Decrease temperature (exothermic reaction), no change in pressure (equal numbers of moles of gases).

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