Worksheet: Le Chatelier’s Principle

In this worksheet, we will practice using Le Chatelier’s principle to predict directions of change for equilibria in response to a stimulus.

Q1:

At equilibrium, a box contains five molecules each of gaseous compounds A, B and C. Shown in the diagram are the equilibrium gas mixtures obtained when molecules of A or B are added to the box without altering the total pressure.

Write a balanced chemical equation for the reaction taking place in the gas mixture.

  • AA+B2C
  • BA+2C2B
  • CA+BC
  • DA+2CB
  • EA+C2B

The total pressure of the original gas mixture is increased and the system is allowed to reach equilibrium. Assuming the temperature remains at the original value and no molecules are added or removed, which of the following situations could be observed?

  • A
  • B
  • C
  • D
  • E

Molecules are added or removed from the original gas mixture and the system is allowed to reach equilibrium. The temperature and pressure are fixed at their original values throughout this process. Which of the following changes cannot result in the situation shown?

  • AAdding B and C
  • BAdding B and removing A
  • CAdding A and C
  • DAdding A and C and removing B
  • EAdding C and removing A

Q2:

Solid calcium carbonate thermally decomposes to form solid calcium oxide and carbon dioxide gas, with an enthalpy change of +192 kJ/mol. The equation for this reaction is as follows: CaCO()CaO()+CO()32ssg

A sample of calcium carbonate is thermally decomposed at a constant temperature under an atmosphere of carbon dioxide in air. The product gases are cooled to room temperature, and a balloon is used to maintain a constant pressure of 1 atm, as shown in the diagram.

Once equilibrium has been reached, the mixture of calcium carbonate and calcium oxide is heated to a higher temperature. How and why does the volume of the gas balloon change?

  • AThe position of equilibrium shifts to the left, decreasing the volume.
  • BThe position of equilibrium does not shift, so the volume is unchanged.
  • CThe position of equilibrium shifts to the left, increasing the volume.
  • DThe position of equilibrium shifts to the right, decreasing the volume.
  • EThe position of equilibrium shifts to the right, increasing the volume.

Once equilibrium has been reached, the total gas pressure is increased to 2 atm. How and why does the mass of solids in the heated flask change?

  • AThe position of equilibrium shifts to the left, increasing the solid mass.
  • BThe position of equilibrium shifts to the left, decreasing the solid mass.
  • CThe position of equilibrium shifts to the right, decreasing the solid mass.
  • DThe position of equilibrium shifts to the right, increasing the solid mass.
  • EThe position of equilibrium does not shift, so the solid mass is unchanged.

Once equilibrium has been reached, a small mass of calcium carbonate is added to the heated flask. How and why does the concentration of carbon dioxide change?

  • AThe position of equilibrium shifts to the left, decreasing the concentration.
  • BThe position of equilibrium shifts to the left, increasing the concentration.
  • CThe position of equilibrium shifts to the right, increasing the concentration.
  • DThe position of equilibrium does not shift, so the concentration is unchanged.
  • EThe position of equilibrium shifts to the right, decreasing the concentration.

Q3:

Gaseous phosphorus pentachloride (PCl)5 decomposes into phosphorus trichloride (PCl)3 and chlorine gases, as shown in the equation: PClPCl+Cl532

The enthalpy change for this reaction is +87.9 kJ/mol. The rates of the forward and backward reactions are shown schematically in the diagram:

Which diagram represents the rates of the forward and backward reactions when the temperature is increased at constant pressure?

  • A
  • B
  • C
  • D
  • E

Which diagram represents the rates of the forward and backward reactions when the pressure is decreased at constant temperature?

  • A
  • B
  • C
  • D
  • E

Q4:

The colorless gas dinitrogen tetroxide (NO)24 decomposes reversibly into the brown gas nitrogen dioxide (NO)2 with a reaction enthalpy of +57.2 kJ/mol: NO2NO242 A student stores a sample of dinitrogen tetroxide in a gas syringe at room temperature and 1 atm pressure and allows the plunger of the syringe to move freely. The gas is left to decompose until equilibrium is reached, as shown.

Why is the position of the plunger at equilibrium different from its position at the start of the reaction?

  • AThe pressure of the gas in the syringe is higher.
  • BThe syringe contains gas molecules with a larger average size.
  • CThe temperature of the gas in the syringe is higher.
  • DThe syringe contains a larger number of gas molecules.
  • EThe syringe contains a larger mass of gas molecules.

The student heats the gas syringe and allows the plunger of the syringe to move freely. Which diagram best illustrates the gas syringe under the new reaction conditions once equilibrium has been reached?

  • A
  • B
  • C
  • D
  • E

The student compresses the gas by pushing the plunger and allows the reaction to reach equilibrium at room temperature, with the plunger fixed in its new position. Which diagram best illustrates the gas syringe under the new reaction conditions once equilibrium has been reached?

  • A
  • B
  • C
  • D
  • E

Q5:

Chloromethane reacts with sodium iodide to produce iodomethane and sodium chloride, as shown by the equation.

The molar solubilities of sodium chloride and sodium iodide in five organic solvents at 20C are shown in the table.

SolventSolubility of NaCl (M)Solubility of NaI (M)
Dimethylformamide6.8×100.96
Formamide1.63.8
Acetone5.5×102.0
Methanol0.244.9
Ethanol1.1×103.1

Assuming that chloromethane and iodomethane are highly soluble in all the solvents listed, which solvent would produce the greatest percentage yield of iodomethane at equilibrium?

  • AEthanol
  • BMethanol
  • CAcetone
  • DFormamide
  • EDimethylformamide

Q6:

Under certain conditions, nitric oxide (NO) and nitrogen dioxide (NO)2 gases react reversibly to produce liquid dinitrogen trioxide (NO)23: NO()+NO()NO()ggl223 The enthalpy change for this reaction is 41 kJ/mol. Below the boiling point of dinitrogen trioxide, which of the following graphs correctly illustrates the effects of temperature (𝑇) and pressure on the equilibrium yield of this reaction?

  • A
  • B
  • C
  • D

Q7:

At equilibrium, a box contains five molecules each of gaseous compounds A, B, C and D. Changing the temperature, 𝑇, or pressure, 𝑃, of the system alters the composition of the gas mixture at equilibrium, as shown in the diagram.

Write a balanced chemical equation for the reaction taking place in the gas mixture.

  • A2AB+C
  • BA+2CD
  • CA+CD
  • DA+2CB+D
  • E2A+2CB

The temperature of the box is decreased and the system is allowed to reach equilibrium. Assuming the pressure remains at the original value, which of the following situations could be observed?

  • A
  • B
  • C
  • D
  • E

The pressure is increased and the system is allowed to reach equilibrium. Assuming the temperature remains at the original value, which of the following situations could be observed?

  • A
  • B
  • C
  • D
  • E

Q8:

Solid blue [Cr(HO)(OH)]233 reacts with excess hydroxide ions in water to produce aqueous [Cr(OH)]63 ions, as shown by the following equation: [Cr(HO)(OH)]()+3OH()[Cr(OH)]()+3HO()233632saqaql Solutions of [Cr(OH)]63 are blue green in color and darken with increasing concentration. All other aqueous ions in this reaction produce colorless solutions. A suspension X is produced by adding [Cr(HO)(OH)]233 to a test tube of aqueous sodium hydroxide. At equilibrium, suspension X contains both solid [Cr(HO)(OH)]233 and aqueous [Cr(OH)]63 ions, as illustrated in the diagram.

Which diagram represents the situation at equilibrium when [Cr(OH)]63 ions are added to suspension X?

  • A
  • B
  • C
  • D
  • E

Which diagram represents the situation at equilibrium when the pH of suspension X is decreased?

  • A
  • B
  • C
  • D
  • E

Which diagram represents the situation at equilibrium when [Cr(HO)(OH)]233 is added to suspension X?

  • A
  • B
  • C
  • D
  • E

The diagram illustrates the situation at equilibrium following one or more changes to suspension X. Which of the following changes could have resulted in the situation shown?

  • AAdding [Cr(OH)]63 ions and acid
  • BRemoving [Cr(OH)]63 ions and adding acid
  • CAdding [Cr(OH)]63 ions and base
  • DRemoving [Cr(OH)]63 ions and adding base
  • EAdding [Cr(OH)]63 ions

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