Lesson: Le Chatelier’s Principle Chemistry

In this lesson, we will learn how to use Le Chatelier’s principle to predict directions of change for equilibria in response to a stimulus.

Lesson Worksheet

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.

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?

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?

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?

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?

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?

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?

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

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