Consider the following gas-phase reaction: 2 A_g + B_g ⇌ C_g + D_g. An equilibrium mixture of reactants and products is subjected to the following changes: (a) A decrease in volume (b) An increase in temperature (c) Addition of reactants (d) Addition of a catalyst (e) Addition of an inert gas. Which of these changes affect the composition of the equilibrium mixture but leave the value of the equilibrium constant Kc unchanged? Which of the changes affect the value of Kc? Which affect neither the composition of the equilibrium mixture nor Kc?

Verified step by step guidance

1. Step 1: Consider the effect of a decrease in volume. According to Le Chatelier's principle, decreasing the volume of a gas-phase reaction shifts the equilibrium towards the side with fewer moles of gas. In this reaction, the left side has 3 moles (2 A_g + B_g) and the right side has 2 moles (C_g + D_g). Therefore, a decrease in volume will shift the equilibrium to the right, affecting the composition but not the equilibrium constant Kc.
2. Step 2: Analyze the effect of an increase in temperature. For an endothermic reaction, increasing temperature shifts the equilibrium to the right, while for an exothermic reaction, it shifts to the left. This change affects both the composition and the value of Kc, as Kc is temperature-dependent.
3. Step 3: Examine the addition of reactants. Adding more reactants shifts the equilibrium to the right, according to Le Chatelier's principle, to consume the added reactants. This affects the composition of the equilibrium mixture but does not change the value of Kc.
4. Step 4: Consider the addition of a catalyst. A catalyst speeds up the rate at which equilibrium is reached but does not affect the position of the equilibrium or the value of Kc. Therefore, it affects neither the composition nor Kc.
5. Step 5: Evaluate the addition of an inert gas. Adding an inert gas at constant volume does not change the partial pressures of the reactants or products, so it does not affect the equilibrium composition or the value of Kc.

Key Concepts

Le Chatelier's Principle

Le Chatelier's Principle states that if a dynamic equilibrium is disturbed by changing the conditions, the position of equilibrium shifts to counteract the change. This principle helps predict how the concentration of reactants and products will change in response to alterations such as volume, temperature, or concentration.

Equilibrium Constant (Kc)

The equilibrium constant (Kc) is a numerical value that expresses the ratio of the concentrations of products to reactants at equilibrium, each raised to the power of their coefficients in the balanced equation. Changes in temperature can affect the value of Kc, while changes in concentration or volume do not alter Kc but can shift the position of equilibrium.

Catalysts and Inert Gases

Catalysts are substances that increase the rate of a reaction without being consumed, and they do not affect the position of equilibrium or the value of Kc. Similarly, the addition of an inert gas at constant temperature and volume does not change the concentrations of the reactants or products, thus having no effect on the equilibrium composition or Kc.