For the water–gas shift reaction CO1g2 + H2O1g2 ∆ CO21g2 + H21g2, ΔH° = - 41.2 kJ does the amount of H2 in an equilibrium mixture increase or decrease when the temperature is increased? How does Kc change when the temperature is decreased? Justify your answers using Le Châtelier's principle.

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Identify the nature of the reaction: The given reaction is exothermic, as indicated by the negative ΔH° value (-41.2 kJ).

Apply Le Châtelier's principle: For an exothermic reaction, increasing the temperature will shift the equilibrium to the left (toward the reactants) to counteract the added heat.

Determine the effect on H2: Since the equilibrium shifts to the left when temperature is increased, the amount of H2 in the equilibrium mixture will decrease.

Consider the effect of decreasing temperature: Decreasing the temperature will shift the equilibrium to the right (toward the products) for an exothermic reaction.

Analyze the change in Kc: When the temperature is decreased, the equilibrium constant Kc will increase because the reaction shifts toward the products, favoring their formation.

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Key Concepts

Here are the essential concepts you must grasp in order to answer the question correctly.

Le Châtelier's Principle

Le Châtelier's Principle states that if a dynamic equilibrium is disturbed by changing the conditions, the position of equilibrium shifts to counteract the change. This means that if a system at equilibrium experiences a change in concentration, temperature, or pressure, the equilibrium will shift in a direction that minimizes the effect of that change.

Equilibrium Constant (Kc)

The equilibrium constant (Kc) is a numerical value that expresses the ratio of the concentrations of products to reactants at equilibrium for a given reaction at a specific temperature. Changes in temperature can affect the value of Kc; for exothermic reactions, increasing temperature typically decreases Kc, while decreasing temperature increases Kc, reflecting the shift in equilibrium position.

Exothermic Reactions

An exothermic reaction is a chemical reaction that releases heat, resulting in a negative change in enthalpy (ΔH). In the context of the water-gas shift reaction, the release of heat means that increasing the temperature will favor the reactants, leading to a decrease in the amount of H2 in the equilibrium mixture, while decreasing the temperature will favor the formation of products, increasing Kc.

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