Question

In which direction will the position of the equilibrium.$$2 \mathrm{HI}(g) \rightleftharpoons \mathrm{H}_{2}(g)+\mathrm{I}_{2}(g).$$,be shifted for each of the following changes? a. \(\mathrm{H}_{2}(g)\) is added. b. \(\mathrm{I}_{2}(g)\) is removed. c. \(\mathrm{HI}(g)\) is removed. d. In a rigid reaction container, some \(\operatorname{Ar}(g)\) is added. e. The volume of the container is doubled. f. The temperature is decreased (the reaction is exothermic).

Short answer

a. The equilibrium position shifts to the right. b. The equilibrium position shifts to the right. c. The equilibrium position shifts to the left. d. No shift in the equilibrium position. e. No shift in the equilibrium position. f. The equilibrium position shifts to the left.

Step-by-step solution

Change in Concentration of Reactants and Products

In each case, use Le Chatelier's Principle to determine the shift in the equilibrium position: a. When more H2(g) is added, the system will consume the added H2(g) by shifting the equilibrium position towards the production of HI(g). Therefore, the equilibrium position shifts to the right. b. When I2(g) is removed, the system will produce more I2(g) by shifting the equilibrium position towards the production of I2(g). Therefore, the equilibrium position shifts to the right. c. When HI(g) is removed, the system will produce more HI(g) by shifting the equilibrium position towards the production of HI(g). Therefore, the equilibrium position shifts to the left.

Change in Pressure or Volume

In each case, examine the number of moles of gas on either side of the equilibrium reaction to determine the shift in the equilibrium position: d. Adding Ar(g) to a rigid reaction container will not change the concentrations of the reactants or products as it is an inert gas. Hence, the equilibrium position would not shift. e. Doubling the volume of the container would decrease the pressure inside. The system will counteract this decrease in pressure by shifting the equilibrium position towards the side with more moles of gas. In this case, there are 2 moles of gas on the left (HI) and 2 moles of gas on the right (H2 + I2). Therefore, there would be no shift in the equilibrium position.

Change in Temperature

In each case, use Le Chatelier's Principle to predict the shift in the equilibrium position based on whether the reaction is exothermic or endothermic: f. If the temperature is decreased and the reaction is exothermic, the system will try to generate more heat to counteract this change. Since the exothermic direction in this case is the production of HI(g), the equilibrium position will shift to the left.

Key concepts

Chemical Equilibrium

Chemical equilibrium occurs when a chemical reaction and its reverse reaction proceed at the same rate, resulting in no net change in the concentration of the reactants and products.

This dynamic state means that, although both reactions continue, their effects balance each other out, maintaining stable proportions of all substances involved. It's important to realize that this balance is not static, but rather a state of constant flux, where reactants are turning into products at the same rate that products regenerate reactants.

Reaction Quotient

The reaction quotient (\( Q \)) is a way to determine the direction in which a reaction mixture will shift to reach equilibrium. It is calculated using the concentrations of the reactants and products at any point during the reaction.

\[\begin{equation}Q = \frac{[products]}{[reactants]}\end{equation}\]
When compared to the equilibrium constant (\( K \)), the reaction quotient tells us if the reaction is at equilibrium (\( Q = K \)), or if it will proceed in the forward (\( Q < K \)) or reverse (\( Q > K \)) direction to reach equilibrium.

Exothermic Reactions

Exothermic reactions release energy in the form of heat to their surroundings. This means the products of the reaction have less enthalpy than the reactants. In terms of Le Chatelier's Principle, an increase in temperature adds heat to the system, favoring the endothermic direction—the direction that absorbs heat.

Conversely, a decrease in temperature implies that the system is losing heat, so the equilibrium will shift towards the exothermic reaction to produce more heat. Understanding whether a reaction is exothermic or endothermic is crucial when predicting the effects of temperature changes on the system's equilibrium.

Shift in Equilibrium

A shift in equilibrium refers to the change in the position of equilibrium in response to an external stress, according to Le Chatelier's Principle. This principle states that if a dynamic equilibrium system is subjected to a change, the system adjusts to minimize that change.

Shifts can occur due to changes in concentration, pressure, volume, or temperature. For example, increasing the concentration of reactants shifts the equilibrium towards the products to reduce the added reactants, while decreasing the temperature of an exothermic reaction shifts the equilibrium towards the products to release heat and counteract the temperature drop.

Effects of Concentration Changes

Concentration changes can significantly affect the position of a chemical equilibrium. Adding more reactants or removing products generally causes a shift towards the product side, increasing the yield of products.

Conversely, removing reactants or adding more products can shift the equilibrium towards the reactants, decreasing the quantity of products formed. In the case of our HI equilibrium, removing HI(g) prompts the system to shift to the left, producing more HI(g), while adding H2(g) causes a rightward shift, using up the added H2(g) to form more HI(g). This adaptability of the system to concentration changes is a practical application of Le Chatelier's Principle.