Question

Le Châtelier's principle is related ultimately to the rates of the forward and reverse steps in a reaction. Explain (a) why an increase in reactant concentration shifts the equilibrium position to the right but does not change \(K ;\) (b) why a decrease in \(V\) shifts the equilibrium position toward fewer moles of gas but does not change \(K ;\) (c) why a rise in \(T\) shifts the equilibrium position of an exothermic reaction toward reactants and changes \(K ;\) and (d) why a rise in temperature of an endothermic reaction from \(T_{1}\) to \(T_{2}\) results in \(K_{2}\) being larger than \(K_{1}\)

Short answer

Increasing reactant concentration shifts equilibrium right but doesn’t change K. Decreasing volume shifts equilibrium toward fewer gas moles but doesn’t change K. Rising temperature in exothermic reactions shifts equilibrium toward reactants and decreases K, while in endothermic reactions it shifts equilibrium toward products and increases K.

Step-by-step solution

Explain Reactant Concentration Increase

Increasing the concentration of reactants in a chemical equilibrium system shifts the equilibrium position to the right (toward the products). According to Le Châtelier's principle, the system will try to counteract the change by consuming the added reactants and forming more products. However, this shift does not change the equilibrium constant, K, because K is dependent only on temperature, not on the concentrations of reactants or products.

Explain the Effect of Decreased Volume

Decreasing the volume of a gaseous reaction system increases the pressure. According to Le Châtelier's principle, the system will respond by shifting the equilibrium position toward the side with fewer moles of gas to reduce the pressure. However, the equilibrium constant, K, remains unchanged because it is a function of temperature, not of pressure or volume.

Explain Effect of Temperature Rise in Exothermic Reaction

For an exothermic reaction (one that releases heat), increasing the temperature adds heat to the system. According to Le Châtelier's principle, the system will shift the equilibrium position toward the reactants to consume some of the added heat. This change also alters the equilibrium constant, K, because K is temperature-dependent and decreases for exothermic reactions as temperature increases.

Explain Effect of Temperature Rise in Endothermic Reaction

For an endothermic reaction (one that absorbs heat), increasing the temperature adds heat to the system. According to Le Châtelier's principle, the system will shift the equilibrium position toward the products to absorb the added heat. This change also increases the equilibrium constant, K. Therefore, at higher temperatures, K_2 is greater than K_1 for an endothermic reaction.

Key concepts

equilibrium shift

Le Châtelier's Principle states that if a system at equilibrium is disturbed, it will adjust itself to counteract the disturbance and re-establish equilibrium. When a change occurs—like adjusting reactant concentration, volume, pressure, or temperature—the system shifts its equilibrium position.

• Adding more reactants: the equilibrium moves to make more products.
• Removing reactants: it shifts to make more reactants.

These shifts restore equilibrium, but the equilibrium constant, K, remains unchanged unless temperature changes. K is solely temperature-dependent, directly linking it to energy changes in the system.

reactant concentration

Increasing reactant concentration causes a shift towards the product side. This occurs because the system tries to reduce the added reactants by creating more products, thus restoring balance. Le Châtelier's Principle ensures that the addition of reactants will lead to a reaction favoring product formation.

However, this change does not affect the equilibrium constant, K. K remains constant as long as the temperature does not change. The formula for K is based on concentration or pressure ratios of products to reactants at equilibrium, which only temperature shifts can alter.

volume and pressure changes

Changes in volume and pressure can also affect the system. Decreasing the volume of a gas-phase reaction increases the pressure. According to Le Châtelier's Principle, the equilibrium position shifts toward the side with fewer moles of gas, reducing pressure.

This does not alter the equilibrium constant K, as K is independent of volume and pressure variations. The system adapts to these changes by shifting its equilibrium position to regain equilibrium under new conditions.

temperature effects

Temperature changes significantly impact the equilibrium because they alter reaction energy. For exothermic reactions that release heat, increasing temperature shifts the equilibrium towards the reactants to absorb the excess heat. This change decreases the equilibrium constant K.

Conversely, in endothermic reactions where heat is absorbed, raising the temperature shifts equilibrium towards products. This shift increases K because it favors forward endothermic reactions, integrating the additional heat into the chemical process.

equilibrium constant (K)

The equilibrium constant, K, is critical for understanding chemical equilibria. It represents the ratio of product concentrations to reactant concentrations at equilibrium and is given by the formula:
\[ K = \frac{[Products]}{[Reactants]} \]
While changes in reactant concentration, volume, or pressure shift equilibrium positions, they do not change K. It is exclusively dependent on temperature.

For exothermic reactions, increasing temperature decreases K since it shifts equilibrium towards reactants. For endothermic reactions, increasing temperature increases K because equilibrium shifts toward products. Thus, understanding K gives valuable insights into how equilibrium shifts with temperature variations.