Question

List four factors that can shift the position of an equilibrium. Only one of these factors can alter the value of the equilibrium constant. Which one is it?

Short answer

Temperature changes can alter the value of the equilibrium constant.

Step-by-step solution

Understanding Equilibrium Shift

In a chemical reaction, equilibrium is the state at which the rate of the forward reaction equals the rate of the reverse reaction. Certain factors can shift this equilibrium position, meaning they change the concentrations of products and reactants at equilibrium.

Identifying Factors Shifting Equilibrium

The main factors that can shift the position of an equilibrium include: 1. Concentration changes: Changing the concentration of reactants or products will shift the equilibrium to the side that opposes the change. 2. Temperature changes: Raising or lowering the temperature can shift equilibrium towards the endothermic or exothermic direction. 3. Pressure changes: For gases, changing the pressure by altering the volume can shift equilibrium towards the side with fewer moles of gas. 4. Addition of a catalyst: A catalyst speeds up both forward and backward reactions but does not change the position of equilibrium.

Determining Which Factor Affects the Equilibrium Constant

Out of all the listed factors, only temperature changes can affect the value of the equilibrium constant (K). This is because the equilibrium constant is a function of temperature, and changes in temperature can favor either the forward or reverse reaction, thereby changing the equilibrium constant's value.

Key concepts

Concentration Changes

When we talk about concentration changes in a chemical equilibrium, we're looking at how adding or removing substances can shift the balance of a reaction. Imagine a chemical reaction where the reactants and products are constantly in a tug-of-war. If you add more reactants, the tug on the product side becomes stronger, shifting the equilibrium towards the products. Conversely, adding more products tips the balance back towards the reactants.

This is a direct result of Le Chatelier's principle, which states that a system at equilibrium will adjust to oppose any changes imposed on it. So, by altering the concentration, the reaction shifts to "counteract" the change, making it a dynamic and adaptable process.

Temperature Changes

Temperature changes can have a significant impact on chemical equilibria. This is because temperature affects the kinetic energy of molecules, thus influencing reaction rates. According to Le Chatelier's principle, if you heat up a reaction mixture, the equilibrium will shift in the endothermic direction, where heat is absorbed to reduce the impact of the added thermal energy. In contrast, cooling down the system shifts the equilibrium towards the exothermic direction, where heat is released.

Importantly, while concentration and pressure changes shift equilibrium without altering the equilibrium constant ( K ), temperature changes can actually change the value of K . This is because temperature impacts the energy profiles of the reactions involved, favoring either the forward or backward reaction, consequently altering the ratio of products to reactants at equilibrium.

Pressure Changes

Pressure changes primarily affect equilibria involving gaseous reactions. If you increase the pressure by decreasing the volume of the reaction vessel, the system will respond by shifting the equilibrium towards the side of the reaction with fewer moles of gas. This helps to alleviate the pressure change. It's like trying to fit too many people in a small room; the system wants to minimize crowding.

Conversely, reducing the pressure by increasing the volume shifts the equilibrium towards the side with more moles of gas. This adjustment helps to "occupy" the newly available space, stabilizing the pressure change imposed.

• Adding an inert gas at constant volume doesn’t affect the equilibrium position as it doesn't change the partial pressures of reactants or products.

Equilibrium Constant

The equilibrium constant (K) is a critical value in chemical reactions, reflecting the ratio of product concentrations to reactant concentrations at equilibrium. Each reaction has its own unique K value, which remains constant as long as the temperature doesn't change.

However, if temperature changes, K will change because the reaction kinetics are affected. For example, suppose a reaction is exothermic (releases heat). In that case, increasing the temperature will shift the equilibrium towards the reactants, resulting in a lower K value. The opposite is true for endothermic reactions.

Therefore, while factors like concentration, pressure, or a catalyst influence the position of equilibrium, they do not alter the equilibrium constant, making temperature the sole factor that can change K .