ACS Chemistry Practice Exam 2025 – The All-in-One Guide to Exam Success!

The equilibrium constant, \( K_{eq} \), is a value that expresses the ratio of the concentrations of products to reactants at equilibrium at a given temperature. It is important to note that this constant is dependent on the temperature of the system.

When the temperature of a reaction is changed, it can affect the equilibrium position depending on whether the reaction is exothermic or endothermic. For example, in an endothermic reaction, increasing the temperature shifts the equilibrium to favor the production of products, consequently increasing the equilibrium constant. Conversely, for an exothermic reaction, increasing the temperature shifts the equilibrium towards the reactants, resulting in a decrease in the equilibrium constant.

In contrast, adding a catalyst does not affect the equilibrium constant; it merely increases the rate at which equilibrium is reached by lowering the activation energy for both the forward and backward reactions. Changing the concentration of products or reactants also does not change \( K_{eq} \); it may shift the equilibrium position to favor one side or the other until a new equilibrium is established, but the value of the \( K_{eq} \) remains constant at a given temperature. Therefore, the only situation among the options provided that will lead to a change in the equilibrium