If an enzyme is added to a solution where its substrate and product are in equilibrium, what will occur? a. Additional substrate will be formed. b. The reaction will change from endergonic to exergonic. c. The free energy of the system will change. d. Nothing; the reaction will stay at equilibrium.

Verified step by step guidance

1. Understand the role of enzymes: Enzymes are biological catalysts that speed up the rate of chemical reactions without being consumed or permanently altered by the reaction.
2. Recognize the concept of equilibrium: In a chemical reaction, equilibrium is the state where the rate of the forward reaction equals the rate of the reverse reaction, and the concentrations of products and reactants remain constant over time.
3. Analyze the effect of enzymes on equilibrium: Adding an enzyme to a reaction mixture at equilibrium will increase the rate of both the forward and reverse reactions equally. This is because enzymes catalyze reactions by lowering the activation energy, making it easier for the reaction to occur in both directions.
4. Determine the impact on substrate, product, and free energy: Since the enzyme speeds up both the forward and reverse reactions, the concentrations of substrate and product will not change, nor will the free energy of the system. The reaction will reach a new equilibrium state more quickly, but the position of the equilibrium does not shift.
5. Select the correct answer: Based on the analysis, the correct answer is 'd. Nothing; the reaction will stay at equilibrium.' because the enzyme catalyzes both the forward and reverse reactions without altering the equilibrium concentrations or the free energy of the system.