Step-by-Step Reasoning: Enzymes and Equilibrium

The Question: Why do enzymes NOT change the equilibrium of a chemical reaction?

Step 1: Define Equilibrium Chemical equilibrium is the state where the forward reaction rate equals the reverse reaction rate. The equilibrium constant (Keq) = [products]/[reactants] at equilibrium. Keq depends on $\Delta G$ (free energy difference between products and reactants).

Step 2: Define What an Enzyme Does An enzyme lowers the activation energy — the energy required to reach the transition state. It does this for BOTH the forward reaction (A→B) and the reverse reaction (B→A) by the same amount (by symmetry, because the transition state is between A and B on the energy diagram).

Step 3: Apply the Relationship Between Rate and Equilibrium Keq = kforward / kreverse (ratio of rate constants). If an enzyme multiplies BOTH kforward and kreverse by the same factor f: New Keq = (f × kforward) / (f × kreverse) = kforward / kreverse = same Keq. The f cancels out — Keq is unchanged.

Step 4: The Thermodynamic Argument $\Delta G$ = -RT ln(Keq). Since $\Delta G$ is determined by the chemical nature of reactants and products (their bond energies, not by kinetics), and the enzyme does not change the chemical identities of reactants or products, $\Delta G$ is unchanged. Therefore, Keq is unchanged.

Step 5: Practical Consequence Enzymes can ONLY catalyse thermodynamically feasible reactions ($\Delta G$ < 0 in the forward direction). A reaction that is thermodynamically forbidden ($\Delta G$ > 0 in forward direction) CANNOT be made to proceed in the forward direction by an enzyme — the enzyme would only accelerate the thermodynamically favoured reverse direction.

Step 6: What Enzymes DO Change

• Rate of reaching equilibrium (faster with enzyme)
• Activation energy (lowered)
• The time needed to reach equilibrium (reduced)

What Enzymes Do NOT Change:

• Equilibrium position (Keq)
• Free energy change ($\Delta G$)
• Enthalpy ($\Delta H$)
• Entropy ($\Delta S$)
• The final concentrations of products and reactants at equilibrium

NEET Summary: "Enzymes are kinetic agents, not thermodynamic agents. They change the RATE but not the DIRECTION or EXTENT of a reaction."