The property of a chemical reaction that is affected by the presence of a catalyst is

Step 1: Analyze the question We need the property of a reaction that a catalyst changes. A catalyst speeds up a reaction without being used up.

Step 2: Explain the concept A catalyst provides an alternative reaction pathway with a lower activation energy (Ea). The activation energy is the minimum energy reactants need for a successful collision. With a lower Ea, more particles have enough energy to react, so the rate increases.

Step 3: Check each option

• A. Activation energy: It is lowered by the catalyst. ✔
• B. Free energy: The Gibbs free energy change (ΔG) depends only on the initial and final states (reactants and products). A catalyst does not change it.
• C. Equilibrium position: A catalyst speeds up the forward and reverse reactions equally. It helps equilibrium to be reached faster but does not shift its position.
• D. Enthalpy change: ΔH is the difference between the energy of products and reactants. It is unchanged by a catalyst, because the catalyst only changes the path, not the starting and ending energy levels.

Step 4: Answer The only property the catalyst alters is the activation energy, so the answer is A.

Exam tip: A catalyst changes the speed (the route and the Ea), not the thermodynamics (ΔH, ΔG, equilibrium position, yield). If an option mentions "rate" or "activation energy," it is usually the right choice for a catalyst question.