How does adding a catalyst increase the rate of reaction?

• A. It offers an alternative reaction pathway with a lower activation energy
• B. It raises the temperature
• C. It increases the surface area of reactants
• D. It changes the equilibrium constant

Answer: (A)

Lowering the activation energy by offering an alternative reaction pathway with a lower energy barrier is how a catalyst speeds up a reaction. Activation energy is the minimum energy molecules must have to transform into products. At a given temperature, only a fraction of molecules have enough energy to get over that barrier, so the reaction rate is limited. A catalyst provides a different route for the reaction and lowers that barrier, so more molecules can react in the same time frame. It does this by providing new steps or stabilizing transition states and intermediates on its surface, and it is not consumed in the process. Because it speeds up both forward and reverse reactions equally, the equilibrium position remains unchanged, even though the overall rate is higher. The other ideas don’t explain how a catalyst works: raising temperature increases the fraction of energetic molecules; increasing surface area helps the reactants meet more often but isn’t a catalyst; changing the equilibrium constant would alter the balance of products and reactants rather than the speed at which equilibrium is reached.