Increasing the concentration of reactants generally has what effect on the rate of a chemical reaction?

• A. Increases the rate
• B. Decreases the rate
• C. Has no effect
• D. Only affects rate if a catalyst is present

Answer: (A)

Increasing the concentration of reactants increases the rate because more molecules are around to collide with each other. Reactions occur when molecules collide with enough energy and the right orientation, so more molecules mean more collisions per unit time, which leads to more productive collisions and a faster overall rate. The exact change depends on the reaction order with respect to each reactant (rate often scales as [A]^m [B]^n), so doubling a reactant can increase the rate by a factor of 2^m. Catalysts speed up reactions by providing a lower-energy pathway, not by changing the concentration, so the rate can rise with concentration regardless of whether a catalyst is present. If a reactant is zero-order for the rate, changing its concentration won’t affect the rate, but that’s a special case rather than the general rule.