What is the reaction rate?

• A. The product of concentrations
• B. The change in concentration of a reactant or a product per unit time
• C. The equilibrium constant
• D. The energy change per mole

Answer: (B)

Reaction rate is the speed at which a reaction proceeds, i.e., how fast concentrations change with time. It’s defined as the change in concentration of a reactant (negative sign) or a product (positive sign) per unit time. For a simple reaction A → products, the rate can be written as rate = -d[A]/dt = d[Product]/dt. The usual units are molarity per second (M/s). Rate can be described as an average rate over a time interval or as an instantaneous rate at a particular moment. The other ideas describe different things: the product of concentrations isn’t the rate itself, the equilibrium constant tells how far the reaction goes at equilibrium, and the energy change per mole is a thermodynamic property, not the speed of the reaction.