7A.7 Express the units for rate constants when the concentrations are in moles per liter and time is in seconds for (a) zeroth-order reactions; (b) first-order reactions; (c) second-order reactions.
Why do you divide by mol/L for first and second order reactions?
The rate of a reaction is usually given as mol L^-1 s^-1 (concentration per unit of time). For first order reactions, the concentration of one reactant takes care of the unit for concentration, so the rate constant is given in units of s^-1. For second order reactions, there are two units of concentration already present, so the rate will be s^-1 divided by concentration unit. Hope that helps:)