Reaction Rate and Concentration
As the reaction proceeds, the concentration of reactant drops, and the rate drops, i.e., rate is somehow related to the concentration.
The order of a reaction is defined as the power to be applied to the reactant in the appropriate rate equation
For first order reactions, Rate is proportional [reactant] where [] is shorthand for concentration, moles/liter, i.e. molarity (M).
For simple second order reactions of the form A ?products has a reaction rate of the form: Rate is proportional to [reactant]2. Technically, the equation is Rate = k[A]2, i.e., the proportionality has been replaced with an equal sign and a constant of proportionality.
For simple reactions Rate = k[A]order. k is the specific rate constant. The order is, generally speaking, an integer. Order =0, 1, and 2 are common.
The order has nothing to do with the balanced equation!