Initial Rate and Concentrations of Reactants for
Rate expressions can take different forms. Forms such as , , and are common. A rate expression is described as having the general form . The sum of the powers that the reactant concentrations are raised to in the rate expression is the reaction order. The reaction order is equal to in the general form.
A zero-order reaction is a reaction in which rate is not dependent on the reactant concentrations. The rate law for a zero-order reaction has the form .
A first-order reaction is a reaction in which the rate is dependent on the concentration of one of the reactants, raised to the power of one. The rate law for a first-order reaction has the form . A first-order reaction may have more than one reactant. For example, it may have the form . The concentration of B does not affect rate in this case. For first-order reactions, .
A second-order reaction is a reaction in which the sum of the powers that the reactant concentrations are raised to in the rate expression is equal to two. The rate depends on the concentrations of two reactants each raised to the power of one, or the rate depends on the concentration of one reactant raised to the power of two. The rate law for a second-order reaction can have the form , or it can have the form . For second-order reactions, .
In a third-order reaction the powers that the concentrations of the reactants are raised to in the rate expression is equal to three. Third-order reactions can have multiple different forms, for example, , , or . For third-order reactions, the sum of the exponents in the rate equation equals 3.
The units of the rate constant are dependent on the reaction order.
Rate Constant Units and Reaction Order
In a chemical reaction of the form , the following experimental data are measured.
|Measurement||[A] (M)||[B] (M)||Measured Rate (M/s)|
For this reaction, determine the following.
1. Rate expression
2. Reaction order
3. Rate constant