Kinetics :  Order and Molecularity

Molecularity - The number of molecules consumed in a chemical reaction

Step 1: N2O5 NO2 + NO3
Step 2: NO2 + NO3 NO2  + NO + O2
Step 3: NO + NO3 2 NO2

• Unimolecular - When a single molecules is consumed.  e.g. Step 1 in the above reaction.
• Bimolecular - When two molecules are consumed.  e.g. Steps 2 and 3 in the above reaction.

Order - Describes the relationship between the rate of a step in a chemical reaction and the concentration of one of the reactants consumed in that step.

• First-Order - The rate of the reaction depends on the concentration of a reactant raised to the first power.
• The #1 for the first power is usually omitted because any number raised to the first power equals itself.

Rate = k[N2O5]

The above reaction is first order with respect to [N2O5] and first order overall.

• The rate of the overall reaction is the sum of the orders of all the reactants

•  If there is only one reactant, then the order with respect to that reactant is equal to the order of the overall reaction.

• Second-Order - The rate of the reaction depends on the concentration of a reactant raised to the second power.

Rate = k[HI]2

The above reaction is second order with respect to [HI] and second order overall.

Rate = [NO]2[O2]

The above reaction is first order with respect to [O2] and second order with respect to [NO].
The overall reaction is third order, or higher order.

• Higher Order - The rate of the reaction depends on the concentration of a reactant raised to a power greater than two.

Rate = [NO]1/2[Cl2]3

The above reaction is mixed order with respect to [NO] and higher order with respect to [Cl2]3.

• Mixed Order - The rate of the reaction depends on the concentration of a reactant raised to a fractional power.

Next:  "Sample Rate Calculations"