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"