Rate of Reaction
Rates of reactions are usually expressed in units of moles per liter per unit time. If we know the chemical equation for a reaction, its rate can be determined by following the change in concentration of any product or reactant that can be detected quantitatively. 

r = ∆ concentration   =  ∆[ ] 
           ∆ time                   ∆t

To describe the rate of a reaction, we must determine the concentration of a reactant or product at various times as the reaction proceeds.

Example:
At some time, we observe that the reaction 2 N2O5 (g) → 4 NO2 (g) + O2 (g) is forming NO2 at the rate of 0.0072 mol / L∙s.

 (a) What is the rate of change of [O2],  ∆ [O2]/ ∆t in mol / L∙s?
 (b) What is the rate of change of [N2O5], ∆ [N2O5]/ ∆t in mol / L∙s?

Plan
We can use the mole ratios from the balanced equation to determine the rates of change of other products and reactants. The rate of reaction can then be derived from any one of these individual rates.

Solution
(a) The balanced equation gives the reaction ratio 1 mol O2 : 4 mol NO2

rate of change of O2 =                  =                                       x                             =   0.0018 mol O2 / L ∙s  

(b) The balanced equation shows that 2 mol N2O5 is consumed for every 4 mol NO2 that is formed. Because [N2O5] is  
       decreasing as [NO2] increases, we should write the reaction ratio as

rate of change of N2O5 =                    =                                       x                             =   - 0.0036 mol N2O5 / L ∙s  

∆[O2 ]              0.0072 mol NO2            1 mol O2   
∆t                             L∙s                           4 mol NO2 

- 2 mol N2O5
   4 mol NO2

     ∆[N2O5 ]        0.0072 mol NO2              -2 mol N2O5   
       ∆t                       L∙s                                 4 mol NO2 

Factors that affect the rate of a reaction

Temperature	As temperature increase, the rate of a reaction increase.
In general, for ever 10oC increase in temperature the rate of a reaction increase.
Temperature affects endothermic reactions more than exothermic reaction.	
Concentration	As concentration increase, the rate of a reaction increase.	
Surface Area	As surface area increase, the rate of a reaction increase.	
Chemical Nature	Some compounds react more easily than other reactants.	
Catalyst	Catalyst increases the rate of a reaction by providing an alternative reaction pathway.