Rate Orders and Rate Laws

Reaction Rates Are measured as the change in concentration over time. ∆[reactants] Are measured as the change in concentration over time. ∆[reactants] ∆time ∆time We can measure rate as the rate of disappearance of reactants or the appearance of products We can measure rate as the rate of disappearance of reactants or the appearance of products The reactants concentration decreases per unit time and the product concentration increases over time. The reactants concentration decreases per unit time and the product concentration increases over time.

Reaction Rate Graph [Products] Increase [Reactants] Decrease H 2 + I 2  2HI

Rate Order – a specific exponential relationship between the concentration of a chemical and the rate of a reaction Rate Order – a specific exponential relationship between the concentration of a chemical and the rate of a reaction We look to see how the rate of a reaction changes in relationship to a change in the concentration of a reactant. We look to see how the rate of a reaction changes in relationship to a change in the concentration of a reactant.

Rate Orders – Tell us how the rate changes with changes in reactant concentration Rate Orders – Tell us how the rate changes with changes in reactant concentration Zero Order (0 o )– as concentration changes, the reaction rate does not change Zero Order (0 o )– as concentration changes, the reaction rate does not change First Order (1 o ) – as the concentration changes, the rate changes by the same factor. First Order (1 o ) – as the concentration changes, the rate changes by the same factor. (double concentration = double rate)

Second Order (2 o ) – as the concentration changes, the rate changes by the same factor, squared. Second Order (2 o ) – as the concentration changes, the rate changes by the same factor, squared. (double concentration = rate increases by a factor of 2 2 (or 4x) (double concentration = rate increases by a factor of 2 2 (or 4x) Or triple concentration = rate increase by a factor of 3 2 Or triple concentration = rate increase by a factor of 3 2 Third Order (3 o )– as the concentration changes, the rate changes by the same factor, cubed. Third Order (3 o )– as the concentration changes, the rate changes by the same factor, cubed. (double concentration = rate increases by a factor of 2 3 ) (double concentration = rate increases by a factor of 2 3 )

Rate Laws A rate law is an equation that tells us the rate order of each reactant in a reaction. Its general form is Its general form is Rate = k [X] a [Y] b Where ‘a’ and ‘b’ represent the order of the reactant X and Y respectively. Where ‘a’ and ‘b’ represent the order of the reactant X and Y respectively. [X] means the concentration of reactant X in Molarity. (same thing for [Y]) [X] means the concentration of reactant X in Molarity. (same thing for [Y]) ‘k’ is the rate constant which is different for every reaction and depends on temperature. ‘k’ is the rate constant which is different for every reaction and depends on temperature. The rate constant and rate orders for a reaction can only be determined by experimentation The rate constant and rate orders for a reaction can only be determined by experimentation

Example For the reaction X + Y  Z For the reaction X + Y  Z The following rate data was obtained. Determine the rate order of each reactant, and then write the rate law. The following rate data was obtained. Determine the rate order of each reactant, and then write the rate law. [X][Y]Rate (M/sec) [X][Y]Rate (M/sec) ___?___ 33___?___