Average rate of reaction: A + B C + 2 D The rate at which [A] and [B] decrease is equal to the rate at which [C] increases and half the rate at which [D] increases.  [A] t =  [B] t  [C] t =  [D] 2t =

The rate of a reaction can be increased by: Increasing temperature Increasing concentration Decreasing particle size (increasing surface area of reactants)

First premise: Reactants must collide in order to react and form products. A 2 + B 2 2 AB Collision Theory

First premise: Reactants must collide in order to react and form products. A 2 + B 2 2 AB Second premise: Reactants must have the correct orientation to form the products upon collision

Collision Theory First premise: Reactants must collide in order to react and form products. A 2 + B 2 2 AB Second premise: Reactants must have the correct orientation to form the products upon collision Third premise: Reactants must have sufficient energy for the collision to result in formation of products E < E a

Collision Theory First premise: Reactants must collide in order to react and form products. A 2 + B 2 2 AB Second premise: Reactants must have the correct orientation to form the products upon collision Third premise: Reactants must have sufficient energy for the collision to result in formation of products E > E a

Reaction progress Energy EaEa Activated complex: an unstable transition state between reactants and products. It can either fall back down on the reactant side or go on to the product side.  H rxn

Reaction progress Energy EaEa  H rxn EaEa A catalyst speeds up the rate of a reaction by lowering the activation barrier

Reaction progress Energy  H rxn EaEa A catalyst speeds up the rate of a reaction by lowering the activation barrier

Reaction rate laws: AB Rate = k[A] k = specific rate constant depends upon temperature unique for every reaction Indicates the probability of a successful conversion of reactants to products Reaction order is given by the sum of the exponents on the molar concentrations in the rate law expression This reaction is first order in [A] and first order overall

General form of rate law: aA + bB products Rate = k[A] m [B] n The only time m=a and n=b is when the reaction proceeds in a single step, which is uncommon. The reaction order must be found experimentally by comparing reactant concentrations. Trial [A] 0 (M) [B] 0 (M) Rate (M/s) x x x10 -3 Double [A]  rate doubles First order in [A] Double [B]  rate quadruples Second order in [B] Rate = k[A][B] 2

Calculating rate constants 1.Determine the rate law. 2.Substitute in the reactant concentrations and measured rates. 3.Solve for k. Trial [A] 0 (M) [B] 0 (M) Rate (M/s) x x x10 -3 Rate = k[A][B] 2 k (M -2 s -1 ) Rate = 2[A][B] 2

Instantaneous rates vs. average rates: Average rates are found using:  [A] tt Example: [NO] 0 = M and [NO] = M after 8 s. What is the average rate? = x M/s M – M 8 s – 0 s Avg. rate = Instantaneous rates are found using rate laws. Example: What is the instantaneous rate if the reaction above has the following rate law: Rate = (-0.2 M -1 s -1 ) [NO] 2 Rate = (-0.2 M -1 s -1 ) [0.150 M] 2 = -4.5 x M/s