RATE LAWS!!! Reaction rate video: http://www.youtube.com/watch?v=kw-Lt9-WmTg

What is different about the following three reactions?

Concentration of products vs. reactants

Data for reaction rates: Average rate Instantaneous rates

Reaction Rates C4H9Cl(aq) + H2O(l) C4H9OH(aq) + HCl(aq) The average rate of the reaction over each interval is the change in concentration divided by the change in time:

Reaction Rates C4H9Cl(aq) + H2O(l) C4H9OH(aq) + HCl(aq) Note that the average rate decreases as the reaction proceeds. This is because as the reaction goes forward, there are fewer collisions between reactant molecules.

Data for reaction rates: Average rate Instantaneous rates

Concentration and Rate Each reaction has its own equation that gives its rate as a function of reactant concentrations. this is called its Rate Law To determine the rate law we measure the rate at different starting concentrations.

Rate Laws rate = k [A]m[B]n A rate law shows the relationship between the reaction rate and the concentrations of reactants. For gas-phase reactants use PA instead of [A]. k is a constant that has a specific value for each reaction. The value of k is determined experimentally. rate = k [A]m[B]n “Constant” is relative here- k is unique for each rxn, and k changes with T “m” and “n” refer to reaction order

Concentration and Rate How is the rate affected when [B] is doubled? How is the rate affected when [A] is doubled?

Concentration and Rate Since the change in [B] does not affect the rate, the reaction is zero order in relation to [B] Since the change in [A] (x2) caused the rate to increase by 4 (22), the reaction is second order in relation to [A]. Rate law: rate = k [A]2[B]0 = k [A]2

Method for Initial Rates Used to find the form of the rate law Choose one reactant to start with Find two experiments where the concentration of that reactant changes but all other reactants stay the same Write the rate laws for both experiments Divide the two rate laws Solve for the order Follow the same technique for other reactants

Example Choose one reactant to start with NH4+ Find two experiments where the concentration of that reactant changes but all other reactants stay the same Exp 2 & 3

Example Write the rate laws for both experiments Exp 2: Rate = 2.70x10-7 = k(0.100)x(0.010)y Exp 3: Rate = 5.40x10-7 = k(0.200)x(0.010)y Divide the two rate laws 0.50 = 0.50x Use log rules to solve for the order x = 1 so the order for NH4+ is one

Example Follow the same technique for other reactants NO2-: Exp 1 & 2 Exp 1: Rate = 1.35x10-7 = k(0.100)1(0.0050)y Exp 2: Rate = 2.70x10-7 = k(0.100)1(0.010)y 0.5 = 0.5y y = 1 So Rate = k[NH4+]1[NO2-]1 Overall Reaction Order – sum of orders of reactants

Finding k We can find k using values from any of the experiments given Units will be different for k depending on order of reactants

Example BrO3- : Exp 1 & 2 Exp 1: Rate = 8.0x10-4 = k(0.10)x(0.10)y(0.10)z Exp 2: Rate = 1.6x10-3 = k(0.20)x(0.10)y(0.10)z 0.50 = 0.50x x = 1

Example Br- : Exp 2 & 3 Exp 2: Rate = 1.6x10-3 = k(0.20)1(0.10)y(0.10)z Exp 3: Rate = 3.2x10-3 = k(0.20)1(0.20)y(0.10)z 0.50 = 0.50y y = 1

Example H+ : Exp 1 & 4 Exp 1: Rate = 8.0x10-4 = k(0.10)1(0.10)1(0.10)z 0.25 = 0.50z OR ¼ = (½)z z = 2

Example So Rate = k[BrO3-]1[Br-]1[H+]2 Solve for rate constant, k Overall order of reaction = 4 Solve for rate constant, k