Rate Law Learning Goals: I will be able to write a rate law equation using a table of data and calculate rate constants .I will become familiar with reaction mechanisms and how they relate to the rate law Rate Law

Rate Law Equation Equation used to connect the rate of reaction to the concentration([ ]) of its reactants (at a give T and P). Rate Law Equation Equation that shows the relationship between rate (r), rate constant (k), initial concentrations of reactants ([X] and [Y]), and the orders of reaction (m and n)

Rate Law Equation Rate Law Equation: For the reaction: 2X + 2Y  2XY Order of Reaction -describes the exponents in the rate law equation -related to the particular reactant it is associated with -not related to the coefficients in the reaction equation Rate Constant -unique for every rxn -determined experimentally -varies with T so needs to be kept constant in each trial Total Order of Reaction -sum of the individual orders of reaction for each reactant

Reactant Order For the reaction equation: 2X + 2B + 3C  products [X]1 If the initial [X] is doubled, rate will: If the initial [X] is tripled, rate will: [Y]2 If the initial [Y] is doubled, the rate will: If the initial [Y] is tripled, rate will: [Z]0 If the initial [Z] is doubled, rate will: If the initial [Z] is tripled, rate will: double triple quadruple 3^2 = 9 No effect No effect

Reactant Order 3  1+2 Eg. What is the overall order of reaction? NO2 is the first order reactant (exponent 1) O2 is the second order reactant (exponent 2) What is the overall order of reaction? 3  1+2

Initial Rate Instantaneous rate determined just after the reaction begins (just after t=0) Used to find the rate law equation for a reaction Use different concentrations of reactants to find the effect on the initial rate

Find the rate law equation for the reaction above Example: Initial rates from three investigations for the reaction at 800 degrees Celsius represented by: 2NO(g) + 2H2(g)  N2 (g) + 2H2O (g) Find the rate law equation for the reaction above Run Initial [NO(g)] mol/L Initial [H2(g)] mol/L Initial rate mol/L.s 1 0.400 0.100 1.10 x 10-5 2 0.200 2.20 x 10-5 3 0.800 8.80 x 10-5

Units of k The order of a reaction can also be determine from the units of the rate constant. Rate is always measured in mol/(L.s). Therefore, a first order reaction has the unit of s-1 because: rate= k[A]  mol = k x mol L.s L Total Reaction Order Units of k mol/(L.s) 1 1/s or s.1 2 L/(mol.s) 3 L2/mol2.s)

Example 2: The initial rate for the following reaction is 3.2x10 mol/(L.s) BrO3- + 5Br- + 6H+  3Br2 + H20 The initial concentration of bromate is 0.10mol/L, hydrogen ion is 0.20mol/L, bromide is 0.10mol/L, and hydrogen ion is 0.20mol/L. It is first order with respect to bromate, first order with respect to bromide, and second order with respect to hydrogen ion. Find the rate constant, k, and the rate law expression.

Example 3: For the reaction H2O2 + 3I- + 2H+ I3- + 2H2O and the data shown below, determine the rate law equation.