Rate Law

1. 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

2. 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)

3. Rate Law Equation • For the reaction: 2X + 2Y  2XY Rate Law Equation: 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

4. 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

5. Reactant Order • Eg. • 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

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

7. 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

8. 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 = kx mol L.s L

9. 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.

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