Reaction Rate Laws Podcast 16.3

16.1-16.2 Review Factors affecting reaction rates include: 1._____________ 2._____________ 3._____________

What’s a Rate Law? A rate law is an expression that relates how the _______ of reaction changes as _____________________ changes.

Consider the decomposition of hydrogen peroxide: 2 H2O2  2 H2O + O2 The reaction is _______________ The rate law is: rate = k [H2O2]1 k is called the _________________ The larger the value for k, the ________ the reaction. (k is ______________dependent) [H2O2] Initial Rate of Decomp

What would a second order reaction look like? Consider A B The reaction is _______________ The rate law is: rate = k [A]2 [A] Initial Rate of Decomp

How do we determine the rate law? You cannot figure out the rate law from the balanced equation. You must look at __________________ -Alternatively, you can look at graphs or mechanism, but that will not be covered here. . .

Method of Initial Rates Allows us to determine reaction order by comparing initial rates of a reaction carried out with varying reactant concentrations. For example: if doubling [X] doubles the rate, the rate law must be first order with respect to X. If doubling [X] quadruples the rate, the rate law must be second order with respect to X.

Example Given the following information, determine the overall rate law for the reaction: A + B  C Experiment [A]0 (M) [B]0 (M) Initial Rate (M/s) 1 1.0 0.0050 2 2.0 0.010 3 0.040

A + B  C When [A] doubles, and [B] is constant, rate ____________ Experiment [A]0 (M) [B]0 (M) Initial Rate (M/s) 1 1.0 0.0050 2 2.0 0.010 3 0.040 When [A] doubles, and [B] is constant, rate ____________ When [B] doubles, and [A] is constant, rate ____________ The experiment is _____ order with respect to A, and _____ order with respect to B. The rate law is: _______________________ Do not compare experiments when both concentrations change, as they do in experiment 1 and 3.

Overall Order The overall order is the sum of the exponents in the rate law. Rate = k[A]1[B]2 represents a _____ order reaction

A + B  C Experiment [A]0 (M) [B]0 (M) Initial Rate (M/s) 1 1.0 0.0050 2 2.0 0.010 3 0.040 We can also determine the rate constant, k, from this data, once the rate law is known. Choose any experiment and substitute values.

Practice Problem What is the rate law and the value of rate constant for the reaction: 2 AB2 + C  CB + BA based on the data below? Experiment [AB2]0 (M) [C]0 (M) Initial Rate (M/s) 1 1.0 1.4 x 10-7 2 0.5 3.5 x 10-8 3 2.0 5.5 x 10-7

Solution

One last note: A reaction can also be ________ order. This means that the rate does not depend on the concentration of that reactant. Mathematically: x0 = ________ For a reaction: A + B  C that is zero order in A, and 1st order for B, the rate law would be:

Chapter 16 -Summary Topics Collision Theory Average Rate Activation Energy Catalysis Rate Law Rate Constant, k Reaction Order Initial Rates Method Not covered - reaction mechanisms/16.4