Ch 14- Chemical Kinetics -The area of chemistry concerned with the speeds, or rates, of reactions.

14.2- Reaction Rates Units of reaction rates are typically expressed as concentration per unit time or (M/s) If the concentration of reactants and products is graphed vs. time you will find that the concentration of reactant decreases over time and the concentration of product increases over time. The slope of the line represents the average rate.

14.3- Concentration and Rate The relationship between rate and concentration can be expressed by a rate law, which has the following form: Rate law: Rate = k [ A ] m [ B ] n forA + B  AB

Rate = k [ A ] m [ B ] n The constant k is called the rate constant and changes with temperature. The reaction orders, represented above as m & n, indicate how the rate is affected by each reactants concentration. The overall reaction order is determined by adding the values for m & n. **m & n will be small whole numbers like: 0, 1, or 2

The units for k will change depending on the overall reaction order. Reaction order Units for kUnits for Rate 0 1 2

***the value of k helps to indicate the speed of the reaction. large k (~ 10 9 or higher) is a fast reaction small k (10 1 or lower) is a slow reaction

The reaction orders and rate constant can be determined using experimental data for a particular reaction. The reaction is carried out multiple times and each time the concentration of the reactants changes. The rate of the reaction is measured in the experiment and the data is gathered. From there the rate law can be determined using the following method of initial rates.

Example Problem and Explanation: The following data were measured for the reaction of nitric oxide with hydrogen: 2 NO (g) + 2 H 2 (g)  N 2 (g) + 2 H 2 O (g) a) Determine the reaction order with respect to NO. b) Determine the reaction order with respect to H 2. c) Write the rate law for this reaction. d) Calculate the overall reaction order for this reaction. e) Calculate the rate constant. f) Calculate the rate when [NO]= M and [H 2 ]= M Experiment # [NO] (M) [H 2 ] (M) Initial rate (M/s) x x x10 -3

Description for part a Compare 2 experiments where [NO] changes and [H 2 ] stays the same. Plug the values from each experiment into the rate law. Cancel out units and values that are the same. Solve for m. Since m= 2 the reaction is 2 nd order with respect to NO

Description for part b Compare 2 experiments where [H 2 ] changes and [NO] stays the same. Plug the values from each experiment into the rate law. Cancel out units and values that are the same. Solve for n. Since n= 1 the reaction is 1 st order with respect to H 2

Plug m and n into the rate law and rewrite.

Rearrange and solve for k. Pick any experiment from the data table and plug the values into the rate law. Be careful when figuring out your units!

C) Plug in given values and rate constant (k) and solve for rate. Units should always cancel to get M/s for rate.