Announcements Handouts on back tables Exams handed back in lab this week. Very Useful Video: How to analyze dye kinetics lab data (link on course web site)

A rate law must be determined by experiment Method 1: Initial Rates Vary one thing at a time, keep temperature constant If concentration doubles and: Rate does not change Rate doubles Rate quadruples General Rule:

Simulation: Concentration dependence of reaction rates Starting Concentration Initial Rate Order of Reaction A→ B C→ D E→ F

Example: The initial rate method with >1 reactant

What is k for 2NO2 + O2 →2NO2? (use experiment 4 data) 284 M/s 7094 M/s 1.4 x 10-4 M/s 0.004 M/s

A rate law must be determined by experiment Method 2: Using Concentration vs. Time Graphs The graph of [R] vs. time will depend upon the reaction order Data from zero, first, and second order reactions will all look different when graphed Take experimental data, graph it different ways, look for which graph looks linear

What does the concentration vs What does the concentration vs. time graph look like for a zero order reaction?

What does the concentration vs What does the concentration vs. time graph look like for a first order reaction?

What does the concentration vs What does the concentration vs. time graph look like for a second order reaction?

We can use rate laws to determine an unknown quantity We MUST know the order of the reaction Four variables: k, rate constant [R]0, initial concentration of reactant [R]t, concentration of reactant after time t elapsed time If you know the reaction order and three of these, you can find the fourth

Example: Using the rate law for a first order reaction to determine [reactant] after an amount of time

Example: Using the rate law for a second order reaction to determine the rate constant k The decomposition of nitrous oxide at 565 oC, 2 N2O  2 N2 + O2 is second order in N2O. If the reaction is initiated with [N2O] equal to 0.108 M, and drops to 0.0940 M after 1250 s have elapsed, what is the rate constant?

Flashback: There are two methods for determining and using the rate law Method 1: Initial rate Method 2: Graphical Collect [R] over an interval of times Make plots of: [R] vs. time ln[R] vs. time 1/[R] vs. time Only one will be linear. → reaction order Slope of linear plot = rate constant

The rate law equations can be rearranged in the form of an equation for a line

What does the concentration vs What does the concentration vs. time graph look like for a zero order reaction?

What does the concentration vs What does the concentration vs. time graph look like for a first order reaction?

What does the concentration vs What does the concentration vs. time graph look like for a second order reaction?

Using the graphical method on a real example- decomposition of H2O2 2 H2O2  2 H2O + O2 Use Excel (or other spreadsheet program) to make graphs of time vs. [H2O2] time vs. ln[H2O2] time vs. 1/[H2O2]