Rate Law Main Concept: The rate law shows how the rate depends on reactant concentrations.

Rate Law Rate Law Defined Reaction Orders How to Determine Graphical Representations

- rate law: expresses rate of reaction as proportional to concentration of each reactant raised to a power - power of each reactant in rate law is order of reaction with respect to that reactant - sum of powers of reactant concentrations in rate law is overall order of reaction

- When rate is independent of concentration for a reactant, reaction is zeroth order for that reactant, since raising reactant concentration to power zero is equal to the reactant concentration being absent from rate law

- In cases where concentration of any other reactants remain essentially constant during reaction, the order of a reaction with respect to a reactant concentration can be inferred from plots of concentration of reactant versus time

Negatively sloped straight line with [A] Zeroth Order Reaction Negatively sloped straight line with [A]

Negatively sloped straight line with ln [A] First Order Reaction Negatively sloped straight line with ln [A]

Positively sloped straight line with 1/[A] Second Order Reaction Positively sloped straight line with 1/[A]

Zeroth Order   First Order Second Order 

Question: Looking at the “Kinetics” section of the reference pages available to you, what order are these two equations that are given to you? What might they allow you to do?

[A]t – [A]0 = – kt 0th order 1st order 2nd order These are presented on your reference page in this order, so… [A]t – [A]0 = – kt 0th order 1st order 2nd order

- method of initial rates is useful for developing conceptual understanding of what a rate law represents - Investigation of data for initial rates enables prediction of how concentration will vary as reaction progresses

Rate Constant Main Concept: The magnitude and temperature dependence of the rate of reaction is contained quantitatively in the rate constant.

- proportionality constant in rate law = rate constant, k - rate constant is a measurable quantity that characterizes a chemical reaction - Rate constants vary over many orders of magnitude because reaction rates vary widely

- temperature dependence of reaction rates is contained in temperature dependence of rate constant - For first-order reactions, half-life is often used as a representation for rate constant because they are inversely proportional, and half-life is independent of concentration. For example, radioactive decay processes

Zero Order First Order Second Order *** Half-life equation doesn’t work for zero or second order reactions Zero Order First Order Second Order