Rate Law Main Concept The rate law shows

  • Slides: 16
Download presentation
Rate Law Main Concept: The rate law shows how the rate depends on reactant

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

Rate Law Defined Reaction Orders How to Determine Graphical Representations

Rate Law Defined Reaction Orders How to Determine Graphical Representations

- rate law: expresses rate of reaction as proportional to concentration of each reactant

- 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

- 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,

- 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

Zeroth Order Reaction Negatively sloped straight line with [A]

Zeroth Order Reaction Negatively sloped straight line with [A]

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

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

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

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

Zeroth Order First Order Second Order

Zeroth Order First Order Second Order

Question: Looking at the “Kinetics” section of the reference pages available to you, what

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?

These are presented on your reference page in this order, so… [A]t – [A]0

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

- method of initial rates is useful for developing conceptual understanding of what a

- 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

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

- 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

- 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

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

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