# Lecture22222

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Published on August 12, 2008

Source: slideshare.net

## Description

a supplemental resource for students

Kinetics: the Rate Law and Effect of Concentration Lecture 22

The rate law for a chemical reaction is an equation which links the reaction rate with concentrations or pressures of reactants and constant parameters (normally rate coefficients and partial reaction orders).

For aA+bB+…  cC+dD+… Rate = k[A] m [B] n …

Rate = k[A] m [B] n … Herein k is the rate constant. It depends on temperature. m and n are reaction orders. They depend on the reaction mechanism.

Herein k is the rate constant. It depends on temperature.

m and n are reaction orders. They depend on the reaction mechanism.

Rate = k[A] m [B] n … If the rate doubles when [A] doubles, m=1. If the rate quadruples when [A] doubles, m=2. If the rate does not change when [A] doubles, m=0.

If the rate doubles when [A] doubles, m=1.

If the rate quadruples when [A] doubles, m=2.

If the rate does not change when [A] doubles, m=0.

Coefficients a and b in aA+bB+…  cC+dD+… may or may not be related in any way to the reaction orders m and n : Rate = k[A] m [B] n …

The reaction mechanism must conform to to the rate law. The rate law is based on experimental fact.

Components of the rate law must be found experimentally: Measure concentrations to find the initial rate. Use initial rates from several experiments to find the reaction orders. Use reaction orders to find the rate constant.

Measure concentrations to find the initial rate.

Use initial rates from several experiments to find the reaction orders.

Use reaction orders to find the rate constant.

How to define the reaction orders: Rate=k[A] . First order overall. Rate=k[A] 2 . Second order overall. Rate=k[A] 0 =k(1)=k . Zero order overall.

Rate=k[A] . First order overall.

Rate=k[A] 2 . Second order overall.

Rate=k[A] 0 =k(1)=k . Zero order overall.

How to define the reaction orders: NO (g) +O 3(g)  NO 2(g) +O 2(g) Rate=k[NO][O 3 ] First order with respect to NO. First order with respect to O 3 . Second order overall.

NO (g) +O 3(g)  NO 2(g) +O 2(g)

Rate=k[NO][O 3 ]

First order with respect to NO.

First order with respect to O 3 .

Second order overall.

How to define the reaction orders: 2NO (g) +2H 2(g)  N 2(g) +2H 2 O (g) Rate=k[NO] 2 [H 2 ] Second order with respect to NO. First order with respect to H 2 . Third order overall.

2NO (g) +2H 2(g)  N 2(g) +2H 2 O (g)

Rate=k[NO] 2 [H 2 ]

Second order with respect to NO.

First order with respect to H 2 .

Third order overall.

Reaction orders cannot be deduced from the balanced equation.

Special cases: If a reaction order is fractional, the rate depends on the square (cubic) root of the concentration: rate=k[A][B] 1/2 If a reaction order is negative, the rate decreases when the concentration of that component increases.

If a reaction order is fractional, the rate depends on the square (cubic) root of the concentration: rate=k[A][B] 1/2

If a reaction order is negative, the rate decreases when the concentration of that component increases.

A sample problem on determining rate order from rate laws.

It we do not know the reaction law, we have to find it from a series of experiments, starting each one with a different set of reactant concentrations and obtaining an initial rate in each case.

A sample problem on determining reaction order from initial rate data.

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