Yuana Elly. A., S.T., M.ScSeptember, 104

Rate laws

Rate law and Reaction orderThe order of a reaction refers to the

powers to which the concentrations are raised in the kinetic rate law.

the reaction in the above equation is α order with respect to reactant A. and β order with respect to reactant B.

The overall order of the reaction, n, isn = α + β

The units of –rA , are always in terms of concentration per unit time while the units of the specific reaction rate, kA, will vary with the order of the reaction.

Consider a reaction involving only one reactant, such asA Products

with a reaction order n. The units of the specific reaction rate constant k are

Then for

time

ionconcentratk

n

1)(

Rate equationSingle and multiple reaction :

Single reaction : A B or A + B C + DMultiple reaction :

Series reaction : A B CParallel reaction : A B

A C

Elementary and non elementary reactions :Elementary reactions : rate equation corresponds

to the stoichiometric reaction A + 2B C, -rA = k. CA. CB

2. where k : rate constantNon elementary reaction : rate equation isn’t

related to the stoichiometric reaction 2A 3B, -rA= k.C2

A/CB, where k : rate constant

Elementary rate lawsFor an elementary reaction such as

the bimoIecuIar reaction between oxygen and methanol

The reaction is first order in molecular oxygen and first order in methanol therefore. we say both the reaction and the rate law are elementary.

Types of Elementary ReactionUnimolecular

O* O + ORate = k. [O*]

BimolecularNO(g) + O3(g) NO2(g) + O2(g)Rate = k. [NO]. [O3]

TermolecularO + O2 + M O3 + M Rate = k. [O]. [O2]. [M]

Termolecular reactions are low probability reactions; require three species to come together simultaneously

2H2O2(aq) 2 H2O(l) + O2(g)

Experimental rate law: Rate = k [H2O2][I-]

(i) H2O2 + I- H2O + OI- slow

(ii) OI- + H2O2 H2O + O2 + I- fast

Rate depends on the slow, rate-determining step, (i)

Here, OI- is the intermediate species

2NO2 + F2 2 NO2F

Experimental rate expressionrate = kobs [NO2] [F2]

Possible mechanism which fits the experimental observationNO2 + F2 NO2F + F slow

NO2 + F NO2F fast

First step: rate = k1 [NO2] [F2] rate determining step

Second step: rate = k2 [NO2] [F]

The rate of the first reaction determines the rate of the overall reaction

Elementary Rate LawsThere are many reactions where the stoichiometric

coefficients in the reaction are identical to the reaction orders but the reactions are not elementary owing to such things as path-ways involving active intermediates and series reactions. For these reactions that are not elementary but whose stoichiometric coefficients are identical to the reaction orders in the rate law, we say the reaction follows an elementary rate law.

For example, the oxidation reaction of nitric oxide

2NO + O2 2NO2

The reaction is not elementary but follows the elementary rate law

Non elementary Rate Laws

A large number of both homogeneous and heterogeneous reactions do not follow simple rate laws.

Examples of Homogeneous ReactionsCO +Cl2 COCl2

This reaction is first order with respect to carbon monoxide, three-halves order with respect to chlorine. and five-halves order overall

Non elementary Rate LawsExample of Heterogeneous Reactions In many pas-solid catalyzed reactions. It

historically has been the practice to write the rate law in terms of partial pressures rather than concentrations.

C6H5CH3+ H2 C6H6 +CH4

Remember