© University of South Carolina Board of Trustees Chapt. 13 Sec. 6 Reaction Mechanisms.

© University of South Carolina Board of Trustees

Chapt. 13

Sec. 6Reaction Mechanisms


Reaction Mechanism

Most overall chemical reactions take place through a series of elementary steps. This series of steps constitutes the mechanism of the reaction.


e-

H2OO2 + H+

[RuII-OH2]2+e- + H+

[RuIII-OH]2+

[RuIV=O]2+

[RuV=O]3+

[RuIII-OOH]2+

[RuIV-OO]2+

[RuV-OO]3+

e- + H+

e-H+

H2O

e- + H+

O2

H2O

2H2O H2O2 + H2

H2O2 H2 + O2


DefinitionsElementary step: A reaction occurring between

molecules in a single encounter.

Overall reaction: The stoichiometric relationship between initial reactants and final products.

Intermediate: Created and destroyed during the reaction. Does not appear in the overall reaction.

Mechanism: A series of elementary steps that lead to an overall reaction.


Overall Rate Laws

The coefficients of an overall reaction do not determine its rate law.

The overall rate law can be determined from the mechanism and the elementary steps.

A + B C Rate = k[A][B]


Elementary Rate Laws

The molecularity of an elementary step does determine its rate law.

● Unimolecular – one reactant molecule– 1st order rate law

● Bimolecular– two reactant molecules– 1st order in each reactant - 2nd order overall

A + B C Rate = k[A][B]

A CRate = k[A]


Student Example

The mechanism of a reaction is

2NO N2O2

N2O2 + O2 2NO2

a) What is the overall reaction?

b) Identify an intermediate in the reaction.

c) What is the rate law for each step in the mechanism?


Mechanism Overall Rate Law I

Rate-Limiting Step: The slowest step in a mechanism.

● The rates of steps after the rate-limiting step are unimportant.

The rate of the overall reaction is determined by the rate of the rate-limiting step.


Determining Rate Limiting Step

For the reaction2NO2 + O3 N2O5 + O2

the experimentally determined rate law is rate = k[NO2][O3].

Identify the rate limiting step in the proposed two-step mechanism:

NO2 + O3 NO3 + O2 step 1NO3 + NO2 N2O5 step 2


Complex Reaction Mechanisms

2NO N2O2

N2O2 + O2 2NO2

If the first step is rapid and the second step is the rate limiting step, the rate law is: rate = k2[N2O2][O2]

With the second step being rate limiting, the first step is able to reach equilibrium

fast

slow



2NO N2O2

N2O2 + O2 2NO2

If the first step reaches equilibriumrate1 (forward) = rate-1 (reverse):

rate1 = k1[NO]2

rate-1 = k-1[N2O2]Because the rate at equilibrium are equal:rate1 = rate-1

k1[NO]2 = k-1[N2O2][N2O2] = (k1/k-1)[NO]2



Substituting into the expression for step 2:rate = k2[N2O2][O2]

rate = k2(k1/k-1)[NO]2[O2]

Combine all the rate constants:rate = k [NO]2[O2]

The reaction is second order in NO and first order in O2.


Chapt. 18Electrochemistry

Sec. 1Assigning Oxidation States


Definitions I

2Na(s) + Cl2(g) 2Na+ + 2Cl−

Oxidized: Loses electrons

● Na is oxidized.

● Cl2 is the oxidizing agent.

Reduced: Gains electrons

● Cl2 is reduced.

● Na is the reducing agent.


Definitions II

Redox reaction: A reaction involving the transfer of electrons (reduction + oxidation).

Oxidation state: Net charge on an atom.

● Used to monitor gain/loss of electrons


Calculating Oxidation State

Oxidation state = Net charge on an atom

● Electrons shared between identical atoms are split evenly

● Electrons shared between different atoms are all assigned to the more electronegative atom


Student Example

Draw the Lewis dot structure of NH3 and use it to find the oxidation state of each atom.


Oxidation-State Short Cuts

● The oxidation state of an element is zero

● The sum of atomic oxidation states

= total charge on an ion or molecule

● The oxidation state of F is always -1 (except in F2)

● The oxidation state of O is always -2 (except with F or O)

● The oxidation state of H is always +1 (except with metals or in H2)


Example: Using Short Cuts

Find the oxidation state of each atom in K2CrO4


Student Example

Assign oxidation states to each atom in

a) MnO42-

b) NH4Cl

© University of South Carolina Board of Trustees Chapt. 13 Sec. 6 Reaction Mechanisms.

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