13.Third order reactions
A number of reactions are found to have third order kinetics. An example is the oxidation of NO,
for which the overall reaction equation and rate law are given below.
2NO + O2 →2NO2
d[NO2] / dt = k [NO]2 [O2]
One possibility for the mechanism of this reaction would be a three-body collision (i.e. a true
termolecular reaction).
14.Enzyme reactions – the Michaelis-Menten mechanism
An enzyme is a protein that catalyses a chemical reaction by lowering the activation energy.
Enzymes generally work by having an active sitethat is carefully designed by nature to bind a
particular reactant molecule (known as the substrate). An example of a substrate bound at the
active site of an enzyme is shown on the left.
The activation energy of the reaction for the enzyme-bound substrate is lower than for the
free substrate molecule, often due to the fact that the interactions involved in binding shift the
substrate geometry closer to that of the transition state for the reaction. Once reaction has occurred, the product molecules arereleased from the enzyme.
15.Chain reactions
Chain reactions are complex reactions that involve chain carriers, reactive intermediates which
react to produce further reactive intermediates. The elementary steps in a chain reaction may be
classified into initiation, propagation, inhibition, and termination steps.For more details
16.Explosions and branched chain reactions
An explosion occurs when a reaction rate accelerates out of control. As the reaction speeds up,
gaseous products are formed in larger and larger amounts, and more and more heat is generated.
The rapid liberation of heat causes the gases to expand, generating extremely high pressures, and
it is this sudden formation of a huge volume of expanded gas that constitutes the explosion. The
pressure wave travels at very high speeds, often much faster than the speed of sound, and the
‘bang’ associated with an explosion is the result of a supersonic shock wave.
There are two different mechanisms that may lead to an explosion. These are related to the fact
that the overall reaction rate depends on both the magnitude of the rate constant and the amounts
of reactants present in the reaction mixture.
If the heat generated in a reaction due to the reaction exothermicity cannot be dissipated
sufficiently rapidly, the temperature of the reaction mixture increases. This increases the rate
constant, and therefore the reaction rate, producing more heat and accelerating the reaction rate
still further, and so on until an explosion results. Such explosions are known as thermal
explosions,and in principle may occur whenever the rate of heat production by a reaction mixture
exceeds the rate of heat loss to the surroundings (often the walls of the reaction vessel).
The second category of explosions arise from chain branching within a chain reaction, and are
known as chain branching explosions(or sometimes, somewhat misleadingly, isothermal
explosions). In this case, one or more steps in the reaction mechanism produce two or more chain
carriers from one chain carrier, increasing the number of chain carriers, and therefore the overall
reaction rate.
In practice, both mechanisms often occur simultaneously, since any acceleration in the rate of an
exothermic reaction will eventually lead to an increase in temperature. However, chain branching
is not a requirement for an explosion. As an example, detonation of TNT (2,4,6-trinitrotoluene) is
simply the result of an extremely fast chemical decomposition that generates huge quantities of
gas. The reaction 2H2(g) + O2(g) →2H2O(g)provides an example of a reaction in which both
mechanisms are important.
17.Temperature dependence of reaction rates
It is found experimentally that the rate constants for many chemical reactions follow the Arrhenius
equation..
18.Simple collision theory
As the name suggests, simple collision theory represents one of the most basic attempts to
develop a theory capable of predicting the rate constant for an elementary bimolecular reaction of
the form A + B →P. We begin by considering the factors we might expect a reaction rate to
depend upon. Obviously, the rate of reaction must depend upon the rate of collisions between the
reactants. However, not every collision leads to reaction. Some colliding pairs do not have
enough energy to overcome the activation barrier, and any theory of reaction rates must take this
energy requirement into account. Also, it is highlylikely that reaction will not even take place on
every collision for which the energy requirement is met, since the reactants may need to collide in
a particular orientation (e.g. SN2 reactions) or some of the energy may need to be present in a
particular form (e.g. vibration in a bond coupled to the reaction coordinate).
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