coming towards end part.
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]
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.
It is found experimentally that the rate constants for many chemical reactions follow the Arrhenius equation..
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.
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 site that 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 are released from the enzyme.
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 are released 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 highly likely 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).
particular form (e.g. vibration in a bond coupled to the reaction coordinate).
