FACTORS AFFECTING RATE OF CHEMICAL REACTIONS

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FACTORS AFFECTING RATE OF CHEMICAL REACTIONS

We all know that chemical reaction involves the linear transformation of all the specie in the reactant or left hand side to the product (right hand side). For chemical reaction to take place effectively, certain conditions must be in order. A certain reaction might occur or proceeds faster than the other, this is due to certain factors. This are the factors i will be highlighting in this discussion.

The rate of a chemical reaction is
affected by several factors like:
Rate of a reaction is directly
proportional to the concentration of reactants. 
i.e. rate (r) ∝ cn 
where 
c = concentration 
n = order of the reaction 
or r = kcn 
where 
k = specific rate 

The
number of collisions and hence the activated collisions between the reactant
molecules increase with increase in concentration. Therefore, according to the
collision theory, the rate of a reaction should increase with increase in the
concentration since the rate is directly proportional to the collision
frequency.
i.e., rate of reaction ∝ collision frequency (Z) 
The rate of a reaction decreases
exponentially with time as the concentration of reactants is decreasing. This
can be shown graphically as follows:
The partial pressure is another way
of expressing the concentration for gases. The number of collisions increases
with increase in the partial pressures of gases. Hence the rate of reactions
involving gaseous reactants increases with increase in partial pressures.
However it has no effect on reactions involving reactants in liquid or solid
phases.
It is important to keep in mind that
the partial pressures of reactants can be increased by increasing the pressure
of overall system. However the partial pressures do not increase when an inert
gas or a non reacting gas is added to the reaction mixture at constant volume.
The average kinetic energy increases
with increase in absolute temperature. Hence the number of molecules with
energy greater than the threshold energy also increases (see the Maxwell
distribution curves shown below). As a result, the number of effective
collisions between reactant molecules also increases. Therefore, usually it is
observed that the rate of reaction increases with increase in
temperature. 
However note that increase in
temperature also increases the number of collisions and hence the number of
effective collisions are also expected to increase. But this is a minor factor
affecting the rate.
Justification: It is known that the collision frequency, Z is proportional
to the square root of absolute temperature, √T of the gas. 
i.e., For example, if the
temperature is increased by 10 oC from 293 K to 303 K, the collision
frequency can be increased by a factor of only 1.7% . But usually the rate of a
reaction is doubled (i.e., increased by 100%) when the temperature is increased
by 10 oC. Hence the effect of collision frequency is minor on the
rate of reaction. 
The major factor is increase in the
fraction of molecules which can cross the energy barrier at higher temperature.
Temperature
Coefficient:
The ratio of rate constants of a reaction at two different
temperatures which differ by 10 oC is called temperature
coefficient.
The relation between rate constant
and temperature can be shown by Arrhenius equation:
Where 
k = specific rate constant 
A = Frequency factor 
Ea = Activation
energy 
R = Gas constant 
T = Absolute Temperature 
Upon multiplying by ‘ln’ (natural
logarithm) on both the sides,
Therefore when a graph is plotted
for log k against 1/T, a straight line with negative slope is obtained.
The relation between two rate
constants (k1& k2) at two different temperatures (T1
& T2) can be given as:
However it is not always true that
the rate of a reaction increases with increase in temperature. Certain
reactions like biological reactions which are catalyzed by enzymes may be
slowed down with increase in temperature since the enzymes may lose their
activity (see below for more explanation about a catalyst).
 Catalyst is a
substance which alters the rate of a reaction without being consumed or without
undergoing any chemical change during the reaction.
 A catalyst increases the rate
of reaction by providing a new path with lower activation energy (Ea’)
for the reaction. 
In case of reversible reactions, the
catalyst lowers the activation energies of both forward and backward reactions
to the same extent and helps in attaining the equilibrium quickly. Note
that, as it is evident from the above diagram, when a catalyst increases the
rate of forward reaction, it also increases the rate of backward reaction.  
Some substances may decrease the
rate of a reaction. These are generally referred to as negative catalysts or
inhibitors. They interfere with the reaction by forming relatively stable
complexes, which require more energy to breakup. Thus the speed of the reaction
is reduced.
Rate of a reaction depends on the
nature of bonding in the reactants. Usually the ionic compounds react faster
than covalent compounds. 
The reactions between ionic
compounds in water occur very fast as they involve only exchange of ions, which
were already separated in aqueous solutions during their dissolution. 
E.g. AgCl is precipitated out
immediately when AgNO3 solution is added to NaCl solution. 
This reaction involves only the
exchange of ions as shown below and hence occurs very fast. 
Whereas, the reactions between
covalent compounds take place slowly because they require energy for the
cleavage of existing bonds. 
E.g., The esterification of acetic
acid occurs slowly since the breaking of bonds requires energy.
The reaction between the reactants
occurs only when they collide in correct orientation in space. Greater the
probability of collisions between the reactants with proper orientation,
greater is the rate of reaction.
The orientation of molecules affect
the probability factor, p. The simple molecules have more ways of proper
orientations to collide. Hence their probability factor is higher than that of
complex molecules. 
The orientation factor also affect
the interaction between reactants and catalysts. For example in case of
biological reactions, which are catalyzed by enzymes, the biocatalysts. The
enzymes activate the reactant molecules (or substrates) at a particular site on
them. These sites are called as active sites and have definite shape and
size. 
The size, stereochemistry and
orientation of substrates must be such that they can fit into the active site
of the enzyme. Then only the reaction will proceed. This is also known as lock
and key mechanism. 
The enzymes lose their activity upon
heating or changing the pH or adding certain chemical reagents. This is due to
deformation of the configuration of active site.
The rate of a reaction increases
with increase in the surface area of solid reactant, if any used. The surface
of a solid can be increased by grinding it to a fine powder. 
E.g. The reaction between zinc and
hydrochloric acid occurs within seconds if the zinc metal is finely powdered.
But the reaction will be slower when a zinc wire is used. 
This is also true with the solid
catalysts, which are usually employed in finely powdered form, while carrying
out a chemical reaction. 
E.g. Finely powdered nickel is used
during the hydrogenation of oils.
The rate of some photochemical
reactions, which occur in presence of light, increases with increase in the
intensity of suitable light used. With increase in the intensity, the number of
photons in light also increases. Hence more number of reactant molecules get
energy by absorbing more number of photons and undergo chemical change. 
E.g. The rate of photosynthesis is
more on brighter days. 
However, some photochemical
reactions involving the free radicals, generated in a chain process, are not
greatly affected by the intensity of the light. Just one photon is sufficient
to trigger the formation a free radical. This in turn initiate a chain process
in which more free radicals are formed repeatedly in each cycle without the
need of extra photons.
The solvent may affect the rate in
many ways as explained below: 
The solvents are used to dissolve
the reactants and while doing so they help in providing more interactive
surface between reactant molecules which may be otherwise in different phases
or strongly bonded in solid phase. 
Usually solvents help in breaking
the cohesive forces between ions or molecules in the solid state. The polar
molecules tend to dissolve more in polar solvents with more dielectric
constants and react faster in them. Whereas non polar molecules prefer non
polar solvents. 
In case of diffusion controlled
reactions, the viscosity of the solvent plays major role. The rate decreases
with increase in the viscosity of the solvent.
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