Chemical Equilibrium:
Chemical
equilibrium refers to the state of a system in which the concentration of the
reactant and the concentration of the products do not change with time and the
system does not display any further change in properties.
When the rate of the forward reaction is equal to the rate of the reverse reaction, the state of chemical equilibrium is achieved by the system. When there is no further change in the concentrations of the reactants and the products due to the equal rates of the forward and reverse reactions, the system is said to be in a state of dynamic equilibrium.
A
graph with the concentration on the y-axis and time on the x-axis can be
plotted. Once the concentration of both the reactants and the products stops
showing change, chemical equilibrium is achieved.
Dynamic Equilibrium:
The
stage at which the rate of the forward reaction is equal to the rate of
backward reaction is called an equilibrium stage. At this point, the number of
reactant molecules converting into products and product molecules into
reactants are the same. The same equilibrium can be carried out with the same
reactants anywhere with similar conditions with continuous interchanging of
molecules hence chemical equilibrium is dynamic.
Types of Chemical Equilibrium:
There
are two types of chemical equilibrium:
· 1) Homogeneous Equilibrium
·
2) Heterogeneous Equilibrium
Homogenous Chemical Equilibrium:
In this
type, the reactants and the products of chemical equilibrium are all in the
same phase. Homogenous equilibrium can be further divided into two types:
Reactions in which the number of molecules of the products is equal to the
number of molecules of the reactants. For example,
H2 (g) + I2 (g) ⇌ 2HI (g)
N2 (g) + O2 (g) ⇌ 2NO (g)
Reactions
in which the number of molecules of the products is not equal to the total
number of reactant molecules. For example,
2SO2 (g) + O2 (g) ⇌ 2SO3 (g)
COCl2 (g) ⇌ CO (g) +
Cl2 (g)
Heterogeneous Chemical Equilibrium
In this type, the reactants and the
products of chemical equilibrium are present in different phases. A few
examples of heterogeneous equilibrium are listed below.
CO2 (g) + C (s) ⇌ 2CO (g)
CaCO3 (s) ⇌ CaO (s) +
CO2 (g)
Thus,
the different types of chemical equilibrium are based on the phase of the
reactants and products.
Factors Affecting Chemical Equilibrium:
According to Le-Chatelier’s
principle, if there is any change in the factors affecting the equilibrium
conditions, the system will counteract or reduce the effect of the overall
transformation. This principle applies to both chemical and physical equilibrium.
There
are several factors like temperature, pressure and concentration of the system
which affect equilibrium. Some important factors affecting chemical equilibrium
are discussed below.
Change in Concentration:
The concentration of the reactants or products
added is relieved by the reaction which consumes the substance which is added.
The concentration of reactants or
products removed is relieved by the reaction which is in the direction that
replenishes the substance which is removed.
When the concentration of the reactant or
product is changed, there is a change in the composition of the mixture in
chemical equilibrium.
Change in Pressure:
Change
in pressure happens due to the change in the volume. If there is a change in
pressure it can affect the gaseous reaction as the total number of gaseous
reactants and products are now different. According to Le Chatelier’s
principle, in heterogeneous chemical equilibrium, the change of pressure in
both liquids and solids can be ignored because the volume is independent of
pressure.
Change in Temperature:
The
effect of temperature on chemical equilibrium depends upon the sign of ΔH of
the reaction and follows Le-Chatelier’s Principle.
As temperature increases the
equilibrium constant of an exothermic reaction decreases.
In an endothermic reaction the
equilibrium constant increases with increase in temperature.
Along with equilibrium constant, the
rate of reaction is also affected by the change in temperature. As per Le
Chatelier’s principle, the equilibrium shifts towards the reactant side when
the temperature increases in case of exothermic reactions, for endothermic
reactions the equilibrium shifts towards the product side with an increase in
temperature.
Effect of a Catalyst:
A catalyst does not affect the chemical
equilibrium. It only speeds up a reaction. In fact, catalyst equally speeds up
the forward as well as the reverse reaction. This results in the reaction
reaching its equilibrium faster.
The same amount of reactants and
products will be present at equilibrium in a catalysed or a non-catalysed
reaction. The presence of a catalyst only facilitates the reaction to proceed
through a lower-energy transition state of reactants to products.
Effect of Addition of an Inert Gas:
When an inert gas like argon is
added to a constant volume it does not take part in the reaction so the
equilibrium remains undisturbed. If the gas added is a reactant or product
involved in the reaction then the reaction quotient will change.
Examples of Chemical Equilibrium
In chemical reactions, reactants
are converted into products by the forward reaction and the products may be
converted into the reactants by the backward reaction. The two states,
reactants and products are different in composition.
After some time of the start of
the reaction, the rate of the forward and the backward reactions may become
equal. After this, the number of reactants converted will be formed again by
the reverse reaction such that the concentration of reactants and products do
not change any more. Hence, the reactants and products are in chemical
equilibrium.
N2O4 ⇌ 2NO2
PCl5 ⇌ PCl3 +PCl2
N2 + H2 ⇌ 2NH3
Importance of Chemical
Equilibrium:
It
is useful in many industrial processes like,
Preparation of ammonia by Haber’s
process: In this nitrogen combines with hydrogen to form ammonia, the yield of
ammonia is more at low temperature, high pressure and in the presence of iron
as catalyst.
Preparation of sulphuric acid by
contacts process: In this process, the fundamental reaction is the oxidation of
sulphur dioxide into sulphur trioxide. This involves chemical equilibrium.
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