What is Chemical Equilibrium?

Chemical equilibrium is identified as a state where the components involved in a reversible reaction namely reactants and product do not change in the concentration during a chemical process. It transpires when the velocity of the forwarding process is same as the velocity of the reverse reaction. In chemistry, the equilibrium position, equilibrium state, and equilibrium equation of a reaction helps to understand a reaction in detail.

Chemical Equilibrium in Real Life

Chemical equilibrium is the point of a revocable reaction where the speed of forward process corresponds to the reverse reaction rate. The amount of the reactants and products is stable when a process is in equilibrium. Chemical equilibrium can be observed in our day-to-day life. One of the most common examples is the cold drink bottle where carbon dioxide is present inside the bottle and you can see there is a constant change of state in carbon dioxide from the liquid to gas but still the overall liquid in the bottle appears to be same maintaining its equilibrium.         

Types of chemical equilibrium           

Chemical equilibrium can be of two types, homogeneous equilibrium or heterogeneous equilibrium.        

Homogeneous equilibrium        

Homogeneous equilibrium is a state where all the components involved in a reaction occur in the same state. A homogeneous equilibrium is observed in solutions where all the components are in a dissolved state. Whereas for gaseous equilibrium all the components are in a gaseous state.  An example is given below,

$${\text{N}}_{\text{2}}\left(\text{g}\right){\text{+3H}}_{\text{2}}\left(\text{g}\right)\rightleftharpoons {\text{2NH}}_{\text{3}}\left(\text{g}\right)$$

Heterogeneous equilibrium

Heterogenous equilibrium is a state where the components participating in a chemical process do not occur in the same state that is the reaction may have components with two or more different states. An illustration of heterogeneous equilibrium can be witnessed in the following reaction:

Chemical Equilibrium as a Dynamic Reaction 

Chemical equilibrium is considered to be a dynamic reaction. The rate of the forward direction and the rate of the reverse direction is equal. Also at an equilibrium state, the composition of the system does not vary with time. Even though the forward and reverse reaction takes place, the rate does not change and remains the same. This dynamic equilibrium can be achieved only in the case of reversible reactions.

 The dynamic character of a reversible chemical reaction can be demonstrated with the help of the following reversible reaction.    

for the above process the forward reaction can be noted down as:     

The reverse reaction can be recorded as:    

At equilibrium, the speed of forward process is equivalent to the speed of reverse process. In the beginning of the reaction process, the speed of forward reaction is high till it reaches equilibrium after which the speed of forward reaction decreases and attains a constant speed.    

But, the speed of the reverse process is slow initially which then increases till it reaches a constant rate.    

The state where both the forward and reverse reaction speeds are the same is known as equilibrium.    

But reaching equilibrium does not mean the reaction process no longer transpires in the system. If it is observed microscopically, it can be seen that at constant rates both onward and opposite continue to occur causing a state of dynamic equilibrium.     

Chemical equilibrium constant        

The equilibrium constant is defined as the fraction of the concentrations of the products to the reactants. It is expressed as Keq 

$\text{aA+ bB}\rightleftharpoons \text{ mC+ nD }$,

where $\left[\text{C}\right]$and $\left[\text{D}\right]$are the concentrations of the product whereas, $\left[\text{A}\right]$and $\left[\text{B}\right]$are the amounts of the reactants in addition to m, n, a, b, being their respective stoichiometric coefficients.

Let us derive the above equilibrium constant with the help of the following equations,   

We can now note down the rate of the above forward and reverse equations as      

As we know the equation is in equilibrium,     

$${\text{k}}_{\text{1}}{\left[\text{A}\right]}^{\text{a}}{\left[\text{B}\right]}^{\text{b }}{\text{ =k}}_{\text{2}}{\left[\text{C}\right]}^{\text{m}}{\left[\text{D}\right]}^{\text{n}}$$which can then be rearranged as $$\frac{{\text{k}}_{\text{1}}}{{\text{k}}_{\text{2}}}\text{=}\frac{{\left[\text{C}\right]}^{\text{m}}{\left[\text{D}\right]}^{\text{n}}}{{\left[\text{A}\right]}^{\text{a}}{\left[\text{B}\right]}^{\text{b }}}$$.

Le Châtelier's Principle  

In 1884, Henry-Louis Le Châtelier, a French chemist and engineer, introduced one of the key principles of chemical balance, which defined what happens to a system whenever anything temporarily prevents this from a state of balance. The condition of balance is due to temperature, pressure or concentration. Le Châtelier principle implies that changing some or all of these variables causes a reaction that partly balances the shift until a new equilibrium is established.     

Features affecting chemical equilibrium    

Several factors affect the chemical equilibrium of a process such as concentration, pressure, catalyst, and temperature   

Effect of concentration   

The Le Châtelier theory expresses that when there is an alteration in the intensity of the reacting elements or products involved in the reaction, the equilibrium mixture modifies its composition such that the effects of the concentration change are minimized.   

Influence of pressure   

The consequence of a rise in pressure or volume is maintained by producing more or less gas moles. For instance, as the pressure of a procedure rises or the volume reduces, the balance changes to support the side of the reaction that contains less gas moles.    

Influence of temperature   

The result of temperature on equilibrium directly correlates with the heat of the process. The heat of a process can generally be either exothermic or endothermic which can be expressed as follows,   

Endothermic reaction: $\text{Heat + A}\rightleftharpoons \text{B }$

Exothermic reaction: $\text{A}\rightleftharpoons \text{B+heat }$

According to the Le Châtelier principle, increasing temperature for an exothermic is like adding more moles on the reactant side which will, in turn, shift the reaction towards the reactant side, whereas, for an exothermic reaction an upsurge in the temperature will swing the reaction towards the product side.   

Effect of catalyst   

Catalyst is described as an element that accelerates the speed of the process. In a reversible process, the catalyst escalates the speed of reaction both in forward and reverse processes such that the equilibrium is attained much faster without affecting the equilibrium of a reaction.     

Importance of Chemical Equilibrium

The most significant example of chemical equilibrium is the transport of oxygen by hemoglobin. Hemoglobin carries oxygen to different tissues in the body but along with carrying oxygen it should also be able to release oxygen at desired location in our body. This is possible due to the variation in chemical equilibrium at different sections of the body.   

Common Mistakes    

• Do not get confused between reversible and irreversible reactions. Chemical equilibrium applies to reversible reactions and not irreversible reactions.    
• While writing equilibrium constant, remember product concentration will always be in the numerator and reactants in the denominator. It should not be reversed.    

Practice Problem

Write down the equilibrium constant for the given reaction at a temperature of $300\text{K}$${\text{H}}_{\text{2}}\left(\text{g}\right){\text{+Cl}}_{\text{2}}\left(\text{g}\right)\rightleftharpoons \text{2HCl}\left(\text{g}\right)$

Solution:

$\begin{array}{c}\\ \text{K }= \frac{\left[\text{H}\right]\left[\text{Cl}\right] }{{\left[\text{HCl}\right]}^2 } \\ =\frac{(5.0\times {10}^{-3})(3.5\times {10}^{-3}) }{{[3\times {10}^{-3}]}^{2 }} \\ =\text{ 0}\text{.194}\times {10}^2 \end{array}$

Context and Applications

This topic is significant in the professional exams for both undergraduate and graduate courses, especially for 

B.Sc Chemistry M.Sc Chemistry