Chemical Equilibrium

In Chemistry, we define chemical equilibrium as a state in which the rate of the forward reaction is equal to the rate of the backward reaction. In other words, we can say it 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. After that, the system will not display any further change in their properties and it becomes constant. Here we will study what is chemical equilibrium, what is equilibrium in Chemistry, and various factors affecting chemical equilibrium.

Equilibrium Meaning in Chemistry

Chemical equilibrium definition refers to the state of a system where the concentration of the reactant and the concentration of the products do not change with respect to time and the system does not display any further change in properties.

Chemical equilibrium is said to be achieved by the system when the rate of the forward reaction is equal to the rate of the reverse reaction. 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, at the time point of time the system is said to be in a dynamic state of equilibrium.

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The graph with the concentration on the y-axis and time on the x-axis is plotted and it is shown in the above diagram. Once the concentration of both the reactants and as well as the products stops showing any change, in that state chemical equilibrium is said to be achieved.

Define Equilibrium 

Let’s understand this with an example. Consider hydrogen and iodine gas. These gases react to form hydrogen iodide. Here the reaction is given below:

H2(g)  +  I2(g)  ⇌  2HI(g) 

Reaction A: Forward reaction H2(g)  +  I2(g) → 2HI(g) 

Reaction B: Reverse reaction 2HI(g) → H2(g)  +  I2(g) 

Initially, only the forward reaction occurs because HI was not present. As soon as some HI is formed, it begins to decompose back into H2 and I2. After that, the rate of the forward reaction decreases while the rate of the reverse reaction keeps on increasing. In the long run, the rate of the combination of  H2 and I2 to produce HI becomes equal to the rate of decomposition of HI into H2 and I2. When the rates of the forward and rate of reverse reactions become equal to one another, then the reaction has achieved its state of balance. 

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The above diagram shows the equilibrium in reaction.

Chemical equilibrium can be attained in any of the cases 

1. when the reaction begins with all reactants and also no products

2. when all products and no reactants

3. some of both are available.

The figure below shows changes in the concentration of H2, I2, and HI for two different reactions. In the reaction represented by the graph on the left side of  (A), the reaction begins with only H2 and I2 present. There is no HI initially. As the reaction proceeds towards equilibrium, the concentrations of the  H2 and I2 gradually start decreasing, while the concentration of the HI gradually starts to increase.  When the curve levels out and the concentrations become constant, that time equilibrium state has been reached. At equilibrium, concentrations of all substances are constant.

As we can see in reaction B, the process begins with only HI and there were no  H2 and I2 present. In this situation, the concentration of HI gradually decreases while the concentrations of H2 and I2 gradually increase until an equilibrium state is reached again.

As we have noticed that in both cases, the relative position of equilibrium is the same, which is shown by the relative concentrations of reactants and products.

The concentration of HI at equilibrium is significantly higher as compared to the concentrations of  H2 and I2. This is true only when the reaction began with all reactants or all products. The position of equilibrium is a property of the particular reversible reaction and it does not depend upon how equilibrium has been achieved.

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The above diagram represents the equilibrium between reactants and products is achieved regardless of whether the reaction starts with the reactants or products.

Types of Chemical Equilibrium

There are two types of chemical equilibrium:

• Homogeneous Equilibrium

• Heterogeneous Equilibrium

Homogeneous Chemical Equilibrium

In this type of reaction, the reactants and the products of chemical equilibrium are all in the same phase

It is also divided into two types: (i) Reactions having 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) 

(ii) Reactions having 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 of reaction, the reactants, as well as the products of chemical equilibrium, are present in different phases. Examples are given below:

CO2(g)  +  C(s)  ⇌  2CO(g) 

CaCO3(s)  ⇌  CaO(s)  +  CO2(g) 

Conditions for Equilibrium

Chemical equilibrium is a dynamic process. The forward and reverse reactions continue to occur even after the equilibrium state has been achieved. However, the rates of the reactions are the same here, and there is no change in the relative concentrations of reactants and products for a reaction that is at equilibrium. Following are the conditions and properties of a system at equilibrium.

• The system must be closed, which means no substances can enter or leave the system.

• Equilibrium is a dynamic process. Even though we don't necessarily see the reactions, both forward and reverse reactions are taking place.

• The forward and reverse rates of reactions must be equal.

• The number of reactants and products need not be equal. However, after equilibrium is achieved, the amounts of reactants and products will always be constant.

Factors Affecting Chemical Equilibrium

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According to Le-Chatelier’s principle, if there is any change in the factors that affect the equilibrium conditions, then the system will counteract or reduce the effect of the overall transformation. This principle is applied to both chemical and physical equilibrium.

There are several factors that affect equilibrium conditions like temperature, pressure, and concentration of the system that affect equilibrium.

• Change in Pressure

Change in pressure occurs due to the change in the volume. If there is a change in pressure it can also affect the gaseous reaction because the total number of gaseous reactants and products are now different. In the heterogeneous chemical equilibrium, according to the principle of Le Chatelier, if there is a change of pressure in both liquids and solids it can be ignored because the volume is independent of the pressure.

• Change in Temperature

The effect of temperature on chemical equilibrium depends upon the sign of ΔH of the reaction and it also follows Le-Chatelier’s Principle.

When the temperature increases, the equilibrium constant of an exothermic reaction decreases.

In the case of an endothermic reaction the equilibrium constant increases with an increase in temperature.

Together with the equilibrium constant, the rate of reaction is also affected by the change in temperature. In the case of exothermic reactions according to Le Chatelier’s principle, the equilibrium shifts towards the reactant side when there is increased temperature. In the case of 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. The same amount of reactants and products will be present at equilibrium in a catalysed in a non-catalysed reaction. The presence of a catalyst only facilitates when the reaction proceeds 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 in an undisturbed state. If the gas added is a reactant or product that is involved in the reaction then the reaction quotient will change.

Examples of Chemical Equilibrium

In chemical reactions, the reactants are converted into products by the forward reaction and the products can be converted into the reactants by the backward reaction. They are two states, reactants, and products both are present in different compositions.

After some time when the reaction starts, 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 so that the concentration of reactants and products do not change anymore. Hence, the reactants and products will be in chemical equilibrium.

Importance of Chemical Equilibrium

It is useful in many industrial processes like,

• It is used in the preparation of ammonia with the help of Haber’s process. Here nitrogen gas combines with hydrogen gas to form ammonia. The yield of ammonia happens more at low temperature, high pressure, and in the presence of iron as a catalyst.

• It is used in the preparation of sulphuric acid by contact process: In this process, the fundamental reaction is the oxidation of sulphur dioxide into sulphur trioxide. This involves chemical equilibrium.

Conclusion

Now we have understood what equilibrium is in Chemistry. We have discussed some very important concepts and principles related to the studies of reaction rates and chemical equilibria. The equilibrium is a state in Chemistry in which there is no net change in the concentrations of reactants and as well as the products. As we know there is no apparent change at equilibrium, this does not mean that all chemical reactions have ceased.