Equilibrium Notes for Class 11 : Definition, Physical, Chemical Equilibrium & Affecting Factors

Equilibrium is a state of balance in the system. Chemical equilibrium is the balance in the chemical reaction. Chemical equilibrium has a lot of applications in the manufacturing industry. Hence, it is important to study them. In this article, we will study What is Chemical Equilibrium? Its Types and Examples and Factors Affecting the Equilibrium.

Also, check out more about Hydrocarbons, here.

What is Equilibrium?

A reversible reaction can proceed in both the forward and backward directions. It means that products can be back converted into reactants.

Equilibrium is when the rate of the forward reaction equals the rate of the reverse reaction. The concentration of product and reactant is constant at equilibrium.

Most of the chemical reactions are reversible. In these reactions, the reactants are not completely converted into products and hence they do not go to completion.

Types of Equilibrium

Equilibrium is mainly of two types: Physical Equilibrium & Chemical Equilibrium

Physical Equilibrium

• The change of the substance from one place to another is a physical process. These changes are reversible.
• Equilibrium between two phases of the same substance is called physical equilibrium.

Example: Water changes into a gas and vice versa.

• The rate of evaporation is constant at any given temperature.
• However, the rate of condensation increases with the increasing concentration of vapour molecules.
• A state is reached where the rates of evaporation and the rates of condensation are equal.
• This is called physical equilibrium.
• The maximum constant pressure exerted by the vapour at equilibrium is the vapour pressure of the liquid at a given temperature.

Read more about Chemical Reactions, here.

Characteristics of Physical Equilibrium

• All the measurable properties become constant at Equilibrium.
• It can be established only in a closed vessel.
• At equilibrium, the opposing forces become equal.
• The magnitude of equilibrium value gives an indication of the extent of the reaction.
• The equilibrium is dynamic in nature. That is the reaction keeps on going only the rate becomes constant.

Dynamic Nature of Physical Equilibrium

• At physical equilibrium between liquid water and water vapour, the rates of both processes evaporation and condensation become equal.
• In other words, the number of molecules returning to the liquid equals the number of molecules escaping from the liquid.
• Hence the total number of molecules in both liquid and vapour phases remains although the rates of two opposing tendencies are equal and the number of molecules in both the phases remains constant, the individual molecules are continuously moving from one phase to another.
• Thus, physical equilibrium has motion in it. This is called the dynamic nature of physical equilibrium.

Read about Emulsions and Suspensions here.

Chemical Equilibrium

For a Chemical Reaction as below

At the start of the reversible reaction, A and B molecules form the product molecules C and D.

As the reaction continues concentration of A and B molecules decreases.

As the concentration of the product molecules increases, the day starts to form the reactant molecules.

• Thus, we can say that the rate of forwarding reaction depends upon the concentration of the reactants while the rate of backward reaction depends on the concentration of the products.
• Eventually, the rate of forwarding and backward reaction becomes equal and there are no changes in the concentrations of the reactants and products. This is because the reactants and products get converted into one another as fast as they are formed.
• This is called Chemical Equilibrium.

Types of Chemical Equilibrium

Chemical Equilibrium is of the following types:

Homogeneous Chemical Equilibrium

The equilibrium in which all the reactants and products are present in a single phase is a homogeneous chemical equilibrium.

Example:

Heterogeneous Chemical Equilibrium

The equilibrium in which reactants and products exist in two or more phases is called heterogeneous equilibrium.

Example:

Learn more about Adsorption, here.

Characteristics of Chemical Equilibrium

• At equilibrium, the forward and reverse reactions proceed at the same rate.
• All the reactants and products are present at equilibrium
• The concentration of reactants and products please constant values and these are called equilibrium concentration.
• At chemical equilibrium, neither the forward nor the reverse reaction is ceased. Both the reactions continue to proceed in opposite directions at equal rates.
• A catalyst does not affect the position of equilibrium.
• The changes in concentration pressure, volume and temperature can change the equilibrium position.

Dynamic Nature of Chemical Equilibrium

• At chemical equilibrium the rates of forwarding and reverse reactions are equal and the concentration of the reactants and products reach a constant value.
• However, this does not mean that the forward and reverse reactions have stopped.
• The reaction is still occurring in both directions with equal speed.
• The individual molecules are still reacting and there is heavy activity at the molecular level.
• The reactant molecules continue to form product molecules and product molecules continue to form reactant molecules.
• Thus, chemical equilibrium has motion in it. This is called the dynamic nature of chemical equilibrium.

Check the Development of the Periodic Table here.

Law of Mass Action

Norwegian chemist and mathematician Cato Maximilian Guldberg and Chemist Peter Wage gave the law of mass action to obtain the correct equilibrium expression.

The law of mass action states that the rate at which a substance reacts is directly proportional to its active mass and the rate of the chemical reaction is directly proportional to the product of active masses of reactants with each active mass term raised to the power equal to its stoichiometric coefficient in the balanced reaction.

Consider the following example

aA + bB → Products

If [A] and [B] are active masses or concentrations of the reactants A and B respectively at equilibrium then,

According to the Law of Mass Action,

Equilibrium Constant

Consider the following chemical reaction of reversible nature.

According to the law of mass action, the rate of forwarding reaction is directly proportional to the product of molar concentrations of the reactants A and B, raised to their stoichiometric coefficient in the balanced reaction.

Similarly, according to the law of mass action, the rate of free verse reaction is directly proportional to the product of the molar concentration of the products C and D raised to their stoichiometric coefficient in the balanced reaction.

At equilibrium, the rate of forwarding reaction is equal to the rate of the reverse reaction.

Where,

Kc: Equilibrium Constant

The equilibrium constant is defined as the ratio of the product of equilibrium concentrations (mol/L) of products to the product of the equilibrium concentration of reactants with the concentration of each substance raised to the power equal to its stoichiometric coefficient in the balanced reaction.

Also, read more Amines, here.

Equilibrium Constant at Partial Pressure

The concentration of reactants and products in reactions involving gases can be expressed in the terms of partial pressures. Therefore, for gas-phase reactions equilibrium constant can also be written using partial pressures.

Consider a gas-phase chemical reaction as follows:

The Equilibrium Constant at Partial Pressure can be given as:

Relationship between Kc and Kp

We know that the Equilibrium Constant at Partial Pressure is given as:

We know that the gas law,

Similarly,

Therefore,

Where,

= number of moles of gaseous products – number of moles of gaseous reactants

Characteristics of An Equilibrium Constant

1. The value of the equilibrium constant depends on the temperature.
2. The changes in the values of concentration of products or reactants, volume pressure does not alter the value of the equilibrium constant.
3. The use of a catalyst does not affect the value equilibrium constant.
4. The equilibrium constant is a unitless quantity.
5. The expression of equilibrium constant may contain the concentrations of gases or molecules and ions in solution, but may not contain the molar concentrations of pure solids or liquids.
6. The form of the equilibrium constant expression and the numerical value of the equilibrium constant depends on the form of the balanced equation.

Know all about Solutions, its Components, Types, Properties here.

Uses of An Equilibrium Constant

The Equilibrium Constant is crucial for the following

To calculate equilibrium concentrations.

By knowing the value of the equilibrium constant we can calculate the equilibrium concentrations.

To know the extent of the reaction

• The numerical value of the equilibrium constant indicates the rate of conversion of the reactants to products.
• If the equilibrium constant is very large then the reaction is in the favour of the products. Such reactions easily undergo completion.
• If the equilibrium constant is very small then the concentration of reactants is much greater than that of the products. This shows that the reaction does not take place easily.
• If the equilibrium constant is intermediate between the above two values, then it shows that both the reactants and products are present in significant amounts.

To predict the direction of reaction

• The value of the equilibrium constant just before achieving the equilibrium is called a reaction quotient.
• If the reaction quotient is less than the equilibrium constant, it should increase in order to achieve the required equilibrium.
• It means that the concentration of the products should increase or the concentration of the reactants should decrease.
• If the reaction quotient is greater than that of the equilibrium constant then it should decrease to achieve the required equilibrium.
• It means that the concentration of the products should decrease or the concentration of the reactants should increase.
• If the reaction quotient is equal to the equilibrium constant then the equilibrium is achieved and no net reaction takes place.

Check out the notes on Colloids, here.

Le-Chatelier’s Principle

Le-Chatelier’s Principle states that if external stress is applied to a reacting systematic breeam the system will adjust itself in such a way that the effect of the stress is reduced.

In short it means that the reaction then proceeds to change the composition until a new equilibrium is achieved.

Check more important topics of Chemistry here.

Factors Affecting Equilibrium

Following are the factors that affect the equilibrium of a reaction.

Effect of Change in Concentration

• At the equilibrium, both the reactants and products are present at equilibrium concentration.
• The addition of one or more reactants or products to the reacting systematic were Librium causes to increase its concentration.
• The stress is applied to the system by the addition of the reactant or product. According to Le-Chatelier’s Principle, the effect of an increase in the concentration of a substance is reduced by shifting the equilibrium in a direction that consumes the added substance.
• Similarly, the removal of a product or reactant from the equilibrium state results in decreasing the concentration of that substance. Again the stresses applied to the system and according to Le-Chatelier’s Principle effect of a decrease in the concentration of the substance is minimized if the reaction shifts in the direction that will replenish the removed substance.

Effect of Change in Pressure and Volume

• The change in pressure does not affect the concentrations of solids and liquids. The concentration of gases is affected appreciably by the change in pressure.
• We know the ideal gas equation,

• Thus, we can conclude that pressure and volume have an inverse relationship and are directly proportional to the molar concentration.
• So if the pressure of a gaseous reaction mixture at equilibrium is increased at constant temperature by decreasing its volume then there will be an increase in the molar concentration.
• Hence according to Le-Chatelier’s Principle, the system will shift its equilibrium in such a way that the number of moles are decreased.

Learn more about the Position of Hydrogen in Periodic Table, here.

Effect of Change in Temperature

• We know that a change in the concentration pressure or volume of a system at equilibrium results in a shift of the equilibrium in such a way that the effect is nullified.
• However, a change in temperature alters the value of the equilibrium constant itself.
• If heat is added to a reaction systematic equilibrium its temperature increases.
• The stress is applied in the form of heat.
• So according to Le-Chatelier’s Principle, the reaction shifts in such a way that the heat is reduced. In short, the equilibrium will tend to shift in the endothermic direction in which heat is absorbed.
• Hence the addition of heat or increase in temperature of a system results in an endothermic reaction.
• Similarly, the removal of heat from the equilibrium mixture causes a fall in temperature.
• Hence, according to Le-Chatelier’s Principle, the reaction proceeds in such a way that the removal of heat is replaced by the release of heat. In other words, the reaction becomes exothermic.
• Removal of heat or decrease in the temperature of a system results in an exothermic reaction.

Effect of Catalyst

• The addition of a catalyst to the reaction system at equilibrium increases the rate of forwarding and backward reaction equally.
• Hence the position of the equilibrium is unaffected by the use of a catalyst. Catalyst is used to simply increase the rate of the reaction or the rate at which the equilibrium is established

Also, read more about Chemical Equations, here.

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