What is the meaning of Equilibrium?
An equilibrium represents a state in a process when the observable properties such as colour, temperature, pressure, concentration etc do not show any change.
The word equilibrium means ‘balance’ which indicates that a chemical reaction represents a balance between the reactants and products taking part in the reaction. The equilibrium state is also noticed in certain physical processes such as the melting point of ice at 0℃, both ice and water are present at equilibrium.
In the case of physical processes such as the melting of solid, dissolution of salt in water etc., the equilibrium is called physical equilibrium while the equilibrium associated with chemical reaction is known as a chemical equilibrium.
Table of Contents
- Equilibrium in Physical changes
- Equilibrium in Chemical changes
- Characteristics of Equilibrium states
- Related topics on chemical equilibrium
- Ionic equilibrium
- Related topic on ionic equilibrium
- FAQs
Equilibrium in Physical changes
This equilibrium is associated with the physical process. These are:
(i) Solid ⇋ Liquid equilibrium
eg. H2O(s) ⇋ H2O(l) rate of melting of ice = rate of freezing of ice
(ii) Liquid ⇋ Gas equilibrium
eg. H2O(l) ⇋ H2O(g)
(iii) Solid ⇋ Gas equilibrium
eg. I2(s) ⇋ I2(vapour)
Also read : Physical equilibrium
Equilibrium in Chemical changes
The chemical equilibrium in a reversible reaction is the state at which both forward and backward reactions occur at the same speed.
The stage of the reversible reaction at which the concentration of the reactants and products do not change with time is called the equilibrium state.
Or
The state in which the measurable properties of the system such as pressure, density, colour or concentration do not undergo any further noticeable changes with time under a given set of conditions is said to be a state of equilibrium.
Characteristics of Equilibrium states
(i) Equilibrium state can only be achieved if a reversible reaction is carried out in closed space.
(ii) Chemical equilibrium at a given temperature is characterised by the constancy of certain properties such as pressure, concentration, density or colour.
(iii) At equilibrium each reactant and each product have a fixed concentration and it is independent of the fact whether we start the reaction with the reactants or with the products.
2HI ⇋ H2 + I2
H2 + I2 ⇋ 2HI
(iv) Equilibrium state attained in a lesser time by the use of a positive catalyst.
(v) It is dynamic in nature. However, the reaction seems to have come to standstill because the concentration of reactants and products do not change.
Related topics on chemical equilibrium
- The law of mass action
- Equilibrium constant
- Standard free energy change and equilibrium constant
- Le Chatelier Principle
Ionic equilibrium
Chemical reactions also take place in a solution in which generally ions participate. The substance which forms an ion in the solution is called the electrolyte. The equilibrium is present between the unionised molecules of a particular substance and the ion formed in the solution is known as ionic Equilibrium.
The ionic compounds are generally acids, bases and salts. Therefore the ionic equilibrium is present in them when dissolved in water and any other solvent. Since all of them are electrolytes, the equilibrium constant is related to the strength of these electrolytes i.e. the number of ions which they furnish in solution.
Strong and Weak electrolytes:
The strength of an electrolyte is expressed in terms of the degree of ionisation(α).
The electrolytes which are almost completely ionized are called strong electrolytes while those ionized to smaller extent are known as weak electrolytes.
- For strong electrolytes, α is equal to 1.
- For weak electrolytes, α is equal to less than 1.
There is no equilibrium in strong electrolytes because when it is dissolved in a solvent it ionises completely. But weak electrolytes are ionized to a small extent. Therefore there is an equilibrium between the unionized electrolytes and ions formed in the solution.
- CH3COOH + H2O ⇌ CH3COO– + H3O+
- NH4OH + H2O ⇌ NH4+ + OH–
Related topic on ionic equilibrium
- Ostwald Dilution Law
- Theories of Acids and Bases
- pH of acid and base
- Salt Hydrolysis
- Buffer solutions
- Solubility Product
- Common ion effect
Frequently Asked Questions on Equilibrium
What is equilibrium and what is an example?
The state in which the measurable properties of the system such as pressure, density, colour or concentration do not undergo any further noticeable changes with time under a given set of conditions is said to be a state of equilibrium. Example: 2HI ⇋ H2 + I2
What are KP and KC in equilibrium?
Kp is the equilibrium constant used when partial pressures of the species are given and Kc is the equilibrium constant used when equilibrium concentrations are expressed in molarity.
Why is equilibrium important in chemistry?
When the quantities of reactants and products are stable – their ratio does not change a chemical reaction is in equilibrium. This suggests that the reaction has reached a point where the reactant and product amounts remain constant over time, since the forward and backward reactions are at the same rate.
What is the pH formula?
pH is described as the negative of the logarithm of the molar hydronium-ion concentration. pH formula is -log [H3O+].
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