# Law of Chemical Equilibrium and Equilibrium Constant

Chemical equilibrium refers to the state wherein both the reactants and the products present in the concentration have no tendency to change with the period of time during a chemical reaction. A chemical reaction achieves chemical equilibrium when the rate of forward reaction and that of the reverse reaction is same. Also, since the rates are equal and there is no net change in the concentrations of the reactants and the products – the state is referred to as a dynamic equilibrium and the rate constant is known as equilibrium constant. Let’s find out more.

## Law of Chemical Equilibrium

Representation of the attainment of chemical equilibrium is –Â

The equilibrium constant is defined as the product of the molar concentration of the products which is each raised to the power equal to its stoichiometric coefficients divided by the product of the molar concentrationsÂ of the reactants, each raised to the power equal to its stoichiometric coefficients is constant at constant temperature. T

his equilibrium constant can be simply expressed in terms of the partial pressures of the reactants and the products. However, if it is expressed in terms of the partial pressure, it is denoted by Kp.

## Equilibrium Constant

### Equilibrium Constant Units and Formula

Law of mass action also forms the basis which states that the rate of a chemical reaction is directly proportional to the product of the concentrations of the reactants raised to their respective stoichiometric coefficients. Therefore, given the reaction –Â

aA(g) + bB(g) â‡”Â cC(g) + dD(g)

By using the law of mass action here,Â

• The forward reaction rate would beÂ k+ [A]a[B]b
• The backward reaction rate would beÂ kâ€“ [C]c[D]dÂ

where, [A], [B], [C] and [D] being the active massesÂ andÂ k+Â andÂ kâˆ’Â areÂ rate constants of forward and backward reactions, also the a, b, c, d are the stoichiometric coefficients related to A, B, C and D respectively. However, at the equilibrium – the forward and the backward rates are equal, stating –Â

Rate of forward reaction = Rate of backward reaction

or,Â

or,Â

where,Â

KcÂ is the equilibrium constant expressed in terms of the molar concentrations. The equationÂ KcÂ = [ C ]cÂ·[ D ]dÂ / [ A ]aÂ·[ B ]bÂ

or, Kc = Kf / KbÂ is the Law of Chemical Equilibrium.Â The equilibrium constant is therefore related to the standard Gibbs free energy change for the reaction which is stated by the equation –

Â§GÂº= -RT ln Keq

where T states the temperature, R is the universal gas constant and Keq is the equilibrium constant.

## Solved Examples for You

Question: Write the equilibrium constant expression for the reaction equation:

NH3 + HOAc = NH4+ + OAc

Solution:

[NH4+] [OAc] ——————— = K (unitless constant) [NH3] [HOAc]

Question: A closed container has N2O4 and NO2 gases in it. It has been placed in the lab for many days. What would you consider the container and the gases to be?

1. an open system
2. a closed system
3. not a system

Solution:Â A closed systemÂ since it has been in the lab, the temperature of the system is the same as its environment.

Share with friends

## Customize your course in 30 seconds

##### Which class are you in?
5th
6th
7th
8th
9th
10th
11th
12th
Get ready for all-new Live Classes!
Now learn Live with India's best teachers. Join courses with the best schedule and enjoy fun and interactive classes.
Ashhar Firdausi
IIT Roorkee
Biology
Dr. Nazma Shaik
VTU
Chemistry
Gaurav Tiwari
APJAKTU
Physics
Get Started

### One response to “Acids, Bases and Salts”

1. sylvester says:

Thanks for this. It’s fantastic