Important Laws of Chemistry Class 12

In Board Examination 2 marks questions can be asked on Laws. So, this article may help you to grab those 2 marks.

1.      Henry’s Law:

At a constant temperature, the solubility of a gas in a liquid is directly proportional to the partial pressure of the gas present above the surface of liquid or solution.

Or

Mole fraction of gas in the solution is proportional to the partial pressure of the gas over the solution.

Or

The partial pressure of the gas in vapour phase (P) is proportional to the mole fraction of the gas (X) in the solution.

P = K_HX

2.   Where K_H is the proportionality constant named as Henry’s Constant which is differenr for different gases.

       Raoult’s Law:

It states that for a solution of volatile liquids, the partial pressure (P) of each component of the solution is directly proportional to its mole fraction (X) present in solution.

P = P0 X

For component 1,

P₁ = P0₁ X

Similarly For component 2,

P₂ = P0₂ X

Where P0 is the vapour pressure of pure component. P0₁ & P0₂ are the vapour pressure of pure component 1 and 2 respectively.

 

3.      Faraday’s first Law of electrolysis: (excluded in AISSCE 2021)

It states that the amount of any substance deposited or liberated at any electrode is directly proportional to the quantity of electricity passed through the electrolytic solution.

m α Q

m = Z Q

Where m is mass of substance deposited, Z is constant of proportionality and called as electrochemical equivalent and Q is the coulombs of electricity passed.

Q = I X t

Where, I is the current in amperes and t is the time in seconds.

m = Z x I x t

 

4.      Faraday’s second Law of electrolysis: (excluded in AISSCE 2021)

It states that when same quantity of electricity is passed through different electrolytic solutions connected in series, the weights of the substances produced at the electrodes are directly proportional to their chemical equivalent weights.

For example, when same amount of current is passing through the electrolytic solution of CuSO₄ and AgNO₃ connected in a series, the weights of Cu and Ag deposited are:

 

Weight of Cu deposited / Weight of Cu deposited = Eq. wt. of Cu / Eq. wt. of Ag

5.     Kohlrausch law of independent migration of ions:

The law states that limiting molar conductivity of an electrolyte can be represented as the sum of the individual contributions of the anion and cation of the electrolyte.

Thus if λ0_Na⁺ and λ0_Cl^- are the limiting molar conductivity of the sodium and chloride ions respectively, then the limiting molar conductivity for sodium chloride is given by the eqastion:

Λ°_m =  λ0_Na⁺ + λ0_Cl^-

In general, if an electrolyte on dissociation gives ν₊ cations and ν_- anions then its limiting molar conductivity is given by:

Λ°_m = ν₊  λ^°₊  + ν_-  λ^°_-

Where λ^°₊  &  λ^°_- are the the limiting molar conductivity of cation and anion respectively.

 

6.     Rate Law:

The rate law is the expression in which reaction rate is given in terms of molar concentration of reactants with each term raised to some power, which may or may not be same as the stoichiometric coefficient of the reacting species in a balanced chemical equation.

For a general reaction,

aA + bB ----à  cC + dD

Rate α [A]x [B]y

Where x and y may or may not be equal to the stoichiometric coefficients (a and b).

Rate = k [A]x [B]y

-d[R]/dt = k [A]x [B]y

 

By: S. K. Nigam