Question

Explain law of mass action taking suitable chemical reaction. Derive the relationship between K_p and K_c for a homogeneous.

Answer 1

Law of Mass Action:
According to this law,
“The rate of reaction of a substance is proportional to the product of molar concentration of reactants at a constant temperature at any given time.”
Molar concentration is called active mass. Active mass is the number of moles dissolved in one litre of solution. Let a reaction;
A + B ➝ C + D

According to law of mass action ;
Rate α [A] [B]
Rate = k₁ [A] [B]
Here, k₁ = constant of proportionality or rate constant for forward reaction.
When the concentration of [A] and [B] is unity, k₁ shows rate of forward reaction
Rate of backward reaction α [C][[D]
Rate = k₂ [C] [D]
Here k₂ is rate constant for backward reaction.
At equilibrium,
Rate of forward reaction = Rate of backward reaction

Relationship between K_p and K_c for a homogeneous reaction,

H₂ + I₂ \(\rightleftharpoons\) 2HI

Suppose ‘a’ moles of H₂ and ‘b’ moles of I₂ be heated in sealed glass bulb having volume V litres in a thermostat till equilibrium is established. If the concentration of HI formed after analysis is 2x, then according to above reaction, 2x moles of HI will be obtained from x moles of each H₂ and I₂.

The equilibrium concentration per litre of various reactions and products may be shown as follows :