Chemical Equilibrium | Law of Mass Action#
Law of Mass Action#
- According to this law, the rate of a reactions is directly proportional to the product of active masses of reactants raised to the power of their stoichiometric coefficients.
Example
Let us consider a reaction:
\[n_1A + n_2B → Products\]
According to law of mass action, we can write:
\[rate ∝ [A]^{n_1}\ [B]^{n_2}\]
\[rate = {k\ [A]^{n_1}\ [B]^{n_2}}\]
Here, k = rate constant and the above expression is called rate law or rate equation.
Active Mass#
- Active mass of a component A is denoted by [A].
- Active mass can be defined in two phases: solution phase and gaseous phase.
- When reaction occurs in solution phase, active mass is taken as molarity.
\[Active\ mass = molarity\]
When reaction occurs in gaseous phase, active mass can be taken as partial pressure.
\[Active\ mass = Partial\ pressure\]
Since, partial pressure = mole fraction x Total pressure
We can write:
\[Active\ mass = Mole\ fraction \times Total\ pressure\]
- For gaseous reactants, both molarity and partial pressure can be used as active mass interchangeably.
- Active mass of solid substance in a chemical reaction is always taken as 1.
Example
\[A_{(aq)} + B_{(s)} ⇌ C_{(aq)}\]
\[[B] = 1\]
- If any one of the substance in reactant or product is solid or aqueous, then the active mass of other liquids will be taken as 1.
Example
\[A_{(l)} + B{(aq)} ⇌ C_{(l)} + D_{(l)}\]
\[[A] = 1, [C] = 1, [D] = 1\]
- For a homogeneous liquid phase reaction, the active mass of pure liquid will not be taken as unity.
Example
\[A_{(l)} + B_{(l)} ⇌ C_{(l)} + D_{(l)}\]
\[[A] ≠ 1, [B] ≠ 1, [C] ≠ 1, [D] ≠ 1\]