Mass Action Expression
Definition and meaning of Mass Action Expression in chemistry.
Mass action expression is a mathematical formula that describes the ratio of the concentrations or partial pressures of products to reactants at any given moment during a reversible chemical reaction.
In more detail
This expression is constructed by multiplying the concentrations of the product species and dividing by the concentrations of the reactant species, with each concentration raised to the power of its corresponding stoichiometric coefficient from the balanced chemical equation. When the chemical system is at equilibrium, the calculated value of this expression is exactly equal to the equilibrium constant (K) for that specific reaction. Before the system reaches equilibrium, the value is referred to as the reaction quotient (Q), which chemists use to predict the direction the reaction will shift to reach equilibrium.
Key facts
| Field | Physical Chemistry |
|---|---|
| Related Constant | Equilibrium constant (K) |
| Usage | Predicts chemical reaction direction |
For the gas-phase synthesis of ammonia represented by the equation N2 + 3H2 <=> 2NH3, the mass action expression is the concentration of NH3 squared divided by the product of the concentration of N2 and the concentration of H2 cubed.
Frequently asked questions
How is the mass action expression practically used?
It is compared to the established equilibrium constant to determine if a reaction mixture will proceed forward or backward.
Does it apply to solid and liquid phases?
Pure solids and pure liquids are omitted from the expression because their effective concentrations remain constant during the reaction.