The equilibrium constant of a chemical reaction describes the relationship between the concentrations of the reactants and the concentrations of the products of the reaction at dynamic equilibrium.
Chemists found that when a reversible reaction reaches dynamic equilibrium, the product of the concentrations (or partial pressures) of the reaction products raised to the power of their stoichiometric coefficients divided by the product of the concentrations (or partial pressures) of the reactants raised to the power of their stoichiometric coefficients, has a constant value at a particular temperature (see this advanced level article for derivation). For the decomposition of PCl_{5} at 620K,
we have
where [i] is the concentration in mol dm^{-3} of the species i.
We called this constant, K_{c}, the equilibrium constant. Since the species in this reaction are in the gaseous state and that the partial pressure of gas is proportional to its concentration, we can also express the equilibrium constant of this reaction in terms of partial pressures of the species:
where p_{i} is the partial pressure of gas i and K_{p} is the equilibrium constant in terms of partial pressures. Note that K_{c} may or may not be equal to K_{p} for a particular reaction (see here for details).
Another example is the reaction:
where
In general, for a reaction
Question
Calculate the equilibrium constant for the dissociation of PCl_{5} in a 250 ml vessel if the initial and equilibrium amounts of the reactant are 0.0175 mol and 0.0125 mol respectively (assuming no products formed initially).
Answer
PCl_{5} | PCl_{3} | Cl_{2} | |
Initial conc, M |
0.0175/0.250 | 0 | 0 |
Equilibrium conc, M |
0.0125/0.250 | (0.0175-0.0125)/0.250 | (0.0175-0.0125)/0.250 |