The ratio of the amount of product raised to its stoichiometric coefficient to the amount of reactants also raised to its stoichiometric coefficient at any point in the reaction is known as Reaction Quotient (Q)
- The amount can be in terms of molar concentration or partial pressure
Equilibrium constant (KEQ)
When the amount present at equilibrium is the value used in Q, it becomes KEQ.
Law of Mass Action - when a system reaches equilibrium at a given temperature, the ratio of the quantities that make up the reaction quotient has a constant numerical value, KEQ
KEQ from Rate Law The following equations show how KEQ expression can be derived from the Rateforward = Ratereverse.
Therefore, at equilibrium, Ratef = Rater kf [N2O4] = kr [NO2]2 Rewriting, it becomes
Rules in writing KEQ and Q expression amounts is be expressed in terms of molar concentration (for both aqueous and gas) or partial pressure (gas). products are always in the numerator, and the reactants are always in the denominator Coefficients in the chemical equations are the exponents used in the expression The stoichiometric coefficient of each molecule or compound is used in place of the experimentally determined exponents because the reaction is assumed to occur at a fast reversible single step. solids and liquids are not included in the expression, why? unit less
Practice writing KCand KP expression for the following reactions: formation of ozone from oxygen gas formation of ammonia from nitrogen and hydrogen gas decomposition of phosphorus pentachloride into phosphorus trichloride and chlorine gas formation of sulphur trioxide from oxygen and sulphur dioxide reaction of the aqueous sodium carbonate and calcium chloride to form solid calcium carbonate and sodium chloride solution decomposition of solid calcium carbonate into lime and carbon dioxide