18 Jan 2019
School
Department
Course
CHMA11 Lecture 6: Reaction Quotient
- In order to compare the progress of a reaction to the equilibrium state of the reaction,
we use a quantity called the reaction quotient
o Reaction quotient has the same definition as the equilibrium constant
▪ Only difference is that it involves reactions that do not have to be at
equilibrium conditions
o Reaction Quotient: the ratio at any point in the reaction of the concentration of
the products, raised to their stoichiometric coefficients, divided by the
concentrations of the reactants raised to their stoichiometric coefficients
▪ For gases: reaction quotient is in terms of partial pressure instead of
concentrations
• Is called Qp
▪ Method of determining Q
• aA + bB → cC + dD
• Q = [C]c [D]d / [A]a [B]b
▪ Main difference between reaction quotient and equilibrium constant
• At a given temperature, the equilibrium constant only has 1 value,
a value that species the relative amounts of reactants and
products at the equilibrium state of the reaction
• The reaction quotient depends on the current state of the
reaction
o Has many different values as the reaction proceeds
▪ The reaction quotient is useful because the value of it relative to the
equilibrium constant is a good measure of the progress of the reaction
towards equilibrium conditions
• Interpreting Q: how do we use Q to track the equilibrium progress
of a reaction?
o There are 3 possible conditional relationships between Q
and K
▪ 1: Q < K
• In this case Q must get larger as the
reaction progresses toward equilibrium
• Q will become larger as the reactant
concentrations decrease and the product
concentrations increase
o Therefore the reaction will proceed
to the right towards products
▪ 2: Q > K
• In this case, Q must get smaller as the
reaction progresses towards equilibrium