Final answer:
The concentration quotient (Q) for the reaction aA + bB ⇌ cC + dD is calculated by the formula Q = ([C]^c · [D]^d) / ([A]^a · [B]^b), considering the concentrations of gaseous and solute species, excluding liquids such as water when it's neither a solvent nor in excess.
Step-by-step explanation:
To write the concentration quotient (Q) for the reaction aA + bB ⇌ cC + dD, we must consider the concentrations of the gaseous and solute species only. Since water is not in excess, nor the solvent in this reaction, and assuming it is a liquid, its concentration does not appear in the Q expression. According to the Law of Mass Action, the expression for Q is as follows:
Q = ([C]^c · [D]^d) / ([A]^a · [B]^b)
Here, [A], [B], [C], and [D] are the molar concentrations of reactants A and B and products C and D, respectively. The exponents a, b, c, and d correspond to the stoichiometric coefficients from the balanced chemical equation for the reaction.
In chemical equilibrium, when Q is equal to the equilibrium constant K, the reaction is at equilibrium. However, the reaction quotient can be used to determine the direction in which a reaction will proceed to reach equilibrium.
Here is an example to illustrate the concept:
For the reaction 2SO2(g) + O₂(g) ⇌ 2SO₃(g), the reaction quotient would be:
Q = [SO₃]^2 / ([SO2]^2 · [O₂])