The reaction quotient, Q, is a measure of the relative amounts of products and reactants present in a reaction mixture at any given point in time. It is calculated using the same expression as the equilibrium constant but with current concentrations or partial pressures.
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Q is calculated using the formula: $Q = \frac{[products]}{[reactants]}$, where concentrations are raised to the power of their coefficients in the balanced equation.
When Q < K (the equilibrium constant), the reaction proceeds in the forward direction to reach equilibrium.
When Q > K, the reaction proceeds in the reverse direction to reach equilibrium.
Q can be used to determine whether a system is at equilibrium, not yet reached it, or has surpassed it.
Unlike K, which is constant for a given reaction at a specific temperature, Q changes as concentrations or partial pressures change.
Review Questions
How does Q compare to K when a reaction mixture is not at equilibrium?
What does it mean if Q = K for a chemical reaction?
In what way do you use current concentrations or partial pressures to calculate Q?
Related terms
Equilibrium Constant (K): A constant that expresses the ratio of concentrations of products to reactants at equilibrium for a given reaction.
The state where the rate of the forward reaction equals the rate of the reverse reaction, resulting in no net change in concentration of reactants and products.