For a chemical reaction with signed stoichiometric vector entries , positive for products and negative for reactants, the reaction quotient is the product of the species' thermodynamic activities raised to these powers. It equals the equilibrium constant at chemical equilibrium, with all quantities using the same standard state convention.
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The reaction quotient, often denoted as \( Q \), is a measure of the relative concentrations of reactants and products in a chemical reaction at any given moment, not necessarily at equilibrium. It is calculated using the same expression as the equilibrium constant (\( K \)), but instead of using equilibrium concentrations, \( Q \) is calculated using actual concentrations or partial pressures at a specific moment in time.