How this calculator works
Enter activities and the matching positive coefficients from the balanced reaction, in the same order on each side. A solution activity is often approximated by concentration divided by a 1 mol/L reference; ideal gas activity uses partial pressure divided by a specified standard pressure. Pure solids and liquids are omitted. Q uses the current state, which need not be at equilibrium. Comparing Q and K predicts the thermodynamic direction at fixed conditions, not reaction speed.
Formula
Q = product(product activity^coefficient)/product(reactant activity^coefficient). Q < K favours forward change; Q > K favours reverse change.
Worked example
Activities 1 and 2 for two unit-coefficient reactants and 6 for the product give Q = 3. A reactant activity 2 with coefficient 2 and product activity 2 gives Q = 0.5.
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Frequently asked questions
Are activities molar concentrations?
Activities are dimensionless. In an ideal solution, divide mol/L by the 1 mol/L reference before entry.
Can a product activity be zero?
Yes, giving Q = 0. Reactant activities in the denominator must be positive.
Does Q below K mean a fast reaction?
No. It gives direction; kinetics determines how quickly equilibrium is approached.