Q1(f). Activity and activity coefficient: units and magnitudes

Answer in brief

Activity a is the effective concentration, μ = μ° + RT ln a; activity coefficient γ = a/(m/m°). Both are dimensionless; γ → 1 at infinite dilution.

Key points

• Activity: the effective (thermodynamic) concentration, defined by μi = μ°i + RT ln ai; ideal solution: a = x (or m/m°).
• Activity coefficient: γi = ai/(mi/m°) (or ai/xi); it measures the departure from ideality; mean γ± for electrolytes.
• Units: both dimensionless; a is a ratio to the standard state, so a value of a has meaning only with its standard state.
• Magnitudes: a > 0; pure solid or pure liquid a = 1; γ → 1 as concentration → 0; ions: γ± < 1 in dilute solution.
• At high concentration γ± passes a minimum and can exceed 1 (HCl: 1.316 at 3 m): hydration ties up solvent.