Advanced Topic 598 of 1102 Acids, Bases & Electrochemistry
Activity and Activity Coefficients
Effective concentrations accounting for interactions among ions.
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In-depth explanation of Activity and Activity Coefficients
The activity a of a species is its effective concentration in a real solution, defined as the activity coefficient gamma multiplied by the molar concentration: a = gamma × c. Ions interact electrostatically, so their behaviour departs from ideality, and thermodynamic equilibrium constants are written in terms of activities rather than concentrations. In dilute solutions the activity coefficient is near 1, so concentrations can be used directly for simple calculations. At high ionic strength the deviations become large and affect solubility, pH and equilibrium. Activity is the correct quantity for precise thermodynamic work.
Going deeper: Activity and Activity Coefficients
Key ideas: Activity a = gamma x (c/c standard); thermodynamic equilibrium constants use activities. Gamma approaches 1 as concentration goes to zero. At 0.1 M ionic strength, gamma for monovalent ions is about 0.78, for divalent 0.37 and for trivalent 0.08. pH is formally -log a(H+), which is what a pH meter measures.
Common mistakes and connections: Mistake: using concentration in Nernst, K and pH calculations for strong electrolytes at high concentration, giving errors of several tenths of a pH unit. Neutral molecules have gamma near 1. Individual ion activity is not strictly measurable, so mean ionic activity coefficients are reported. Linked to Debye-Huckel and ion-selective electrodes.
Visualization of Activity and Activity Coefficients
The activity coefficient is near 1 in dilute solution and falls with ionic strength, more for 2+ ions, so activity a = gamma c is below concentration. The interactive lesson shows this as a live animation or chart.
Worked example and use case
The pH is strictly -log of the hydrogen ion activity rather than its concentration, which is why a pH meter responds to activity.
Frequently asked questions about Activity and Activity Coefficients
What is Activity and Activity Coefficients?
Effective concentrations accounting for interactions among ions.
Is Activity and Activity Coefficients a beginner, intermediate or advanced topic?
Activity and Activity Coefficients is a advanced topic in Acids, Bases & Electrochemistry. It builds on beginner and intermediate topics, so review those first if needed.
What should I learn before Activity and Activity Coefficients?
Study these earlier topics in Acids, Bases & Electrochemistry first: Transport Number, Debye-Huckel Theory, Ionic Strength.
What comes after Activity and Activity Coefficients?
The next topics in Acids, Bases & Electrochemistry are Overpotential and Pourbaix Diagram.
Can I practise Activity and Activity Coefficients with quizzes?
Yes. The interactive lesson includes a self-check question on Activity and Activity Coefficients, and the Acids, Bases & Electrochemistry quiz draws on every topic in this area.
Where Activity and Activity Coefficients fits in Acids, Bases & Electrochemistry
Activity and Activity Coefficients is topic 88 of 105 in Acids, Bases & Electrochemistry.