Which chemist introduced the thermodynamic concept of activity and coined the term fugacity?

• A. Linus Pauling
• B. Gilbert N. Lewis
• C. Antoine Lavoisier
• D. John Dalton

Answer: (B)

The concept being tested is why thermodynamics uses alternative measures to pressure to account for real-system behavior. Gilbert N. Lewis introduced the idea of activity and coined the term fugacity to handle non-ideal interactions in phases and mixtures. Fugacity acts like an effective pressure that makes the thermodynamic relations for real substances look like those for an ideal system; in equations, the chemical potential is expressed with f (the fugacity) instead of P, so μ = μ° + RT ln f. This lets us extend the ideal-gas framework to liquids and real gases by correcting for molecular interactions.

For solutions, the idea carries over as activity, a_i, which quantifies how closely a component behaves like an ideal dilute solution. It’s written as a_i = γ_i x_i, where γ_i is the activity coefficient that captures non-ideality due to interactions with other species. When the solution is ideal, γ_i = 1 and a_i reduces to the mole fraction x_i. So, Lewis’s contributions lay the foundation for how we treat non-ideal behavior in both gases and solutions.

The other chemists listed are renowned for different breakthroughs: one helped establish atomic theory and precise mass relationships, another advanced bonding concepts, and the last is a foundational figure in early chemistry, none of whom introduced fugacity or the formal activity concept.