Question

Activity of pure metal is

• A. Zero
• B. 1
• C. -1
• D. Depends on temperature

Hint:

A simple rule to remember for thermodynamics problems: the activity of any pure solid or pure liquid is always taken to be 1. The activity of an ideal gas is its partial pressure in bars, and the activity of a solute in an ideal solution is its molarity.

Answer 1

The Correct Option is B

Step 1: Understanding the Concept:
Activity (\(a\)) is a measure of the "effective concentration" of a species in a mixture or solution. It is a dimensionless quantity that accounts for the deviation of a real system from ideal behavior. For chemical reactions involving pure solids or liquids, we need to define a standard state to compare their thermodynamic properties.

Step 2: Detailed Explanation:
The activity of a substance is defined as the ratio of its fugacity (for gases) or chemical potential in a given state to its fugacity or chemical potential in a standard state.
For pure solids and liquids, the standard state is conventionally defined as the pure substance itself at a pressure of 1 bar (or 1 atm) and the temperature of interest.
The activity \(a_i\) of a component \(i\) is given by: \[ a_i = \frac{f_i}{f_i^\circ} \] where \(f_i\) is the fugacity of the component in the mixture and \(f_i^\circ\) is the fugacity in its standard state.
For a pure solid or liquid (like a pure metal), its state is its standard state. Therefore, its fugacity \(f_i\) is equal to its fugacity in the standard state, \(f_i^\circ\).
\[ a_{\text{pure metal}} = \frac{f_{\text{pure metal}}}{f_{\text{pure metal}}^\circ} = 1 \] This convention simplifies thermodynamic calculations, such as writing equilibrium constant expressions. The activity of a pure solid or liquid does not appear in the mass action expression because its value is unity. While its properties (like molar volume) do change slightly with temperature and pressure, for most practical purposes, the activity of a pure condensed phase is taken as 1.

Step 3: Final Answer:
By convention, the standard state for a pure solid or liquid is the pure substance itself. Therefore, the activity of a pure metal in its standard state is defined as 1.