Standard state simply refers to the form of the substance at 1 atm pressure and (usually) 25°C. Here are some helpful definitions from the textbook by Nivaldo Tro:
1. Standard State
- For a Gas: The standard state for a gas is the pure gas at a pressure of exactly 1 atmosphere.
- For a Liquid or Solid: The standard state for s liquid or solid is the pure substance in its most stable form at a pressure of 1 atm and at the temperature of interest (often taken to be 25°C).
- For a Substance in Solution: The standard state for a substance in solution is a concentration of exactly 1 M.
2. Standard Enthalpy Change (ΔH°)
- The change in enthalpy for a process when all reactants and products are in their standard states. The degree sign indicates standard states.
3. Standard Enthalpy of Formation (ΔHf°)
- For a Pure Compound: The change in enthalpy when 1 mole of the compound forms from its constituent elements in their standard states.
- For a Pure Element in its Standard State: ΔHf° = 0.
The standard state of iron metal is Fe (s) since iron is a solid at 1 atm and 25°C. The standard state of mercury is Hg (l) since it is a liquid at 1 atm and 25°C. The standard state of oxygen is O2 (g) since oxygen is a diatomic molecule in the gaseous state at 1 atm and 25°C. The standard state of carbon is taken to be the solid graphite form, C (s, graphite).
Steve
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