Zero enthalpy, in the context of thermodynamics, typically refers to the standard enthalpy of formation being conventionally set to zero for pure elements in their most stable form under standard state conditions.
Understanding Enthalpy of Formation
The concept of "zero enthalpy" is linked to the enthalpy of formation ($\Delta H_f^\circ$), also known as the heat of formation.
- Definition: The enthalpy of formation is defined as the energy change that occurs when one mole of a substance is formed from its constituent elements in their standard states.
- Standard State: The standard state refers to a set of conditions typically defined as a temperature of 25°C (298.15 K) and a pressure of 1 atmosphere (101.325 kPa) for gases, or 1 M concentration for solutions.
- Why Elements Have Zero Enthalpy of Formation: As stated in the reference, the enthalpy of formation for elements in their standard states is zero because they are in their standard states, requiring no energy to form. An element in its standard state is considered to be formed from itself, which involves no chemical change and therefore no energy change relative to this defined baseline.
The Role of Zero Enthalpy as a Reference Point
Setting the standard enthalpy of formation to zero for elements provides a crucial reference point for calculating the enthalpy of formation of compounds and the enthalpy changes of chemical reactions.
- It allows us to determine the relative energy content of compounds compared to their constituent elements.
- Enthalpy changes for reactions ($\Delta H_{rxn}^\circ$) can be calculated using the standard enthalpies of formation of the products minus the standard enthalpies of formation of the reactants.
Examples of Zero Enthalpy
Here are some examples of substances that have a standard enthalpy of formation of zero:
- Oxygen gas ($\text{O}_2(g)$)
- Nitrogen gas ($\text{N}_2(g)$)
- Hydrogen gas ($\text{H}_2(g)$)
- Solid graphite ($\text{C}(graphite)$)
- Solid iron ($\text{Fe}(s)$)
- Liquid mercury ($\text{Hg}(l)$)
Note: Allotropes of elements that are not the most stable form in the standard state do not have a zero enthalpy of formation (e.g., diamond, ozone $\text{O}_3$). Compounds also do not have a zero enthalpy of formation; their values are determined relative to the zero baseline set by the elements.
In summary, "zero enthalpy" is a convention within thermochemistry referring specifically to the assigned standard enthalpy of formation for elements in their stable forms under standard conditions, serving as a fundamental reference point for thermodynamic calculations.
