Why is the enthalpy change of an element in its standard state zero?

The enthalpy change of an element in its standard state is zero because it is the reference point for enthalpy changes.

In more detail, enthalpy change, denoted as ΔH, is a measure of the amount of energy absorbed or released by a system as it undergoes a physical or chemical change under constant pressure. It is a fundamental concept in thermodynamics, the branch of physical science that deals with the relationships between heat and other forms of energy.

The standard state of an element is defined as its most stable physical state at a pressure of 1 bar and a specified temperature, usually 298K. For example, the standard state of oxygen is a gas, while that of iron is a solid. When an element is in its standard state, it is in its most stable form, meaning it does not need to undergo any change to exist in these conditions. Therefore, there is no energy absorbed or released, and the enthalpy change is zero.

This is a convention in thermodynamics that allows for a consistent reference point when calculating enthalpy changes. By setting the enthalpy change of an element in its standard state to zero, it simplifies the calculation of enthalpy changes for chemical reactions. This is because the enthalpy change for a reaction is calculated as the sum of the enthalpy changes for the products minus the sum of the enthalpy changes for the reactants. If the reactants and products are in their standard states, their enthalpy changes are zero, and they do not contribute to the overall enthalpy change for the reaction.

In essence, the reason why the enthalpy change of an element in its standard state is zero is a matter of definition and convenience. It provides a consistent and simple way to calculate and compare the enthalpy changes of different reactions, which is crucial in many areas of chemistry, including chemical engineering and the design of chemical processes.