Standard Enthalpy of Reactions

Standard Enthalpy of Reactions

Enthalpy of a reaction depends on the conditions under which the reaction is carried out. Temperature, pressure, and physical states all affect the value. Therefore, we must specify standard conditions.

The standard enthalpy of reaction is the enthalpy change for a reaction when all participating substances are in their standard states.

The standard state of a substance at a specified temperature is its pure form at 1 bar pressure. For example:

• Standard state of liquid ethanol at 298 K is pure liquid ethanol at 1 bar
• Standard state of solid iron at 500 K is pure iron at 1 bar

Although the standard state can be defined at any temperature, data are usually reported at 298 K (25°C).

Standard conditions are denoted by adding the superscript $^\circ$ to the symbol $\Delta H$, for example $\Delta H^\circ$. …