The standard change of enthalpy (ΔH°) is a measure of the heat energy released or absorbed during a chemical reaction at standard conditions (25°C and 1 atm). It is also known as the heat of reaction.
The standard change of enthalpy can be calculated using Hess's Law, which states that the total enthalpy change of a reaction is independent of the pathway taken. This means that the initial and final states are the same, regardless of the number of steps taken to get there.
Hess's Law is a principle in thermodynamics that states the total enthalpy change of a reaction is independent of the pathway taken, as long as the initial and final states are the same. This allows for the calculation of the standard change of enthalpy by combining known enthalpy changes of other reactions.
To apply Hess's Law, the known enthalpy changes of other reactions are used to create an overall reaction that is equivalent to the desired reaction. The enthalpy changes of the individual reactions are then added algebraically to determine the overall enthalpy change, which is equivalent to the standard change of enthalpy for the desired reaction.
The standard change of enthalpy is typically measured in kilojoules per mole (kJ/mol) or joules per mole (J/mol). These units represent the amount of heat energy released or absorbed per mole of reactant or product in the reaction at standard conditions.