Chemistry: Enthalpy and Hess's Law

Enthalpy

Enthalpy of Reaction (ΔH)

• The enthalpy of reaction (ΔH) is the total energy change in a system during a chemical reaction.
• It is a measure of the total heat absorbed or released during a reaction.
• ΔH is typically measured in units of kJ/mol.

Hess's Law

• Hess's Law states that the total enthalpy change (ΔH) in a reaction is the same regardless of the number of steps involved.
• It allows us to calculate the enthalpy change for a reaction that occurs in multiple steps.
• Hess's Law is based on the principle that enthalpy is a state function, meaning that only the initial and final states of the system matter, not the pathway.

Standard Enthalpy Change (ΔH°)

• The standard enthalpy change (ΔH°) is the enthalpy change for a reaction under standard conditions (25°C, 1 atm, and 1M concentrations).
• ΔH° is a measure of the energy change for a reaction at standard conditions, and it is used as a reference point for comparing reactions.
• Standard enthalpy changes can be used to calculate the enthalpy change for a reaction at non-standard conditions using Hess's Law.

Key Points:

• Enthalpy of reaction (ΔH) measures the total energy change in a system during a reaction.
• Hess's Law allows us to calculate the enthalpy change for a reaction that occurs in multiple steps.
• Standard enthalpy change (ΔH°) is a measure of the energy change for a reaction under standard conditions.