Hess’s law

Definition

Hess's law states that the total enthalpy change for a chemical reaction is the same, no matter how many steps the reaction is carried out in. It relies on the fact that enthalpy is a state function.

5 Must Know Facts For Your Next Test

1. Hess's law can be used to determine the enthalpy change of reactions that are difficult to measure directly.
2. The law is based on the principle of conservation of energy.
3. It allows for the calculation of enthalpy changes by summing the enthalpies of multiple intermediate reactions.
4. Enthalpy changes are additive according to Hess's law.
5. This law applies only to processes where all reactants and products are in their standard states.

Review Questions

• How does Hess's Law utilize the concept of enthalpy being a state function?
• Why is Hess's Law useful for calculating enthalpy changes indirectly?
• What must be true about intermediate steps for Hess's Law to hold?

Related terms

Enthalpy:

A thermodynamic quantity equivalent to the total heat content of a system. It is equal to the internal energy of the system plus the product of pressure and volume.

State Function:

A property whose value does not depend on the path taken to reach that specific value but only on the current state of the system.

Standard State: The physical state of a substance under standard conditions (1 atm pressure and a specified temperature, usually 25°C).