2.5/6 Hess's Law and Enthalpy of Formation

What is Hess’s law and what does it enable us to do as chemists

Hess’s law states that the ΔH of a reaction can be determined by adding the individual enthalpies of different reactions, whose sum provides us our desired reaction.

This enables us to find the ΔHr value for a reaction that we may not be able to complete experimentally.

What are the three rules when manipulating chemical equations and their enthalpies

1. If a reaction is multiplied by a constant, then its enthalpy value must also be multiplied as well.

2. If a reaction is reversed, then its enthalpy must change sign.

3. the product of one reaction will cancel with the reactant of another if they are identical in molecular composition, and vice versa

• products of different reactions add together if they are identical, this goes for two reactants as well.

What is enthalpy of formation

The enthalpy of formation (ΔHf) is the change in enthalpy when one mol of a compound if formed from its elements in standard state

• ΔHf = - ΔHd

what is an element in standard state

Standard state is the most stable form of the element at atmospheric pressure and at 25 degrees celsius

• H is a gas

• O is a gas

• Fe is a solid

When writing the ΔHf of a mol of a compound, how do you know whether its component elements are diatomic, or monatomic

Diatomic elements, which are elements which pair up together, are known using the HOFBrINCl acronym.

How to use Hess’s law to find the ΔHr of a reaction using the ΔHf of its individual components

When you are not provided reactions to use for hess’s law, you can use the ΔHf of each compound within the reaction.

The ΔHf values for the individual compounds can be found on a standardized table