Hess' law of Additivity

Sometimes the heat of a reaction for a chemical change is not easily measured due to WHAT of a reaction, cost, rarity of WHAT etc so we use Hess’ law to calculate WHAT

Sometimes the heat of a reaction for a chemical change is not easily measured due to TIME of a reaction, cost, rarity of REACTANTS etc so we use Hess’ law to calculate ΔrH

Because of the law of conservation of energy, the heat of reaction is the WHAT whether the reactants are converted to the products in a WHAT or in a WHAT

Because of the law of conservation of energy, the heat of reaction is the SAME whether the reactants are converted to the products in a SINGLE REACTION or in a SERIES of REACTIONS

Hess suggested that if two or more thermochemical equations are added to give a final equation then the enthalpies can be added to give the WHAT for the WHAT equation

Hess suggested that if two or more thermochemical equations are added to give a final equation then the enthalpies can be added to give the ENTHALPY for the FINAL equation

Steps

1. Write the WHAT, if not given

2. WHAT the given equations so they will WHAT to yield the WHAT

• If you multiply or divide an equation, multiply or divide the WHAT by the WHAT

• If you flip and equation than you WHAT the sign on ΔH

3. WHAT the reactants and products where possible…SUBTRACT if WHERE sides and ADD if on WHAT sides (must end up with your net equation)

4. WHAT the component enthalpy changes to get the WHAT

Steps

1. Write the NET EQUATION, if not given

2. MANIPULATE the given equations so they will ADD to yield the NET EQUATION

• If you multiply or divide an equation, multiply or divide the ΔH by the SAME FACTOR

• If you flip and equation than you FLIP the sign on ΔH

3. SIMPLIFY the reactants and products where possible…SUBTRACT if on OPPOSITE sides and ADD if on SAME sides (must end up with your net equation)

4. ADD the component enthalpy changes to get the NET ENTHALPY CHANGE