Standard enthalpies of formation. Hf° is the change in enthalpy associated with the formation of a compound from the elements. Everything is in its standard state

The heat of formation of NO2 is

From a table of Hf° values reactions can be put together using Hess's Law.

Determine the H for the following reaction

Construct each compound from its elements and the sum the Hf °.

The reactions are...

Use the Hf ° reactions to make the reaction of interest.

Since MgO is a reactant the MgO formtation reaction must be turned around. Since the reaction is turned around the DH must also be turned around; i.e., the additive inverse is used.

You may have noticed that we do not have to write down all of the reactions.

You may have noticed that the Hf° of the reactants is always subtracted from the Hf° of the products.

H = Hf°prod - Hf°reactants

Calculate H for the following reaction using Hf ° values.

construct each compound from its elements and the sum the Hf°'s

H = Hf°prod - Hf°reactants

H = 6(-242 kJ) + 2(135.2 kJ) - 2(-75 kJ) - 3(0 kJ) - 2(-46 kJ) = -940 kJ