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.
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