(2) What is the enthalpy of formation of dinitrogen tetroxide?
Equation (2) gives the standard reaction enthalpy of the formation of 1 mole of dinitrogen tetroxide from its elements. The enthalpy of formation of is 9.16 kJ/mol.
(3) What is the standard reaction enthalpy of equation (3)?
Equations (1) and (2) must be combined in a way such that is the lone reactant and
is the lone product. Running equation (2) in reverse gives as a reactant. When this equation is then added to equation (1), the result is:
The nitrogen and oxygen can be canceled from both sides of the equation to yield equation (3) as a result. Hess's law says that the enthalpy of the reaction in equation (4) is the sum of the enthalpies of the sequence of reactions used to create equation (4). Since equation (2) runs in reverse, the sign of its enthalpy is changed to the opposite sign. Therefore, the standard reaction enthalpy of equation (3) is given by
