ΔG = ΔH − TΔS
The free energy change due to the reaction can be calculated by substituting the information
given for ΔH, T, and ΔS. Be sure that the units are consistent when solving the equation.Thus, 64.4 kJ can be done by the system.10 . B
The change in enthalpy, or heat of reaction, for any reaction is the sum of the enthalpies of
formation of the products minus the sum of the enthalpies of formation of the reactants:
ΔHrxn = ΣΔHf (products) − ΣΔHf (reactants)
Substituting the heats of formation of BrF 5 (g), Br (g), and F (g) into the equation, the heat of
reaction can be calculated:Since no bonds were broken in this reaction, the (magnitude of the) heat of formation here is
equal to the sum of the bond energies of the product. As there are five equivalent Br–F bonds,
each one would contribute to one-fifth of the total bond energy. This means that for each
bond formed, an average of 187 kJ/mol of energy was released (note the negative sign in front
of the enthalpy). The bond energy is defined to be the energy required to break the bond, the
reverse of the process we have just described. The Br–F bond energy is therefore 187 kJ/mol.
As it always requires energy to break a bond, all bond enthalpies are positive.11 . B
Heats of reaction can be calculated from bond energies using the following equation: