Example
Using bond enthalpies, what is the enthalpy change when hydrogen is added to ethyne to produce ethane?
\(C_{2}H_{2} (g) + 2H_{2} (g)\rightarrow C_{2}H_{6} (g) \)
To answer this we must look at what types of bonds must be broken in the reactants and formed in the products.
Follow these simple steps:
Step one
Draw the full structural formulae of all the molecules from the equation. This will show exactly what bonds are involved.
Step two
Make a list of all the bonds being broken in the reactants and their bond enthalpies using the data book.
Bond breaking
\(1\times{C}\equiv{C}=838kJ\)
\(2 \times C - H = 2 \times 412 = 824 kJ\)
\(2 \times H - H = 2 \times 436 = 872 kJ\)
\(Total\,\, put\,\, in = 2534 kJ\)
Step three
Repeat this process for the bond making steps and all new bonds formed.
Bond making
\(1 \times C - C = 348 kJ\)
\(6 \times C - H = 6 \times 412 = - 2472 kJ\)
\(Total\,\, given\,\, out = -2820 kJ\)
Step four
Calculate the enthalpy change for the reaction, remembering that bond breaking is an endothermic process and bond making is an exothermic process.
\(\Delta H = 2534 + (-2820) = 2534 - 2820 = -286 kJ mol^{-1}\)