Question

Use the data in Figure 7-18 and information from Section 3-7 to establish possible relationships between the molecular structure of the hydrocarbons and their standard enthalpies of formation.

Answer 1

Most clearly established in Figure 7-18 is the point that the enthalpies of formation for alkane hydrocarbons are negative and that they become more negative as the length of the hydrocarbon chain increases (~20 kJ per added CH2 unit). For three hydrocarbons of comparable chain length, C2H6, C2H4, and C2H6, we can also infer that the one having only single bonds (C2H6) has the most negative enthalpy of formation. The presence of a carbon-to-carbon double bond (C2H4) makes the enthalpy of formation more positive, while the presence of a triple bond (C2H2), makes it more positive still.