An alkane hydrocarbon has the formula C_nH_2n+2. The enthalpies of formation of the alkanes decrease (become more negative) as the number of C atoms increases. Starting with butane, for each additional CH₂ group in the formula, Δ_fH°, the enthalpy of formation, changes by about -21 kJ/mol. Use this fact and data from Table 7.2 to estimate the heat of combustion of heptane, C₇H₁₆(l).

Table 7.2

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TABLE 7.2 Some Standard Molar Enthalpies of Formation, AH° at 298.15 K Substance CO(g) CO₂(g) CH4(g) C₂H₂(g) C₂H4(g) C₂H6(g) C3H8(g) C4H10(g) CH₂OH(1) C₂H5OH(1) HF(g) HCl(g) kJ/mola -110.5 -393.5 -74.81 226.7 52.26 -84.68 -103.8 -125.6 -238.7 -277.7 -271.1 -92.31 Substance HBr(g) HI(g) H₂O(g) H₂O(1) H₂S(g) NH3(g) NO(g) N₂O(g) NO₂(g) N₂O4(g) SO₂(g) SO3(g) kJ/mola -36.40 26.48 -241.8 -285.8 -20.63 -46.11 90.25 82.05 33.18 9.16 -296.8 -395.7