Complete the following Hess's law energy cycle relating butane to its standard enthalpy change of formation, ΔHϴf, and standard enthalpy change of combustion, ΔHϴc.
On your diagram:
include the relevant species in the two empty boxes
label each enthalpy change with its appropriate symbol
complete the remaining two arrows to show the correct direction of enthalpy change
Use the data in Table 1.1 to calculate the standard enthalpy change of combustion of butane, ΔHϴc. Show your working.
Table 1.1
Substance | Standard enthalpy change of formation, ΔHϴf / kJ mol-1 |
|---|---|
CO2 (g) | −393.5 |
C4H10 (g) | −125 |
H2O (l) | −286 |
ΔHϴc (C4H10) = .........................................kJ mol-1
Use the bond energies given in Table 1.2 to calculate the standard enthalpy change of combustion of butane. Show your working.
Table 1.2
Bond | Bond energy / kJ mol-1 |
|---|---|
C−C | 348 |
C−H | 410 |
C=O | 743 |
H−O | 463 |
O=O | 495 |
ΔHϴc (C4H10) = .........................................kJ mol-1
Suggest, with a reason, which of the two values for the standard enthalpy change of combustion from parts (b) and (c) is more accurate.
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