Complete the following Hess' Law energy cycle relating butane to enthalpies of formation and combustion.
enthalpy of formation, ΔHϴf
enthalpy 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 showing the correct direction of enthalpy change
Use the data in Table 1.1 to calculate the enthalpy of combustion of butane, ΔHθc
Table 1.1
Substance | Enthalpy of formation, ΔHθf / kJ mol-1 |
CO2 (g) | -393.5 |
C4H10 (g) | -125 |
H2O (g) | -242 |
ΔHθc (C4H10) = ............................................kJ mol-1
Using the values given in Table 1.2, calculate the enthalpy of combustion of butane.
Table 1.2
Bond | Bond Energy / kJmol-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 of enthalpy of combustion from parts (b) and (c) is likely to be more accurate.
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