Question

1 g of C6H6(l) (benzene) is burned in an adiabatic bomb calorimeter (constant volume). T before ignition was 20.826 °C and 25.000 °C after combustion (remember that the energy change of the bomb is opposite the energy change of the combustion). The heat capacity of the calorimeter was 10.000 kJ K-1. Calculate fHm for benzene at 298.15 K from this data (make sure to convert your enthalpies to molar values). Use the tables in the back of the book (resource section) for the fHm of the non-C6H6 species, and don’t forget the PV terms. Compare your calculated value for benzene to the one listed in the table.

Answer 1

2 C₆H₆ (l) + 12O₂(g) ------> 12 CO₂(g) + 6H₂O(l).

T before ignition= 20.826 °C

T after ignition = 25.000 °C

ΔT = 25.000 °C - 20.826 °C = 4.174 °C

Convert 4.174 °C into Kelvin by adding 273.15

4.174 + 273.15 = 277.324 K

The heat capacity of the calorimeter = 10.000 kJ K-1.     

q rxn = - qcal ΔT

= -10.000 kJ K-1 x 277.324 K = - 2773.240 kJ

The ΔH calculated above was for 1g of benzene, now convert 1g of benzene to moles of benzene by dividing with molar mass of benzene.

Molar mass of benzene = 78 g/mol

1g of benzene = 1g /(78 g/mol) = 0.0128 mol

ΔHm for benzene = - 2773.240 kJ/0.0128 mol

ΔHm for benzene = -216659.375 kJ/mol