Question

1. When 1.550 g of liquid hexane (C6H14) undergoes combustion in a bomb calorimeter, the temperature rises from 25.87∘C to 38.13∘C. Find ΔErxn for the reaction in kJ/mol hexane. The heat capacity of the bomb calorimeter, determined in a separate experiment, is 5.73 kJ/∘C. Express your answer in kilojoules per mole to three significant figures.

2. The combustion of toluene has a ΔErxn of –3.91×103 kJ/mol. When 1.55 g of toluene (C7H8) undergoes combustion in a bomb calorimeter, the temperature rises from 23.12∘C to 37.57∘C. Find the heat capacity of the bomb calorimeter. Express the heat capacity in kilojoules per degree Celsius to three significant figures.

Answer 1

1.
ΔErxn = -C * delta T
          = - 5.73*(38.13 - 25.87)
          = -70.25 KJ

number of moles = mass/molar mass
           = 1.55/ (6*12 + 14*1)
           =0.018 mol

ΔErxn = -70.25 KJ / 0.018 mol
          = -3.90*10^3 KJ/mol

2.
number of moles = mass/molar mass
           = 1.55/ (7*12 + 8*1)
           =0.017 mol

ΔErxn = ΔErxn per mol * number of moles
         = -3.91*10^3*0.017
         = -65.875 KJ

ΔErxn = C * delta T
-65.875= -C*(37.57 - 23.12)
C = 4.56 KJ/oC