When a 3.80-g sample of liquid octane (C8H18) is burned in a bomb calorimeter, the temperature of the calorimeter rises by 26.5 ∘C. The heat capacity of the calorimeter, measured in a separate experiment, is 6.21 kJ/∘C . You may want to reference (Page 265) Section 6.5 while completing this problem. Part A Determine ΔE for octane combustion in units of kJ/mol octane. Express your answer using three significant figures.
Combustion reaction at 298 K of octane
2 C8H18 (l) + 25 O2 (g) 16 CO2 (g) + 18 H2O (l)
Now, heat change in calorimeter( Qcal) = Cm × temperature change
{Cm is heat capacity of calorimeter. }
Then, Qcal = 6.21× 26.5 = 164.565 KJ.
Now, Qcombustion = - Qcal
then Qcombustion = - 164.565 KJ
Moles of octane = mass ÷ molar mass = 3.80÷114 = 0.03247
Then, = - 164.565÷0.03247 = - 5068.21 KJ/mole
Now,
= + RT. (1)
At , T =298 K
R = 8.314 ×10-3 KJ/mole K
= total moles of gaseous products - total moles of gaseous reactants
n =( 16 - 25)
n = - 9
Then, putting the above values in Eq.1
H = E - 9× (8.314×298)×10-3 (KJ/mole)
Or, - 5068.21 = E - (9×8.314×298)×10-3
Or, E = (- 5068.21) + (9×8.314×298)×10-3
= ( -5068.21 + 22.29 ) KJ/mole
Or, E = - 5045.92 KJ /mole
Or, E = - 5.05×103 KJ/mole ( upto 3 significant figures)
When a 3.80-g sample of liquid octane (C8H18) is burned in a bomb calorimeter, the temperature...
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