Question

Use Hess’s Law to calculate the heat of combustion of dodecane using heat of formation data. For the calculation you will need the following:

 Heat of formation data taken from BLBMW Appendix C: fH of formation of carbon dioxide gas is –393.5 kJ/mol, and water (liquid) is –285.83 kJ/mol. and heat of formation of dodeane is -352.1 ± 1.4 kJ/mol

Answer 1

we know that

dodecane is C12H26

the combustion of dodecane is given by

C12H26 + 18.5 02 ---> 12 C02 + 13 H20

now

according to Hess`s law

dHo rxn = dHof products - dHof reactants

so

in this case

dHo rxn = ( 12 x dHof C02) + ( 13 x dHod H20) - ( dHof C12H26 ) - ( 18.5 x dHof 02)

now

heat of formation of elements in their standard state is zero

so

heat of formation of oxygen gas is zero

dHof 02 = zero

now

using given values

we get

dHo rxn = ( 12 x -393.5) + ( 13 x -285.83) - ( -352.1) - ( 18.5 x 0)

dHo rxn = -8085.69

now

in this case , the heat of reaction is heat of combustion

so

heat of combustion of dodecane is -8085.69 kJ