Question

What is the heat of combustion (Δ H ° rxn ) for acetylene (C 2 H 2 ) , assuming that the combustion products are CO 2 ( g ) and H 2 O( g ). Hint make sure to balance the equation. Instead of determining heats of reaction from heats of formation we sometime use average bond dissociation energies to calculate heats of reaction. Why would we need to use bond dissociation energies and how would your formula change?

Species Δ H ° f , kJ/mol:

C 2 H 2 ( g )= 227.4

CO 2 ( g )= - 393.5

H 2 O( g )= - 241.8

Instead of determining heats of reaction from heats of formation we sometime use average

bond dissociation energies to calculate heats of reaction. Why would we need to use bond

dissociation energies and how would your formula change?

Answer 1

the reaction

C2H2 + O2 = CO2 + H2O

balance

C2H2 + 5/2O2 = 2CO2 + H2O

the heat of combustion

HRXN = Hproducts - Hreactant = 2CO2 + H2O - (C"H2 + 5/2O2)

HRxn = 2*-393.5 + - 241.8 - (227.4+5/2*0)

HRxn = -1256.2 kJ/mol

Why would we need to use bond dissociation energies and how would your formula change?

Heat of bond dissociation requires us to find all the bonds formed/broken.

Recall that we will have dissociated energies of several compounds so this is just "average" of bonding, therefore, not 100% precise, but still very proximate.

Fromula changes:

HRxn = H-bonds formed - H-bonds broken