Question

Use bond energies to estimate the heat for the combustion of 2.6 g of acetylene gas C2H2 to form carbon
dioxide and water vapor.

Answer 1

Balanced combuiton reaction is

2C₂H₂ + 5O₂ 4CO₂ + 2H₂O

Bond enthalpy of

C C = 963 kJ/mol

O = O = 498 KJ/mol

C - H = 414 kJ/mol

O - H = 465 kJ/mol

C = O = 532 kJ/mol

H⁰ = H⁰ (Reactant bonds) - H⁰ (product bonds)

H⁰ = [2 H⁰(C C) + 4H⁰(C - H) + 5H⁰(O = O) ] - [8H⁰(C = O) + 4H⁰(O - H) ]

Substitute the bond enthalpy in above equation

[2(963 kJ/mol) + 4( 414 kJ/mol) + 5(498)] - [8(532 kJ/mol) + 4( 465 kJ/mol) ]

(6072 kJ/mol) - (6116) = -44 kJ/mol

H⁰ for your reaction is = -44 kJ/mol

-44 kJ/mol is for combution of 2 mole of C₂H₄

molar mass of C₂H₄ = 28.05 gm/mole then 2 mole C₂H₄ = 2 X 28.05 = 56.1 gm

for combution of 56.1 gm C₂H₄H⁰ for your reaction is = -44 kJ/mol then for 2.6 gm C₂H₄H⁰ for your reaction is = -44 X 2.6 / 56.1 = -2.04 KJ

for coombution of 2.6 gm acetylene gas H⁰ = -2.04 KJ