Estimating Enthalpy Of Reaction With Average Bond Enthalpies
TECTechnics Classroom   TECTechnics Overview

Estimating Enthalpy Of Reaction With Average Bond Enthalpies

Average bond enthalpies are use to estimate the enthalpy of reaction when the reactants and products are gases and their standard enthalpies of formation are unavailable. In such reactions reactants' bonds are broken and new bonds are made to form the products. The estimated enthalpy of reaction can be used to determine whether the reaction will be endothermic (ΔH > 0) or exothermic (ΔH < 0).
Natural gas is sometimes coverted to other useful hydrocarbons. The coversion of methane to ethane is one such reaction:
2CH3(g) -------> C2H6(g) + H2(g)
(a) Use bond enthalpies to estimate ΔH for the reaction.

In practice, the conversion of methane to ethane is carried out in the presence of oxygen as follows:
2CH3(g) + (1/2)O2(g) -------> C2H6(g) + H2O(g)
(b) Use bond enthalpies to estimate ΔH for the reaction.
(c) Why is the conversion of methane to ethane more favorable when oxygen is used?

The strings: S7P6A66 (grouping/interaction - chemical).

The math:
Pj Problem of Interest is of type grouping/interaction (chemical).

Equation of interest:
Enthalpy of reaction = ΔHrxn = Σ(bond enthalpies of bonds broken) - Σ(bond enthalpies of bonds formed).

(a) CH4 has 4 C-H bonds
From average bond enthalpies table:
Bond enthalpy of a C-H bond = 413 Kj/mol
So, bond enthalpies of bonds broken = (2x4)(413) = 3304 kJ
C2H6 has 6 C-H bond and 1 C-C bond
H2 has 1 H-H bond
From average bond enthalpies table:
Bond enthalpy of C-C bond = 348 kJ/mol
Bond enthalpy of H-H bond = 436 kJ/mol
So, bond enthalpies of bonds formed = 6(413) + 348 + 436 = 3262 kJ
So, ΔHrxn = 3304 - 3262 = +42 kJ.

(b) CH4 has 4 C-H bonds; O2 has 1 O-O bond
From average bond enthalpies table:
Bond enthalpy of a C-H bond = 413 Kj/mol
Bond enthalpy of a O-O bond = 495 kJ/mol
So, bond enthalpies of bonds broken = (2x4)(413) + (1/2)(495) = 3304 + 247.5 = 3551.5 kJ
C2H6 has 6 C-H bond and 1 C-C bond
H2O has 2 H-O bond
From average bond enthalpies table:
Bond enthalpy of C-C bond = 348 kJ/mol
Bond enthalpy of H-O bond = 463 kJ/mol
So, bond enthalpies of bonds formed = 6(413) + 348 + 2(463) = 3752 kJ
So, ΔHrxn = 3551.5 - 3752 = -200.5 kJ.

(c) The conversion of methane to ethane in the presence of oxygen is more favorable because the formation of 2 mols of H-O bond is more exothermic than the formation of 1 mol of H-H bond.

Math

Blessed are they that have not seen, and yet have believed. John 20:29

TECTechnic Logo, Kimberlee J. Benart | © 2000-2021 | All rights reserved | Founder and Site Programmer, Peter O. Sagay.