Use the bond energies provided to estimate ΔH°rxn for the reaction below. CH3OH(l) + 2 O2(g) → CO2(g) + 2 H2O(g) ΔH°rxn = ?

Question

Use the bond energies provided to estimate ΔH°rxn for the reaction below.

                        CH3OH(l) + 2 O2(g) → CO2(g) + 2 H2O(g)            ΔH°rxn = ?

Bond         Bond Energy (kJ/mol)

C-H           414

C-O           360

C=O          799

O=O          498

O-H           464

Answer 1

CH3OH(l) + 2O2(g) ==> CO2(g) + 2H2O(g)

Let's look at the bond energies on the right side (products) and the left side (reactants):

PRODUCTS:

O=C=O is CO2, bond energy = 2x799 = 1598 kJ/mole x 1 mol = 1598 kJ

H2O is H-O-H, bond energy = 2x464 = 928 kJ/mole x 2 moles = 1856 kJ

Total products = 1598 + 1856 = 3454 kJ

REACTANTS:

CH3OH has 3 C-H bonds, 1 C-O bond and 1 O-H bond, bond energy = 3x414 kJ + 360 kJ + 464 kJ = 2066 kJ

O2 is O=O, bond energy = 498 kJ/mol x 2 moles = 996 kJ

Total reactants = 2066 + 996 = 3062 kJ

∆Hrxn = ∑reactants - ∑products = 3062 - 3454 = -392 kJ

(note: the sign is negative indicating an exothermic reaction, i.e. heat is given off during the combustion of methanol)