Question

Ethanol (CH3CH2OH) can be produced from sugars and starches in crops such as corn or sugarcane. Ethanol is used as a gasoline additive and when completely burned, it combines with oxygen to form CO2 and H2O. Write the balanced equation for the complete combustion reaction and calculate the net energy change per mole of ethanol. Bond Energy C – H → 416 kJ/mol O – H → 467 kJ/mol C – C → 356 kJ/mol C – O → 336 kJ/mol O=O → 498 kJ/mol C=O → 803 kJ/mol

Answer 1

Balanced Combustion reaction of Ethanol -

CH3CH2OH(g) + 3O2(g) ------> 2CO2(g) + 3H2O(g)

ethanol oxygen carbondioxide water

Calculating net energy change using given bond energies -

CH3CH2OH = 5(C-H) + 1(C-C) + 1(C-O) + 1(O-H)

= 5*(416kJ/mol) + 356kJ/mol + 336kJ/mol + 467kJ/mol

= 3239kJ/mol

O2 = O=O = 498kJ/mol

CO2 = 2(C=O) = 2*803kJ/mol = 1606kJ/mol

H2O = 2(O-H) = 2*267kJ/mol = 534kJ/mol

total bond energies of the reactants = DeltaH(CH3CH2OH) + 3*DeltaH(O2)

= 3239kJ/mol + 3*498kJ/mol

= 4733kJ/mol

total bond energies of products = 2*deltaH(CO2) + 3*deltaH(H2O)

= 2*1606kJ/mol + 3* 534kJ/mol

= 3212kJ/mol + 1602kJ/mol = 4814kJ/mol

net energy change per mole = DeltaH(reactants) - DeltaH(products)

= 4733kJ/mol - 4814kJ/mol

net energy change = -81kJ/mol