Question

A 1:3 mixture of CO and H2 is passed through a catalyst to produce methane at 500 K. CO(g) + 3H2(g) = CH4(g) + H2O(g) How much heat is liberated in producing a mole of methane? How does this compare with the heat obtained from burning a mole of methane at this temperature? How does the heat of combustion of CH4 compare with the heat of compustion of CO+3H2? (Answers: 206.11 kJ is liberated; Burning CH4 liberates 802.34 kJ; Burning CO + 3H2 liberates 1008.45 kJ).

Answer 1

The heat of combustion (ΔH⁰c) is the energy released in the form of heat when a compound undergoes complete combustion with oxygen under standard condition. Combustion is an oxidation reaction.

It is generally expressed in units of higher heating value, lower heating value and gross heating value. The calorific value of a substance (fuel or food ) is the amount of heat released during the combustion of a specified amount of that subatance.. It may be calculated as the difference between the heat of formation (ΔH⁰f) of the products and reactants.

Combustion is exothermic reaction because the double bond in molecular oxygen is much weaker than other double bonds or pairs of single bonds, specially those in carbon dioxide and water, conversion of the weak bonds in O₂ to the stronger bonds in CO₂ and H₂O releases energy as heat.

Bond energy of CO (triple bond) is 1079 KJ/mol and methane(single bond) is 414KJ/mol.

1 mole of CO = 28 g   and   1 mole of CH₄ = 16 g

CO(g) + 3H₂(g) CH₄(g) + H₂O(g)                   Heat = 1008.45 KJ
CH_4(g)   + 2O_2(g)         CO_2(g) + 2H₂O_(l)    Heat = 802.34 KJ         

During combustion all the covalent bonds in the reactant are broken, amount of heat liberated in the reaction is related to the strength of these bonds and the strength of the bonds formed in the products So it can provide information about the structure of molecules and differences in this heat of combustion value shows structural variations within the molecules.