When gaseous methane is made to react with water vapour, carbon monoxide and hydrogen gas is formed. This is a way of large scale manufacture of hydrogen. This is an example of chemical hydrocarbon cracking in which thermal hydrocarbon cracking is combined with oxidation of the carbon, the oxygen being extracted from water.
CH4(g)+H2O(g)→ CO(g)+3H2 ΔH = + 206.2 kJ/mol
The equation shows that methane and hydrogen form carbon monoxide and water only when some energy is added. High temperatures thus favour the reaction (formation of CO/H2). Subsequent reaction of CO with steam can convert it to CO2. The reaction is carried out under elevated pressure at 700– 830ᴼC in presence of Nickel catalyst, or at 1200-1500ᴼC. This reaction is also called steam reforming of methane.
Enthalpy is an extensive property. For two moles of methane, the change in enthalpy value will be double that of one mole.
Thus for 2 moles of methane, the enthalpy change would be 2*206.2 = 412.4 kJ.