The standard enthalpy change for a reaction is the enthalpy change when all reactants and products are in their standard state.
Chemists find it convenient to tabulate and compare enthalpy changes for various defined processes. An example of such a process is combustion, that is the reaction of a substance with oxygen to form the oxides of the elements.
The standard enthalpy of combustion of a substance is equal to the enthalpy change for reaction of one mole of a substance with oxygen where all reactants and products are in their standard state at the temperature specified.
As you will be aware from your everyday experience with combustion reactions, product formation is accompanied by release of heat. Therefore combustion reactions are exothermic and standard enthalpies of combustion are negative.
Chemists find it convenient to tabulate and compare enthalpy changes for various defined processes. An example of such a process is combustion, that is the reaction of a substance with oxygen to form the oxides of the elements.
The standard enthalpy of combustion of a substance is equal to the enthalpy change for reaction of one mole of a substance with oxygen where all reactants and products are in their standard state at the temperature specified.
Consider below the reactions for which the enthalpy change is equal to the standard enthalpy of combustion at 25 °C of two fuels, hydrogen and methane.
H2(g) + ½O2(g)
H2O(l)
ΔH° = ΔcH°(H2) = –286 kJ mol–1
CH4(g) + 2O2(g) CO2(g) + 2H2O(l)
ΔH° = ΔcH°(CH4) = –802 kJ mol–1
Fractional coefficients may be required to balance the equation for these reactions because the coefficient of the substance reacting with oxygen must be one.
H2(g) + ½O2(g)
H2O(l)ΔH° = ΔcH°(H2) = –286 kJ mol–1
CH4(g) + 2O2(g)
CO2(g) + 2H2O(l)ΔH° = ΔcH°(CH4) = –802 kJ mol–1
Fractional coefficients may be required to balance the equation for these reactions because the coefficient of the substance reacting with oxygen must be one.
As you will be aware from your everyday experience with combustion reactions, product formation is accompanied by release of heat. Therefore combustion reactions are exothermic and standard enthalpies of combustion are negative.