Question

Which of the following statement is correct regarding the heat of combustion of combustible substances?

• A. It may be exothermic as well as endothermic
• B. It is applicable to gaseous substances only
• C. It is always an exothermic reaction
• D. Its value does not change with temperature

Answer 1

The correct option is C It is always an exothermic reaction

Combustion reactions are always exothermic reactions since they are accompanied by release of heat, so the heat of combustion is always negative in sign. It is applicable to all substances.
Heat of combustion value changes with temperature. Every reaction enthalpy changes with temperature as individual enthalpy of reactants and products vary differently with temperature. Hence the enthalpy change in a reaction will vary with temperature. Kirchoff equation deals with this enthalpy change with temperature.