In combustion reactions, some substances will release more energy than others. Enthalpies of combustion can be used to compare which fuels or substances release the most energy when they are burned. They can be calculated using a bomb calorimeter.

Why is enthalpy of combustion always negative?

The definition of combustion reactions as reactions that release heat tells us that the enthalpy change of the system, ΔH , will always be negative. This is because enthalpy is the total heat content of a system. Since heat is released for combustion reactions, the total heat content must decrease.

Why combustion is always exothermic?

Combustion is an oxidation reaction that produces heat, and it is therefore always exothermic. All chemical reactions first break bonds and then make new ones to form new materials. ... If the energy released by the new bonds is greater than the energy needed to break the original bonds, the reaction is exothermic.