Question

Is combustion enthalpy change always negative?

Answer 1

Yes.

Explanation:

A combustion reaction usually refers to the reaction of a hydrocarbon with oxygen that produces carbon dioxide, water, and heat.

Here's an example of a combustion reaction! This is the complete combustion of propane. :)

`C_3H_8 + 5O_2 -> 3CO_2 + 4H_2O + "heat"`

The definition of combustion reactions as reactions that release heat tells us that the enthalpy change of the system, `DeltaH`, 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.

If it decreases, the final value would be less than the initial value—giving us a negative number for `DeltaH`.