The complete combustion of methanol proceeds according to the equation 2CH₃OH plus 3O₂ react to form 2CO₂ plus 4H₂O. The reaction enthalpy is negative 715 kilojoules per mole. Is the combustion of methanol endothermic or exothermic?
Generally speaking, when we say combustion, we mean reaction with oxygen. In the complete combustion of methanol, two molecules of methanol react with three molecules of O₂, oxygen, to form two molecules of carbon dioxide and four molecules of water. The question references reaction enthalpy. Strictly speaking, a reaction enthalpy is a change in enthalpy of the system. When I say system, I’m referring just to the reactants and the products. We can think about the total enthalpy of the system before and after the chemical reaction. The enthalpy change because of the reaction is negative. Meaning that the enthalpy after the reaction is less than the enthalpy before it. So what this is telling us is that, because of the reaction, overall energy has left the system and gone into the surroundings.
The value for the reaction enthalpy tells us that, for every two moles of methanol and three moles of oxygen reacted, 715 kilojoules of energy will be transferred from the system to the surroundings. An endothermic reaction is one where more energy comes in, then goes out. While an exothermic reaction is the reverse where more energy comes out, then goes in. As shown in the question, the complete combustion of methanol has a negative reaction enthalpy. This means that the energy out is greater than the energy in.
Therefore, the combustion of methanol is an exothermic reaction.