In a reaction, both endothermic and exothermic processes happen, BUT, the overall description of the reaction depends on which is greater.
In the reactants... bonds are broken = EXOTHERMIC
In the products ... bonds are made = ENDOTHERMIC
Exothermic, releases energy, and is -H
Endothermic, takes in energy, and is +H
Combustion is always exothermic is heat is always released, so there is more energy in the reactants than in the products. Energy has been lost.
Overall enthalpy change in reaction =
Reactants - products
*in an exam, they usually give you the standard enthalpy of the reactants and products. If it tells you in the question that it is combustion then you should get a negative answer.
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