Chemistry · Reactivity 1: What drives chemical reactions?. The idea in plain words, the rules to remember, the mistake to avoid and a worked example, written and checked by Mega Lecture.
Breaking a bond always takes in energy, so it is endothermic. Making a bond always gives out energy, so it is exothermic. A reaction is exothermic overall when the new bonds release more energy than was needed to break the old ones.
A bond enthalpy is the energy needed to break one mole of a bond in gaseous molecules. The values in the data booklet are averages over many compounds, so answers from them are only estimates.
Hess's law says the enthalpy change depends only on the reactants and the products, not on the route. So you can add equations together, like an algebra sum, to find a ΔH that is hard to measure directly.
Students work out bonds formed − bonds broken and get the wrong sign. It is always broken − formed. In Hess's law questions, they forget to change the sign of ΔH when they reverse an equation.
Answer: ΔH = −114 kJ
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