Hess's Law & Bond Enthalpy Calculations (AQA A Level Chemistry): Video

Hi, I'm Eleanor with 3 years of experience teaching Chemistry, and this video is about Hess's Law and bond enthalpy calculations.

Both let you calculate the enthalpy change of a reaction without measuring it directly, and they give different answers because bond enthalpies are averaged across many compounds, while Hess's Law uses real data specific to this reaction.

Every reaction has an enthalpy change because bonds are broken and formed: breaking bonds takes in energy, forming bonds releases it, and the balance between the two is the enthalpy change. Hess's Law and bond enthalpies are two different ways of calculating that change without measuring it directly, and because they use different kinds of data, they give different answers for the same reaction.

This video starts with why bond breaking and forming causes an enthalpy change, then covers Hess's Law and how it's applied using real formation and combustion data, and finishes with bond enthalpies, which estimate the same enthalpy change from averaged bond data.

Breaking a bond takes in energy from the surroundings, so bond breaking is endothermic. Forming a bond releases energy to the surroundings, so bond forming is exothermic. Whether a reaction is overall exothermic or endothermic depends on the balance between the energy needed to break the reactants' bonds and the energy released forming the products' bonds. This balance is the enthalpy change that Hess's Law and bond enthalpies each calculate in a different way.

Products can be formed from elements either directly, or indirectly by first forming the reactants and then converting them to products. Because both routes start and end in the same place, the enthalpy change is the same either way: the direct route's enthalpy change equals the indirect route's. Rearranging that equation lets you calculate the enthalpy change of a reaction, delta H r, from data that can be measured experimentally.

Hess's Law cycles are set up differently depending on whether you're given formation or combustion data. With formation data, the arrows point upward from the elements to each compound. With combustion data, the arrows point downward from each substance to its combustion products. A quick way to remember this is form up, burn down. Once the cycle is drawn, going from reactants to products and reversing the sign of any arrow pointing the wrong way gives the enthalpy change of the reaction.

Bond enthalpies can't be measured directly for a specific bond in a specific molecule, so a mean bond enthalpy is used instead — this is the enthalpy needed to break one mole of a given bond, averaged across a range of different compounds containing that bond. The enthalpy change of a reaction can then be estimated as the total bond enthalpies of the bonds broken minus the total bond enthalpies of the bonds formed, assuming everything is in the gaseous state. Because this method uses averaged values rather than the specific bonds broken in this reaction, the answer it gives differs from the value calculated using Hess's Law, which uses real data for this particular set of compounds.

When setting up a Hess's Law cycle, remember: form up, burn down. And when applying bond enthalpies, always calculate bonds broken and bonds formed separately before subtracting — subtracting the wrong way round is a common and easy mistake to make.

Every enthalpy change comes from the balance between the energy needed to break bonds and the energy released forming new ones. Hess's Law calculates that balance using real formation or combustion data specific to the reaction. Bond enthalpies estimate the same balance using mean values averaged across many different compounds. Because one method uses real, specific data and the other uses an average, the two calculations give different answers for the same reaction.

On Save My Exams, you'll find exam specific revision, made by trusted examiners. Study with revision notes, practice with exam questions, and improve your grade by targeting your weak spots. Visit the links in the description.