Bond Energies and the Overall Energy Change
Breaking bonds takes energy in and making bonds gives energy out. Use a table of bond energies to calculate the overall change for a reaction and check the sign against the profile.
What a learner can do afterwards
- Totals the energy needed to break the bonds in the reactants and the energy released making the products
- Gets the sign right and links a negative overall value to an exothermic reaction
- Explains why burning a hydrocarbon in oxygen releases so much energy in terms of the bonds made
- Gives a reason a calculated value differs from one measured in a school experiment
1 · Read
When a hydrocarbon burns in oxygen, the C=O and O-H bonds made pay back more than breaking its C-H and O=O bonds costs. Breaking a bond takes energy in, so it is endothermic. Making a bond gives energy out, so it is exothermic. A bond energy is the cost of breaking one mole of that bond in a gas. To find the overall change, total the break cost and subtract the make payback.
The overall change is the break total minus the make total. Total the break cost across every bond in the reactants. Total the make payback across every bond in the products. Then subtract the second total from the first. Count every bond present: one O2 holds one double bond, while each water holds two O-H bonds.
Take H2 + Br2 -> 2HBr and sort the bonds. Broken on the left: one H-H plus one Br-Br. Made on the right: two H-Br. Look up each bond energy, multiply by its count, total both sides, and subtract. A negative answer means exothermic and warms its surroundings. A positive answer means endothermic.
Tables hold average bond energies, not exact values for your flask, so a table value can differ from a school measurement. Heat escaping to the surroundings adds to the gap. Compare with the profile: a negative value matches the exothermic drop.
Break totals minus make totals gives the change, and the sign names the reaction.
2 · Watch
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Where this leads
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8 questions wait behind this lesson, each with its answer explained. Every answer feeds the sky: stars light as they are learned, and dim when it is time to come back.