Bond energy calculations
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The teaching part (1 min) is free to watch. A free account opens the whole lecture: 4 worked examples, 7 practice questions, 2 real past-paper questions with the answers explained, the mistakes that lose marks, the full-marks tip, a one-card summary and a quick quiz.
What this lecture covers
- The cost of a reaction
- Bond energy
- The formula
- The method
Read the transcript of the teaching part
Last time
Before we start: last time, in Electrolysis, you learned these points. Here they are again on one card. Keep them in mind, because today builds on them.
The cost of a reaction
Every chemical reaction is a rebuild. Old bonds are broken, which costs energy. New bonds are made, which gives energy back. Compare the two bills, and you know whether the reaction is exothermic or endothermic, and by how much.
Bond energy
Bond energy is the energy needed to break one mole of a particular bond. It is given in kilojoules per mole. The same amount of energy is released when that bond forms. A strong bond has a high bond energy. A double bond is stronger than a single bond between the same atoms. C=C is bigger than C–C.
The formula
Energy in is the total of the bond energies of the bonds broken. That is in the reactants. Energy out is the total of the bonds made. That is in the products. Delta H equals energy in minus energy out. Negative means exothermic. Positive means endothermic.
The method
First, write the equation, and check that it is balanced. Second, draw the structures, so that you can see every bond. Third, list the bonds broken, and multiply the number of each by its energy. Fourth, list the bonds made, in the same way. Fifth, subtract: broken minus made. Write the sign and the unit.
How often it comes up
How often Cambridge has asked each part, counted by ExamYard from every past paper in its question bank: Bond energy calculations · Bond energies and enthalpy change.
The whole chapter: Chemical Energetics, Chemical Energetics.
Chapter 5: Chemical energetics
- Exothermic and endothermic reactions, pathway diagrams 6 min
- Bond energy calculations 8 min
Also for IGCSE Chemistry 0620
Which topics come up most in IGCSE Chemistry 0620 past papers, counted from every paper since 2004.