3.1.5Physical Chemistry
Bond Energy — Edexcel Linear - OL (Chemistry) Revision Notes & Topical Questions
5 min read 6 parts Lesson 7 of 9 Unit quiz
Section 1 of 6Breaking and Making Bonds
Every chemical reaction involves breaking old bonds and making new ones.
Bond breaking
- Energy must be taken in from the surroundings to break bonds, so bond breaking is endothermic.
Bond making
- Energy is released when new bonds form, so bond making is exothermic.
Overall change
- Whether a reaction is exothermic or endothermic depends on which of these two energy amounts is larger.
Bond energy
- The bond energy is the energy needed to break one mole of a particular bond, or released when it forms, in kJ/mol.
Section 2 of 6Exothermic and Endothermic Reactions
Exothermic and Endothermic Reactions
Exothermic
- More energy is released making new bonds than is taken in breaking old ones, so ΔH is negative.
Endothermic
- More energy is needed to break the old bonds than is released making new ones, so ΔH is positive.
Meaning of sign
- A negative ΔH shows energy leaves the system, so the surroundings warm up; a positive ΔH does the reverse.
Memory aid
- Bond breaking is ENDothermic, because it takes energy in and leads to the END of the bond.
| Type | Energy in vs out | ΔH |
|---|---|---|
| Exothermic | In < out | Negative |
| Endothermic | In > out | Positive |
Section 3 of 6Calculating ΔH from Bond Energies
Remember to multiply by the balancing numbers in the equation.
1
Draw the displayed formulae of all reactants and products so every bond can be counted.
2
List and add the energies of all bonds broken in the reactants; this is the energy in.
3
List and add the energies of all bonds formed in the products; this is the energy out.
4
Use ΔH = energy in − energy out.
5
Interpret the sign: negative means exothermic, positive means endothermic.
Section 4 of 6Typical Question Types
Typical Question Types
Finding ΔH
- Given all the bond energies, calculate the overall energy change and classify the reaction.
Finding a bond energy
- If ΔH is given, rearrange ΔH = energy in − energy out to find the missing bond energy.
Reversing the reaction
- The reverse reaction has the same size of energy change but the opposite sign.
Check
- A reaction that makes stronger bonds than it breaks must be exothermic.
Section 5 of 6Worked Example
Question Calculate ΔH for N₂ + 3H₂ → 2NH₃ using bond energies in kJ/mol: N≡N 945, H–H 436, N–H 391.
- Bonds broken1 × N≡N + 3 × H–H = 945 + (3 × 436) = 945 + 1308 = 2253 kJ.
- Bonds formed2 NH₃ each have 3 N–H bonds, so 6 × 391 = 2346 kJ.
- ΔH = energy in − energy out = 2253 − 2346 = −93 kJ.
- The sign is negative, so the reaction is exothermic.
Answer ΔH = −93 kJ/mol (exothermic)
Section 6 of 6Exam Checklist & Tips
3.1.5 – Key Points for the Exam
- Bond energy Energy required to break a particular bond, in kJ/mol.
- Exothermic A reaction that releases energy to the surroundings (ΔH negative).
- Endothermic A reaction that takes in energy from the surroundings (ΔH positive).
- ΔH The overall energy change of a reaction.
Common Mistakes
- Subtracting the wrong way round; it is energy in minus energy out.
- Forgetting to multiply bond energies by the coefficients in the equation.
- Counting bonds in a formula incorrectly, for example missing that NH₃ has three N–H bonds.
- Saying bond breaking releases energy.
Last updated:
\n