Bond energy calculations
Introduced| English | Meaning |
|---|---|
| chemical bonds/ˈkemɪkl bɒndz/ | chemical bonds |
| bond energies/bɒnd ˈenədʒiz/ | bond energies |
| endothermic/ˌendəʊˈθɜːmɪk/ | Describing a process that takes in heat from its surroundings. |
| exothermic/eɡzəˈðɜːmɪk/ | Describing a process that transfers heat to its surroundings. |
Energy in the bonds
- Where does a reaction's energy actually come from? From breaking and making chemical bonds 化学键.
- We can calculate the overall energy change from bond energies 键能.
Breaking chemical bonds:
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Breaking bonds takes energy in.
Making chemical bonds releases energy.
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Bond formation is exothermic.
Breaking and making bonds
- Breaking bonds absorbs energy (endothermic 吸热反应).
- Making bonds releases energy (exothermic 放热反应).
Breaking bonds takes energy in; making bonds gives energy out; the difference is the enthalpy change
Bonds broken need 500 kJ; bonds made release 700 kJ. What is the energy change (kJ)?
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500 − 700 = −200 kJ.
Energy change = energy to break bonds − energy released ______ bonds.
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Broken minus made gives the overall change.
The energy change
- Energy change = energy to break bonds − energy released making bonds.
- A negative answer means the reaction is exothermic; positive means endothermic.
Worked example. Bonds broken need 600 kJ; bonds made release 800 kJ. Energy change = 600 − 800 = −200 kJ. Negative, so the reaction is exothermic.
Breaking and making bonds transfers energy; the flame supplies it here
Read the energy profile
Breaking bonds takes energy IN; making bonds gives energy OUT. If more is released than absorbed the reaction is exothermic and ΔH is negative — products sit below reactants.
A negative energy change means the reaction is:
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Negative = energy released = exothermic.
Why the sign matters
- A negative energy change = energy released = exothermic.
- A positive energy change = energy absorbed = endothermic.
Breaking bonds takes energy in; making bonds gives energy out. Students often get this backwards. If more energy is released (making) than used (breaking), the reaction is exothermic overall.
You've got it
- breaking bonds absorbs energy; making bonds releases energy
- energy change = bonds broken − bonds made
- negative = exothermic; positive = endothermic