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 takes in energy.
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False. Making bonds releases energy (exothermic); breaking bonds takes in energy (endothermic).
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
Write from an energy change of plus 150
- Breaking bonds takes in 400 kJ.
- Making bonds releases 250 kJ. The lesson case of 500 broken and 700 made is minus 200.
What is the energy change, and is the reaction exo or endo?
Energy change is energy in to break bonds minus energy out from making bonds.
What is the energy change?
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400 minus 250 is plus 150.