Bond energies and calorimetry
Introduced| English | Meaning |
|---|---|
| bond energy/bɒnd ˈenədʒi/ | The energy required to break one mole of a specified covalent bond in gaseous molecules. |
| endothermic/ˌendəʊˈθɜːmɪk/ | Describing a process that takes in heat from its surroundings. |
| exothermic/eɡzəˈðɜːmɪk/ | Describing a process that transfers heat to its surroundings. |
| specific heat capacity/spəˈsɪfɪk hiːt kəˈpæsɪti/ | The energy needed to raise the temperature of one kilogram of a substance by one kelvin or one degree Celsius. |
| calorimetry/ˌkælɔːˈrɪmətri/ | Measurement of heat transferred during a physical or chemical process. |
Counting energy in bonds
- During a reaction, old bonds break and new bonds form.
- Breaking takes energy in; making gives energy out.
- We can also measure $\Delta H$ in the lab.
Which statement is correct?
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Energy is needed to break bonds and released when bonds form; ΔH is the difference.
Breaking bonds is endothermic and making bonds is exothermic.
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ΔH = bonds broken − bonds made.
Bond energy 键能 calculations
- Breaking a bond is endothermic 吸热; making a bond is exothermic 放热.
- The bond energy is the energy to break one mole of a bond (gas state) — always positive.
- Some values are exact; others are averages over many molecules, so the answer is approximate.
Breaking bonds takes in energy and making bonds gives it out
Energy in, energy out
ΔH = (energy to break bonds) − (energy released forming bonds). Adjust the activation energy and the overall change to compare exothermic and endothermic profiles.
Using bond energies, ΔH of reaction equals:
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ΔH = Σ(bonds broken) − Σ(bonds made); energy in to break minus energy out to make.
Why are calculations using average bond energies only approximate?
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Average bond energies are means over many molecules, so they don't match any single molecule exactly.
Measuring in the lab
- When a reaction heats a known mass of solution, the heat transferred is:
- ($m$ = mass, $c$ = specific heat capacity 比热容, $\Delta T$ = temperature change.) Then per mole:
- The minus sign makes $\Delta H$ negative when the temperature rises (exothermic).
Burning fuel is exothermic, releasing energy to the surroundings
The heat transferred to a solution is calculated with:
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q = mcΔT (mass × specific heat capacity × temperature change); ΔH per mole = −mcΔT/n.
Match each idea to what it means.
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Each item links the term to its correct meaning.
A bond-energy calculation
Worked example. ΔH = (bonds broken) − (bonds made). If breaking needs +1500 kJ and making releases −1800 kJ, ΔH = 1500 − 1800 = −300 kJ/mol (exothermic).
Measuring the temperature change of a known mass of solution
You've got it
- breaking bonds is endothermic; making bonds is exothermic
- $\Delta H_r = \sum(\text{bonds broken}) - \sum(\text{bonds made})$
- average bond energies give only an approximate answer
- calorimetry 量热法: $q = mc\Delta T$, then $\Delta H = -\dfrac{mc\Delta T}{n}$