Bond energies and calorimetry
| English | Chinese | Pinyin |
|---|---|---|
| bond energy | 键能 | jiàn néng |
| endothermic | 吸热 | xī rè |
| exothermic | 放热 | fàng rè |
| specific heat capacity | 比热容 | bǐ rè róng |
| calorimetry | 量热法 | liàng rè fǎ |
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?
Energy is needed to break bonds and released when bonds form; ΔH is the difference.
Breaking bonds is endothermic and making bonds is exothermic.
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:
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?
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:
q = mcΔT (mass × specific heat capacity × temperature change); ΔH per mole = −mcΔT/n.
Match each idea to what it means.
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 $\Delta H = -\dfrac{mc\Delta T}{n}$