Question 1
How many N–H bonds are broken when 2NH₃ decomposes?
Need a hint for Question 1?
Write two separate totals: energy in for breaking and energy out for making.
Check Question 1
Six.
STUDENT SPACE · TOPIC 2
Bond energies
Calculate enthalpy changes from bonds broken and bonds formed.
The essentials
- Breaking bonds absorbs energy. Making bonds releases energy.
- Calculate the energy needed to break bonds and subtract the energy released when bonds form. Balance the equation before counting.
- H–H = 436, Cl–Cl = 242 and H–Cl = 431 kJ mol⁻¹.
Key vocabulary
| English | Chinese |
|---|---|
| bond breaking | 断键 |
| bond making | 成键 |
| bond energy | 键能 |
| energy released | 释放的能量 |
Quick practice
Try each question before opening its hint or answer. Use paper for calculations and diagrams.
Question 2
For H₂ + Br₂ → 2HBr, use H–H 436, Br–Br 193 and H–Br 366 kJ mol⁻¹ to calculate ΔH.
Need a hint for Question 2?
Add energies of bonds broken, then subtract energies of bonds formed.
Check Question 2
ΔH = 436 + 193 − 2(366) = −103 kJ mol⁻¹.
Question 3
Explain why an endothermic reaction can still involve energy release when bonds form.
Need a hint for Question 3?
Compare the total energy absorbed with the total released.
Check Question 3
Bond formation releases energy, but bond breaking absorbs more, so the net transfer is into the system.
Use Lesson 2 in your worksheet for written practice. Your teacher will discuss those answers in class.
PREDICT · TRY · EXPLAIN · 3–5 MINUTES
The bond energy account
Predict whether the reaction releases energy. Break the reactant bonds, then form the product bonds.
H₂ + Cl₂ → 2HCl
Average gas-phase bond energies in kJ mol⁻¹ of bonds. A simplified energy account per reaction as written; it does not show the reaction mechanism or activation energy.
Break: H–H = 436; Cl–Cl = 243. Form: 2 × H–Cl = 862.
Energy absorbedPredict first
Energy releasedWaiting for bond formation
Breaking bonds always requires energy; forming bonds releases energy. Explain the overall change using both parts of the account.