AP Chemistry · Topic 6.7
Bond Enthalpies Practice
Part of Thermodynamics.(ENE-3.B)
Practice questions
8
Sample questions
5 of 8 — sign in to practice the rest with adaptive difficulty and mastery tracking.
Sample 1difficulty 2/5
Bar heights show bond dissociation energies in kJ/mol.
Which bond requires the most energy to break?
- Acheck_circle
C≡C
- B
C-C
- C
C-O
- D
C=C
Why
The triple bond C≡C (839 kJ/mol) has the highest bond energy because three shared electron pairs make it the strongest of the bonds shown.
- A
Sample 2difficulty 3/5
ΔH_rxn ≈
- A
Σ(bonds broken) + Σ(bonds formed)
- Bcheck_circle
Σ(bonds broken) − Σ(bonds formed)
- C
0
- D
Σ(bonds formed) − Σ(bonds broken)
Why
Breaking bonds requires energy (positive); forming bonds releases energy (negative).
- A
Sample 3difficulty 3/5
For H₂(g) + Cl₂(g) → 2 HCl(g)
Using bond energies (kJ/mol), what is ΔH for the reaction?
- A
+679 kJ
- Bcheck_circle
-185 kJ
- C
-247 kJ
- D
+185 kJ
Why
ΔH = (bonds broken) - (bonds formed) = (436 + 243) - 2(432) = 679 - 864 = -185 kJ.
- A
Sample 4difficulty 3/5
Using bond enthalpies (kJ/mol): H-H 436, Cl-Cl 243, H-Cl 432. For H₂ + Cl₂ → 2 HCl, ΔH ≈
- A
+247
- Bcheck_circle
−185
- C
−247
- D
+185
Why
Bonds broken: 436 + 243 = 679. Bonds formed: 2 × 432 = 864. ΔH ≈ 679 − 864 = −185 kJ/mol (exothermic).
- A
Sample 5difficulty 4/5
For 2 H₂(g) + O₂(g) → 2 H₂O(g) (each H₂O has 2 O-H bonds)
Estimate ΔH using bond energies (kJ/mol).
- A
+482 kJ
- B
-1852 kJ
- Ccheck_circle
-482 kJ
- D
-241 kJ
Why
Bonds broken: 2(436) + 498 = 1370. Bonds formed: 4(463) = 1852. ΔH = 1370 - 1852 = -482 kJ.
- A