Bond energy: breaking takes energy, forming releases it

Answer Key - Grade 10 Science Worksheet - Challenge Me

Circle the best answer, or write your answer on the line.

1. 1.

   H₂ + Cl₂ → 2 HCl breaks 1 H–H (436 kJ/mol) and 1 Cl–Cl (243), and forms 2 H–Cl (432). Estimate ΔH in kJ, Ari?

   -185 kJ
2. 2.

   2 H₂ + O₂ → 2 H₂O breaks 2 H–H (436 kJ/mol) and 1 O=O (498), and forms 4 O–H (464). Estimate ΔH in kJ, Bo?

   -486 kJ
3. 3.

   N₂ + 3 H₂ → 2 NH₃ breaks 1 N≡N (946 kJ/mol) and 3 H–H (436), and forms 6 N–H (390). Estimate ΔH in kJ, Esme?

   -86 kJ
4. 4.

   The bond-energy estimate for N₂ + 3 H₂ → 2 NH₃ is −86 kJ; measured data give about −92 kJ. Why the gap, Jun?

   • Bond energies are averages
   • Energy was lost in the sum
   • Ammonia has no N–H bonds
5. 5.

   2 H₂ + O₂ → 2 H₂O has an estimated ΔH of −486 kJ. What is the estimate for 2 H₂O → 2 H₂ + O₂, Kira?

   • −486 kJ
   • +486 kJ
   • +1856 kJ
6. 6.

   Which bonds break in H₂ + Cl₂ → 2 HCl, Luz?

   • One H–Cl bond
   • One H–H and one Cl–Cl
   • Two H–Cl bonds
7. 7.

   Noah says burning methane gives heat because its C–H bonds break. Agree or disagree?

   • Agree: breaking C–H bonds gives the heat
   • Disagree: forming new bonds gives it
   • Agree: carbon itself is hot
8. 8.

   Pia says ΔH ≈ bonds formed − bonds broken. Agree or disagree?

   • Agree: the order doesn't matter
   • Agree: products are written first
   • Disagree: broken minus formed
9. 9.

   C–C is 345, C=C is 611 and C≡C is 837 kJ/mol. Which order runs from weakest to strongest, Ren?

   • C=C, C–C, C≡C
   • C≡C, C=C, C–C
   • C–C, C=C, C≡C
10. 10.

    C=O is 741 kJ/mol and C≡O is 1080 kJ/mol. Which takes more energy to break, Zee?

    • They're equal
    • C≡O
    • C=O