Bond energy: breaking takes energy, forming releases it

Answer Key - Grade 10 Science Worksheet

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

1. 1.

   In H₂ + Cl₂ → 2 HCl, breaking bonds takes 679 kJ and forming bonds releases 864 kJ. Estimate ΔH, Ari?

   -185 kJ
2. 2.

   In 2 H₂ + O₂ → 2 H₂O, breaking bonds takes 1370 kJ and forming bonds releases 1856 kJ. Estimate ΔH, Bo?

   -486 kJ
3. 3.

   In N₂ + 3 H₂ → 2 NH₃, breaking bonds takes 2254 kJ and forming bonds releases 2340 kJ. Estimate ΔH, Esme?

   -86 kJ
4. 4.

   In 2 H₂ + O₂ → 2 H₂O, how many O–H bonds form, Kira?

   • 3
   • 2
   • 4
5. 5.

   In N₂ + 3 H₂ → 2 NH₃, how many N–H bonds form, Luz?

   • 2
   • 6
   • 3
6. 6.

   Breaking takes 679 kJ and forming releases 864 kJ. Is the reaction exothermic or endothermic, Milo?

   • Endothermic: breaking comes first
   • Endothermic: forming gives more
   • Exothermic: forming gives more
7. 7.

   Why is a ΔH found from bond energies only an estimate, Noah?

   • The table gives averages
   • Bonds can't be measured
   • Energy isn't conserved
8. 8.

   H₂ + Cl₂ → 2 HCl has ΔH ≈ −185 kJ. What is ΔH for 2 HCl → H₂ + Cl₂, Pia?

   • About 0 kJ
   • About +185 kJ
   • About −185 kJ
9. 9.

   On an energy diagram, which way does the bond-breaking step go, Ren?

   • Flat
   • Uphill
   • Downhill
10. 10.

    Why does burning methane release energy overall, Zee?

    • Oxygen's bonds release energy
    • Methane has no bonds in it
    • Its products' bonds release more