Chemistry · Grades 10–12
Bond-enthalpy estimate
Bond breaking requires energy; bond formation releases it. Average bond energies give an approximate reaction enthalpy.
Rule and assumptions
Bond breaking requires energy; bond formation releases it. Average bond energies give an approximate reaction enthalpy.
ΔH ≈ bonds broken − bonds formed
Worked examples
For one reaction as written, a bond model requires 463 kJ/mol to break bonds and releases 756 kJ/mol on forming bonds. Estimate ΔH. Round to 3 decimal places if needed. Answer in kJ/mol.
ΔH ≈ bonds broken − bonds formed 463 − 756 = -293
-293 kJ/mol
For one reaction as written, a bond model requires 1048 kJ/mol to break bonds and releases 854 kJ/mol on forming bonds. Estimate ΔH. Round to 3 decimal places if needed. Answer in kJ/mol.
ΔH ≈ bonds broken − bonds formed 1048 − 854 = 194
194 kJ/mol
Try two questions
For one reaction as written, a bond model requires 1188 kJ/mol to break bonds and releases 1176 kJ/mol on forming bonds. Estimate ΔH. Round to 3 decimal places if needed. Answer in kJ/mol.
For one reaction as written, a bond model requires 361 kJ/mol to break bonds and releases 849 kJ/mol on forming bonds. Estimate ΔH. Round to 3 decimal places if needed. Answer in kJ/mol.
Answers and reasoning
Answers and marking notes
- 12 kJ/mol
ΔH ≈ bonds broken − bonds formed 1188 − 1176 = 12
- -488 kJ/mol
ΔH ≈ bonds broken − bonds formed 361 − 849 = -488
Further reading and review references
These are reference links, not copied exercises, institutional endorsement or proof of independent review.
Learning preview · independent educational and translation review required. Suggested grades are not official curriculum alignment.