Chemistry · Grades 9–12
Reaction energetics and system boundaries · Revision sheet
Track energy transferred to the surroundings separately from reaction enthalpy.
Learn the idea
System
Define the reacting system before assigning a sign to heat transfer.
Exothermic
At constant pressure in the school model, an exothermic reaction has negative ΔH and transfers heat out.
Bond model
Breaking bonds requires energy; forming bonds releases energy.
Estimate
With average bond enthalpies, ΔH ≈ energy to break bonds − energy released on forming bonds.
Explain without looking
- Explain why a large negative ΔH does not by itself guarantee a fast reaction.
- Two stated reaction steps have ΔH values −40 and +15 kJ/mol and sum exactly to a target reaction. What is the target ΔH?
Further reading and review references
These are reference links, not copied exercises, institutional endorsement or proof of independent review.
Independent educational review required. Grade bands are approximate; these are not officially aligned curricula.