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Chemistry · Grades 9–12

Reaction energetics and system boundaries

Track energy transferred to the surroundings separately from reaction enthalpy.

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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.

Worked examples

Which sign normally describes ΔH for an exothermic reaction?

The reacting system transfers energy to the surroundings.

Negative.

A bond-enthalpy model requires 450 kJ/mol to break bonds and releases 610 kJ/mol forming bonds. Estimate ΔH.

450 − 610 = −160 kJ/mol.

-160 kJ/mol

Check your understanding

  1. Does breaking a chemical bond itself release energy?

    1. No; energy is required to separate the bonded atoms.
    2. Yes, every broken bond releases energy.
    3. Only because atoms vanish.

  2. Why can the surroundings warm while the reaction enthalpy is negative?

  3. A process absorbs 75 kJ from its surroundings. What is its heat transfer q for the system under the stated sign convention?

Answers and marking notes

Answers and marking notes
  1. No; energy is required to separate the bonded atoms.

    Energy release is associated with forming favorable new bonds.

  2. Energy leaving the reacting system enters the surroundings.

    The sign refers to the system, not the thermometer reading alone.

  3. 75 kJ

    Heat entering the system is positive.

Further reading and review references

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Independent educational review required. Grade bands are approximate; these are not officially aligned curricula.