Mini Chemistry
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Bond Breaking and Bond Forming

Explain qualitatively why reactions are exothermic or endothermic by comparing energy absorbed in bond breaking with energy released in bond making.

  • SEC G3 Pure Chemistry 2027
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Learning objectives

  • describe bond breaking as an endothermic process and bond making as an exothermic process
  • explain qualitatively overall enthalpy changes in terms of the energy changes associated with the breaking and making of covalent bonds.

This lesson explains the energy change of a reaction by following two events: old bonds are broken and new bonds are formed. The direction of both energy transfers matters.

1. Definition

  • Bond breaking is endothermic: energy is absorbed to overcome the attraction between bonded atoms.
  • Bond forming is exothermic: energy is released when atoms form new, stable bonds.

The overall reaction is exothermic or endothermic according to which transfer is greater. Numerical bond-energy calculations are not required.

2. Key Ideas

  • Reactant bonds must be broken before atoms can be rearranged into products.
  • Breaking bonds always absorbs energy—even when the overall reaction is exothermic.
  • Forming product bonds always releases energy—even when the overall reaction is endothermic.
  • Exothermic overall: bond making releases more energy than bond breaking absorbs.
  • Endothermic overall: bond breaking absorbs more energy than bond making releases.
The four-part explanation

State both transfers, compare their sizes, then give the overall result. Avoid saying only “more energy is released” without identifying bond making.

3. Detailed Explanations

A. Follow the energy through a reaction

Use this sequence:

  1. energy is absorbed to break bonds in the reactants;
  2. the atoms rearrange;
  3. energy is released when bonds form in the products;
  4. compare the energy absorbed with the energy released.
Overall reactionComparisonEnergy transfer to surroundingsSign of Δ H
exothermicbond making releases more than bond breaking absorbsnet releasenegative
endothermicbond breaking absorbs more than bond making releasesnet absorptionpositive

B. Why an exothermic reaction still needs energy to start

An exothermic reaction can have an activation energy. Some reactant bonds must begin to break before the more stable product bonds can form. Once product bond formation releases enough energy, the reaction can continue and release energy overall.

Use the Energy Changes in Chemical Reactions hub to connect this bond-level account to reaction profiles and other energy-change evidence.

C. Keep one bond event separate from the overall reaction

“Breaking bonds is endothermic” describes one part of the reaction. “The reaction is exothermic” describes the result after both bond breaking and bond making have been considered. These statements do not contradict each other.

4. Common Mistakes

  • Saying bond breaking releases energy because atoms “escape”. Overcoming an attraction requires energy.
  • Calling bond forming endothermic. Forming a stable bond releases energy.
  • Looking only at the reactants or only at the products instead of comparing both transfers.
  • Using the sign of Δ H backwards: negative is exothermic; positive is endothermic.
  • Trying to calculate bond energies. For this topic, compare the two energy transfers in words.

5. Exam Tips

For an exothermic reaction, write:

Energy is absorbed to break reactant bonds and released when product bonds form. Forming the product bonds releases more energy than breaking the reactant bonds absorbs, so the reaction is exothermic.

For an endothermic reaction, reverse only the comparison and conclusion: breaking absorbs more than forming releases.

6. Worked Examples

Modelled example 1

Explain an exothermic reaction

Core

Problem

Methane combustion is exothermic. Explain this using bond breaking and bond making.
Study the worked solution

  1. State both transfers

    Method

    State that breaking reactant bonds absorbs energy and forming product bonds releases energy.

    Reason

    Both processes occur as atoms rearrange from reactants into products.

    Working

    Break bonds: energy in; form bonds: energy out.

  2. Compare and conclude

    Method

    State that product-bond formation releases more energy than reactant-bond breaking absorbs.

    Reason

    The excess energy is transferred to the surroundings.

    Working

    Net energy release → exothermic; Δ H < 0.

Guided practice 2

Compare two energy transfers

About 5 min

Problem

A reaction profile shows that the energy released when product bonds form is greater than the energy absorbed to break reactant bonds. Classify the reaction and predict the sign of Δ H.

Try this before viewing the solution

Reaction type
ΔH sign

Hints

Hint 1: compare directions
Decide whether more energy leaves or enters the reacting system overall.
View solution step by step

  1. Find the net direction

    Method

    Compare released energy with absorbed energy.

    Reason

    The release is greater, so energy leaves the system overall.

    Working

    release > absorption.

  2. Classify

    Method

    State exothermic and negative Δ H.

    Reason

    Net energy release is the definition of an exothermic change.

    Working

    Exothermic; Δ H < 0.

Common misconception 3

Check a bond-breaking claim

Find and correct the mistake

Learner claim

“Breaking fuel molecules releases the energy in the fuel.” Correct this statement.

Try this before viewing the solution

Breaking bonds
Source of net release

View solution step by step

  1. Correct the first event

    Method

    State that breaking fuel and oxygen bonds absorbs energy.

    Reason

    A bond is an attraction that must be overcome.

    Working

    Bond breaking: endothermic.

  2. Locate the overall release

    Method

    State that forming product bonds releases more energy.

    Reason

    The difference between the two transfers gives the net exothermic change.

    Working

    Greater product-bond release → exothermic overall.

Examiner practice 4

Explain an endothermic reaction

4 marks

Examination question

A reaction is endothermic. Explain its overall energy change using bond breaking and bond making. [4 marks]

Try this before viewing the solution

View solution step by step

  1. State the two processes

    2 marks

    Method

    State that bond breaking absorbs energy and bond making releases energy.

    Reason

    These are the two energy transfers in a reaction.

    Working

    Break: absorb; make: release.

  2. Compare and conclude

    2 marks

    Method

    State that breaking reactant bonds absorbs more energy than forming product bonds releases.

    Reason

    The reacting system therefore gains energy overall.

    Working

    Net absorption → endothermic; Δ H > 0.

Challenge 5

Add a catalyst without changing the conclusion

Minimal support

Problem

A catalyst is added to an exothermic reaction. Explain what changes and what stays the same on its energy profile.

Try this before viewing the solution

Activation energy
ΔH

Hints

Hint 1: pathway
A catalyst changes the route taken by the reaction.
Hint 2: endpoints
Compare the reactant and product energy levels before deciding whether Δ H changes.
View solution step by step

  1. Change the pathway

    Method

    Lower the activation-energy peak.

    Reason

    The catalyst offers an alternative pathway.

    Working

    Lower Eₐ.

  2. Keep the endpoints

    Method

    Leave reactant level, product level and Δ H unchanged.

    Reason

    The overall bond-energy balance and reaction type do not change.

    Working

    Still exothermic; same Δ H.

7. Mind Stretchers

Mind stretcher 1: Activation energy in an exothermic reactionExtension

Why might a fuel need a spark even though its combustion is exothermic?

Show Answer

The spark supplies activation energy so that some reactant bonds begin to break. Forming the product bonds then releases more energy than was absorbed, so combustion is exothermic overall.

Mind stretcher 2: One event versus the overall changeExtension

A student says, “Bond making is exothermic, so every reaction must be exothermic.” What is missing?

Show Answer

The student has ignored the energy absorbed to break reactant bonds. Both transfers must be compared. If bond breaking absorbs more than bond making releases, the reaction is endothermic overall.

8. Quiz

Practise this lesson

Check that you can state both bond-energy transfers, compare them and reach the correct overall conclusion.

Practice next

Lesson quizCheck this lesson quickly with targeted MCQs.Structured practiceSwitch to exam-style written responses after the quiz.