D3.2 Explain energy absorbed in bond breaking and released in bond formation

Ontario Grade 12 Chemistry

Energy Changes and Rates of Reaction

Ontario Grade 12 Chemistry | Study topic D3.2

A chemical reaction can warm or cool its surroundings. For example, if the nearby solution becomes warmer during a reaction, energy has been transferred to the surroundings. To explain this at the particle level, consider the bonds in the reacting substances. Breaking bonds absorbs energy. Forming bonds releases energy. A reaction may include both changes, so both matter when explaining the direction of energy transfer.

What you will learn

Explain why breaking a chemical bond absorbs energy.
Explain why forming a chemical bond releases energy.
Describe how bond breaking and bond formation both contribute to energy transfer during a chemical reaction.

1. Bridge from atoms and bonds

Atoms are the small particles that make up elements and compounds. In a molecule, atoms are joined by chemical bonds. A chemical bond is an attractive connection between atoms. You have already learned that a chemical reaction rearranges atoms: the atoms present in the reactants are still present in the products, but they are connected differently.

Energy is transferred when physical or chemical changes occur. The reacting substances are the chemicals taking part in the reaction. The surroundings are everything outside those substances, such as the solution, container, and nearby air. Energy can move between the reacting substances and their surroundings.

A reaction does not create or destroy atoms. Instead, bonds in the reactants change, and bonds in the products form. Looking at these bond changes helps explain energy transfer without needing to imagine atoms as tiny solid objects being snapped apart.

A chemical reaction rearranges atoms by changing their bonds.
Energy can be transferred between reacting substances and their surroundings.

2. Breaking a bond absorbs energy

Imagine pulling apart two objects that attract each other. You must supply energy to separate them. A bonded pair of atoms also needs energy input before the atoms can be separated. At the particle level, energy is transferred to the bonded particles as the bond is broken.

A process that absorbs energy from its surroundings is called endothermic. Bond breaking is endothermic: energy must be supplied to overcome the attraction that holds the bonded atoms together. This rule applies to breaking a bond, whether or not the overall reaction releases energy.

In a reaction, bond breaking occurs as the atoms in reactant particles separate and become available to form new arrangements. Saying that the bond breaks does not mean that energy is released. The energy transfer into the particles is what allows the separation to occur.

\text{bond breaking: energy absorbed}

Breaking a bond requires an input of energy.
The bond-breaking process absorbs energy from its surroundings.

3. Forming a bond releases energy

When atoms form a bond, they move into a bonded arrangement. As that bond forms, energy is released to the surroundings. A process that releases energy to its surroundings is called exothermic.

Bond breaking and bond formation have opposite directions of energy transfer. Breaking a particular bond absorbs energy; forming that same type of bond releases energy. The surroundings may receive energy during bond formation, while energy must be supplied to break bonds.

Many reactions include both processes. Bonds in reactants break, and bonds in products form. Therefore, it is not enough to say that a reaction releases energy just because new bonds form, or that it absorbs energy just because old bonds break. A complete explanation identifies both changes and compares their contributions. If bond formation releases more energy than bond breaking absorbs, energy is transferred overall to the surroundings. If bond breaking absorbs more than formation releases, energy is transferred overall from the surroundings to the reacting substances.

\text{bond formation: energy released}

Forming a bond releases energy to the surroundings.
A reaction can involve energy absorption during bond breaking and energy release during bond formation.
The overall direction of energy transfer depends on the balance between these processes.

4. Applying the model to a reaction

Consider hydrogen reacting with chlorine to form hydrogen chloride. In this reaction, the hydrogen atoms are bonded to each other in the hydrogen molecules, and the chlorine atoms are bonded to each other in the chlorine molecules. In the product, each hydrogen atom is bonded to a chlorine atom.

The balanced equation shows that one hydrogen molecule reacts with one chlorine molecule to form two hydrogen chloride molecules. At the particle level, the hydrogen–hydrogen and chlorine–chlorine bonds break. Two hydrogen–chlorine bonds form. The first changes absorb energy; the second change releases energy.

The equation helps identify which bonds change, but it does not by itself tell us how much energy is absorbed or released. For this lesson, the important explanation is the direction of energy transfer for each bond change and the fact that both changes occur. Do not treat the energy associated with breaking bonds as energy released by the reaction; bond breaking takes in energy.

\mathrm{H_2(g)+Cl_2(g)\rightarrow 2HCl(g)}

Use a balanced equation to identify the particles and bonds before and after a reaction.
In the example, breaking reactant bonds absorbs energy, while forming product bonds releases energy.
Describe both processes before explaining the reaction's overall energy transfer.

Worked example

Explaining energy changes in hydrogen chloride formation

For the balanced reaction below, explain which bond changes absorb energy and which release energy. Do not calculate an energy value.

Read the balanced equation
The reactants are hydrogen gas and chlorine gas. The product is hydrogen chloride gas. The coefficient 2 means that two hydrogen chloride molecules form.
$\mathrm{H_2(g)+Cl_2(g)\rightarrow 2HCl(g)}$
Identify bonds that break
Each reactant molecule contains a bond between two atoms of the same element. Those hydrogen–hydrogen and chlorine–chlorine bonds must break as the atoms rearrange. Breaking each bond absorbs energy. H-H\ and\ Cl-Cl\ bonds\ break
Identify bonds that form
Each product molecule contains one hydrogen–chlorine bond. Since two product molecules form, two hydrogen–chlorine bonds form. Bond formation releases energy.
$\mathrm{2\ H-Cl\ bonds\ form}$
State the energy explanation
The reaction includes energy absorption when the reactant bonds break and energy release when the product bonds form. Without energy values, do not claim which contribution is larger or state a numerical overall energy change.

Answer: Breaking the hydrogen–hydrogen and chlorine–chlorine bonds absorbs energy. Forming the two hydrogen–chlorine bonds releases energy.

Check: The answer accounts for both types of bond change and does not infer a numerical energy change from the equation alone.

Common mistakes and how to avoid them

Saying that breaking a bond releases energy because the atoms separate.

Correction: Energy must be supplied to separate bonded atoms. Bond breaking absorbs energy.

Saying that forming a bond absorbs energy.

Correction: Bond formation releases energy to the surroundings.

Explaining a reaction's energy transfer by describing only the bonds that break or only the bonds that form.

Correction: Describe both bond breaking and bond formation. They have opposite directions of energy transfer.

Assuming the balanced equation alone gives the size or direction of the overall energy change.

Correction: The equation identifies the reaction participants and bond changes. Without energy information, it does not show which contribution is larger.

Lesson summary

Breaking a chemical bond absorbs energy because energy must be supplied to separate bonded atoms.
Forming a chemical bond releases energy to the surroundings.
A reaction can include both bond breaking and bond formation.
An explanation of energy transfer should identify both processes and avoid claiming which is larger when no energy information is given.

Check your understanding

Question 1

What happens to energy during bond breaking?

Energy is absorbed.
Energy is released.
Energy transfer is impossible.
The direction depends only on the product formula.

Show answer and explanation

Energy is absorbed.

Energy must be supplied to separate bonded atoms, so bond breaking absorbs energy.

Question 2

A reaction forms new bonds after bonds in the reactants break. Which statement describes the energy changes?

Breaking bonds releases energy, and forming bonds absorbs energy.
Both breaking and forming bonds absorb energy.
Breaking bonds absorbs energy, and forming bonds releases energy.
Neither process involves energy transfer.

Show answer and explanation

Breaking bonds absorbs energy, and forming bonds releases energy.

Bond breaking absorbs energy, while bond formation releases energy. Both processes can occur in the same reaction.

Key terms

Chemical bond: An attractive connection between atoms in a molecule.
Endothermic: Describes a process that absorbs energy from its surroundings.
Exothermic: Describes a process that releases energy to its surroundings.
Surroundings: Everything outside the reacting substances, such as the solution, container, and nearby air.

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About this lesson and its review

Published by DoAssignment. This reviewed lesson follows Ontario Grade 12 Chemistry (SCH4U), expectation D3.2. It is a study resource, not an official curriculum publication.

Before publication, content is checked for structure, mathematical or chemical notation, calculations, course boundaries, and readability. Errors can still occur, so corrections are welcomed.

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D3.2 · Explain energy absorbed in bond breaking and released in bond formation