How does the energy of a reaction depend on the bonds broken and formed?
Explain that breaking bonds absorbs energy and forming bonds releases it, and use bond energies to decide whether a reaction is exothermic or endothermic (MA STE HS-PS1-4, energy from changes in total bond energy).
A standard-level answer on bond energy and reaction energy for Massachusetts high school chemistry: why breaking bonds absorbs energy and forming bonds releases it, using bond energies to find the net energy change, and deciding whether a reaction is exothermic or endothermic, grounded in HS-PS1-4.
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What this topic is asking
Standard HS-PS1-4 asks you to show that the energy released or absorbed by a reaction depends on the change in total bond energy. This page explains the rule at the heart of thermochemistry: breaking bonds absorbs energy and forming bonds releases it, and the balance between the two decides whether a reaction is exothermic or endothermic.
Breaking and forming bonds
This pair of rules is the key to the whole topic, and students often get them the wrong way round. Think of a bond as holding atoms in a stable, low-energy state: you must put energy in to break that stability, and energy comes out when a new stable bond forms. A reaction is a two-stage energy exchange: first energy is absorbed to break reactant bonds, then energy is released as product bonds form.
The net energy change
To compare, add up the bond energies on each side:
- Energy to break reactant bonds is the total bond energy of the reactants (energy in).
- Energy released forming product bonds is the total bond energy of the products (energy out).
If the products' bonds release more energy than the reactants' bonds needed, the reaction gives out energy overall and is exothermic. The fuel-burning reactions of Module 3 are exothermic because the strong bonds in carbon dioxide and water release a great deal of energy when they form. This connects the chemistry of bonding from ionic and covalent bonding to the energy of reactions.
Why this matters
Bond energy explains why a reaction is exothermic or endothermic, going one level deeper than just measuring a temperature change. The energy "released" by an exothermic reaction was stored in the chemical bonds of the reactants; it is converted, not created, in line with the conservation of energy from energy changes in chemical reactions. This bond-energy view is also what a potential-energy diagram pictures, the subject of the next topic.
Try this
Q1. Does breaking the bonds in the reactants absorb or release energy? [1]
- Cue. Absorb (breaking bonds always takes in energy).
Q2. A reaction absorbs 500 kJ breaking bonds and releases 400 kJ forming bonds. Is it exothermic or endothermic? [1]
- Cue. Endothermic; net change kJ absorbed overall.
Exam-style practice questions
Practice questions written in the style of MA DESE exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
MA Chemistry (style)3 marks(a) State whether breaking a bond absorbs or releases energy. (b) State whether forming a bond absorbs or releases energy. (c) A reaction releases more energy forming bonds than it absorbs breaking them. Is it exothermic or endothermic?Show worked answer →
A 3-point bond-energy item.
(a) 1 point: breaking a bond absorbs (takes in) energy.
(b) 1 point: forming a bond releases (gives out) energy.
(c) 1 point: exothermic, because more energy is released forming bonds than is absorbed breaking them, so there is a net release. Markers reward the break-absorbs and form-releases rules and the net comparison.
MA Chemistry (style)3 marksA reaction absorbs 800 kJ breaking bonds and releases 950 kJ forming bonds. (a) Calculate the net energy change. (b) Classify the reaction. (c) State what happens to the surroundings.Show worked answer →
A 3-point net-energy item.
(a) 1 point: net change kJ (150 kJ released overall).
(b) 1 point: exothermic (net energy is released; the negative sign shows a release).
(c) 1 point: the surroundings gain the 150 kJ and warm up. Markers reward energy in minus energy out, the negative sign for exothermic, and the warming surroundings.
Related dot points
- Classify reactions as exothermic or endothermic, describe energy transfer as heat, and apply the conservation of energy to chemical and physical changes (MA STE HS-PS3-4(MA), thermal energy transfer).
A standard-level answer on energy changes in chemical reactions for Massachusetts high school chemistry: exothermic and endothermic reactions, energy transferred as heat, the conservation of energy, and the link to temperature change, grounded in HS-PS3-4(MA).
- Interpret a potential energy diagram to identify activation energy, the energy change of reaction, and the effect of a catalyst, and classify the reaction as exothermic or endothermic (MA STE HS-PS1-4 and HS-PS1-5, energy and rate).
A standard-level answer on potential energy diagrams and activation energy for Massachusetts high school chemistry: reading the reactant and product energy levels, the activation energy barrier, the energy change of reaction, and how a catalyst lowers the barrier, grounded in HS-PS1-4 and HS-PS1-5.
- Use collision theory to explain how temperature, concentration, surface area, and catalysts affect the rate of a reaction (MA STE HS-PS1-5, effect of temperature and concentration on reaction rate).
A standard-level answer on reaction rates and collision theory for Massachusetts high school chemistry: how collision theory explains rate, and the effects of temperature, concentration, surface area, and catalysts, grounded in HS-PS1-5.
- Explain how ionic bonds form by transfer of electrons and covalent bonds by sharing, predict which forms from the elements involved, and relate bond type to properties (MA STE HS-PS1-2, bonding from electron states).
A standard-level answer on ionic and covalent bonding for Massachusetts high school chemistry: how electron transfer makes ions and ionic bonds, how sharing makes covalent bonds, predicting bond type from metal versus nonmetal, and the resulting properties, grounded in HS-PS1-2.
- Describe dynamic equilibrium in a reversible reaction and use Le Chatelier's principle to predict the effect of changing concentration, temperature, or pressure (MA STE HS-PS1-6(MA), shifting equilibrium to increase product).
A standard-level answer on chemical equilibrium and Le Chatelier's principle for Massachusetts high school chemistry: dynamic equilibrium in a reversible reaction and predicting the shift when concentration, temperature, or pressure changes, grounded in HS-PS1-6(MA).
Sources & how we know this
- Massachusetts Science and Technology/Engineering Curriculum Framework (2016) — Massachusetts Department of Elementary and Secondary Education (2016)
- Science and Technology/Engineering (STE) Test Design and Development — Massachusetts Department of Elementary and Secondary Education (2024)