Why do some reactions release energy while others absorb it, and how is energy conserved?
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).
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What this topic is asking
The Massachusetts standard HS-PS3-4(MA) is the framework's home for the energy of chemical change: it asks you to reason about how thermal energy transfers and spreads. This page sets up thermochemistry by classifying reactions as exothermic or endothermic, describing energy transfer as heat, and applying the conservation of energy.
Exothermic and endothermic reactions
The simplest way to tell them apart in the lab is to feel the container or read a thermometer:
- In an exothermic reaction, energy flows out of the reacting system into the surroundings, raising their temperature. Burning fuel, neutralizing an acid with a base, and most oxidation reactions are exothermic; a hand warmer works this way.
- In an endothermic reaction, energy flows in from the surroundings to the system, lowering their temperature. Dissolving certain salts and thermal decomposition are endothermic; an instant cold pack works this way.
Energy transferred as heat
Most chemical energy changes show up as heat. The "system" is the reaction itself; the "surroundings" are everything else (the solution, the beaker, the air). In an exothermic reaction the system is the source of heat and the surroundings warm; in an endothermic reaction the surroundings are the source and they cool. The amount of heat absorbed or released can be measured from the temperature change of a known mass of water, a method called calorimetry, which builds on phase changes and heating curves.
The conservation of energy
This is the bedrock of thermochemistry. When a reaction "releases" energy, that energy is not made from nothing; it comes from the chemical potential energy stored in the bonds of the reactants, the idea developed in bond energy and reaction energy. When a reaction "absorbs" energy, the energy is not destroyed in the surroundings; it is stored in the products. Tracking energy in and out of the system, never losing any, is how you reason about every energy change.
Try this
Q1. A cold pack feels cold when activated. Is the process exothermic or endothermic? [1]
- Cue. Endothermic (it absorbs energy from the surroundings, including your skin, so it feels cold).
Q2. When fuel burns and releases energy, where did that energy come from? [1]
- Cue. From the chemical potential energy stored in the bonds of the reactants; it is released, not created.
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 marksA reaction in a beaker makes the beaker feel cold. (a) Classify the reaction as exothermic or endothermic. (b) State the direction of energy transfer. (c) Explain what happens to the temperature of the surroundings.Show worked answer →
A 3-point energy-change item.
(a) 1 point: endothermic (the beaker feels cold because the reaction takes in energy).
(b) 1 point: energy is transferred from the surroundings into the reaction system.
(c) 1 point: the surroundings lose energy to the reaction, so the temperature of the surroundings (and the beaker) falls. Markers reward linking the cold beaker to energy absorbed from the surroundings.
MA Chemistry (style)2 marks(a) State the law of conservation of energy. (b) When fuel burns and releases 1000 kJ, where does this energy go if the products are at the same temperature as the start?Show worked answer →
A 2-point conservation-of-energy item.
(a) 1 point: energy cannot be created or destroyed, only transferred or transformed from one form to another.
(b) 1 point: the 1000 kJ is transferred to the surroundings (as heat, and possibly light), so it is not lost but spread into the surroundings. Markers reward stating the energy is conserved by being transferred out, not destroyed.
Related dot points
- 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.
- 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.
- Name the phase changes, interpret a heating curve, and explain why temperature stays constant during a change of state (MA STE supporting content, energy and changes of state).
A standard-level answer on phase changes and heating curves for Massachusetts high school chemistry: naming the six phase changes, reading the flat and sloping sections of a heating curve, and explaining why temperature is constant during melting and boiling, grounded in the framework's energy and matter content.
- 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)