Thermodynamics & Reaction Energy: Where Energy Goes in a Reaction
Understand energy in chemical reactions — endothermic vs. exothermic, conservation of energy, activation energy, and energy diagrams — the chemistry background for ACT Science.
The Short Version
- Energy is conserved — it isn't created or destroyed, only transferred or transformed.
- Exothermic reactions release energy (get warmer); endothermic reactions absorb it (get colder).
- An energy diagram shows reactants, products, and the energy hill between them.
- Activation energy is the minimum energy needed to start a reaction. ACT Science / chemistry background.
Chemical reactions are really about energy as much as matter. When bonds break and reform, energy is either let out into the surroundings or pulled in from them. A reaction that releases energy makes its surroundings warmer; one that absorbs energy makes them cooler. The branch of science that tracks this energy bookkeeping is thermodynamics, and for tests it comes down to two categories and one kind of diagram.
This guide covers conservation of energy, exothermic and endothermic reactions, energy diagrams, and activation energy, with worked and practice questions matched to the level seen in ACT Science and chemistry at Northside Tutoring.
Why Reaction Energy Matters
Energy changes drive chemistry and physics, and ACT Science passages frequently show energy diagrams or temperature data. Understanding the categories lets you interpret these quickly. (The SAT has no science section.)
Energy Is Conserved
The first law of thermodynamics: energy cannot be created or destroyed, only transferred or changed in form. In a reaction, the energy doesn't vanish — it moves between the chemicals and their surroundings, or shifts between stored (chemical) energy and heat.
Exothermic vs. Endothermic
| Exothermic | Endothermic | |
|---|---|---|
| Energy | released | absorbed |
| Surroundings | get warmer | get colder |
| Products vs. reactants | lower energy | higher energy |
| Examples | burning, explosions | cold packs, photosynthesis |
Reading Energy Diagrams
An energy diagram plots energy as the reaction proceeds. Reactants start at one level, climb over a hump, and settle at the products' level. If the products end lower than the reactants, energy was released — exothermic. If the products end higher, energy was absorbed — endothermic. Just compare the start and end heights.
Activation Energy
The hump in the middle of an energy diagram is the activation energy — the minimum energy needed to get the reaction started, even for an exothermic one. It's why a fire needs a spark: wood burning releases energy overall, but you have to supply the initial push.
Catalysts lower the hump
A catalyst speeds up a reaction by lowering the activation energy — making the hump smaller — without being consumed itself. It doesn't change whether the reaction is exo- or endothermic, only how easily it gets going.
Everyday Examples
Exothermic reactions are all around you: combustion (a campfire), the hand warmer that heats up, and most explosions. Endothermic processes absorb heat: an instant cold pack, melting ice, and photosynthesis, which stores the sun's energy in sugars. Noticing whether something heats or cools its surroundings tells you the type.
Where You'll See This — Test by Test
Reaction energy supports ACT Science passages, which often show energy diagrams; the SAT has no science section and the SSAT doesn't test it. It's core high-school and AP Chemistry and physics.
ACT Science
Energy diagrams and temperature-change data appear in ACT Science chemistry and physics passages.
Explore ACT Tutoring → K-12 CurriculumChemistry
Thermodynamics and reaction energy are central to high-school and AP Chemistry.
Explore Science Tutoring → College AdmissionsSAT
No SAT science section; this content isn't tested there among admissions exams.
Explore SAT Tutoring → K-12 CurriculumSchool Science
A foundational energy concept spanning chemistry and physics.
Explore Science Tutoring →Watch the Lesson
Sometimes a diagram needs a voice. In the short video below, one of our Northside tutors walks through the core idea and works through test-style problems in real time.
Reaction Energy — In Plain English
A live walkthrough from our tutoring team.
— Featuring a Northside Tutoring instructor
Worked Example Problems
These problems are calibrated to the difficulty you'll actually see on test day. Try each one before opening the solution.
A reaction makes its container feel warm. Is it exothermic or endothermic?
Show solution
It releases energy to the surroundings (they warm up) — exothermic.
On an energy diagram, the products are higher than the reactants. What type of reaction is it?
Show solution
Products higher means energy was absorbed — endothermic.
What does the peak of an energy diagram represent?
Show solution
The activation energy — the minimum energy needed to start the reaction.
How does a catalyst affect a reaction's energy diagram?
Show solution
It lowers the activation energy (the hump) without changing the reactant or product energies.
An instant cold pack gets cold when activated. What kind of process is this?
Show solution
It absorbs energy from the surroundings — endothermic.
Common Mistakes to Avoid
Three points students often miss
- Swapping exo- and endothermic. Exothermic releases energy (warms surroundings); endothermic absorbs it (cools them).
- Thinking exothermic reactions need no start-up energy. Even energy-releasing reactions need activation energy to begin.
- Believing a catalyst changes the energy released. A catalyst only lowers the activation energy, not the overall energy change.
Practice Problems — You Try
Three problems below. Work each before checking the solution.
Burning wood releases heat and light. Exothermic or endothermic?
Show solution
Exothermic — energy is released.
Can energy be created or destroyed in a reaction?
Show solution
No — energy is conserved; it only transfers or changes form.
On an energy diagram, reactants are at 50 kJ, the peak is at 90 kJ, and products are at 30 kJ. State the activation energy and whether the reaction is exo- or endothermic.
Show solution
Activation energy = peak − reactants = 90 − 50 = 40 kJ. Products (30) are lower than reactants (50), so energy is released — exothermic.
The Northside Method — How We Teach This 1-on-1
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