An endothermic reaction absorbs heat from its surroundings, so the surroundings feel colder as the reaction proceeds. An exothermic reaction releases heat into its surroundings, so the surroundings feel warmer.
Both are common types of chemical reactions, and the difference between endothermic and exothermic reactions ultimately traces back to whether breaking and forming chemical bonds takes in more energy than it gives out, or the other way around. This endothermic vs. exothermic reactions comparison shows up across chemistry, biology, and everyday life, from cooking to combustion.
What Is an Endothermic Reaction?
An endothermic reaction is a chemical reaction that absorbs energy from its surroundings, usually as heat. Because the reaction pulls heat in rather than releasing it, the surrounding environment cools while the reaction takes place.
The word comes from the Greek roots “endo,” meaning within, and “thermic,” meaning heat, describing a process in which heat moves inward into the reaction.
A cold pack used for sports injuries is a practical demonstration of an endothermic reaction. Squeezing the pack triggers ammonium nitrate to dissolve in water, and this dissolution absorbs heat from the surroundings, which is why the pack feels cold to the touch. Photosynthesis is another example: plants absorb light energy to convert carbon dioxide and water into glucose and oxygen, storing that energy in glucose’s chemical bonds.
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What Is an Exothermic Reaction?
An exothermic reaction is a chemical reaction that releases energy into its surroundings, usually as heat and sometimes as light. Because the reaction gives off heat, the surroundings warm up as the reaction proceeds. The prefix “exo” means outward, describing a process where heat moves outward from the reaction into the environment.
Combustion is the clearest everyday example of an exothermic reaction. Burning wood, natural gas, or any hydrocarbon fuel releases a large amount of energy as heat and light, which is exactly why fire feels hot.
Neutralization reactions, in which an acid reacts with a base, are also exothermic, releasing heat as new bonds form between ions in solution.
According to the MIT OpenCourseWare lecture notes on thermochemistry, the heat of reaction is the change in enthalpy between the products and reactants of a reaction carried out at constant pressure, and a negative value indicates heat has been released, making the reaction exothermic.
How Do Endothermic Reactions Work?
Endothermic reactions work by absorbing more energy to break the existing bonds in the reactants than is released when new bonds form in the products. Every chemical reaction involves two energy-related steps: breaking the bonds in the starting materials, which always requires an input of energy, and forming new bonds in the products, which always releases energy. In an endothermic reaction, the energy needed to break the original bonds is greater than the energy given off when the new bonds form.
That leftover energy deficit comes from the surroundings. This is why touching an endothermic reaction in progress, like a cold pack activating or citric acid reacting with baking soda, feels cold: the reaction actively pulls heat energy from your hand and the surrounding air to complete the bond-breaking step.
How Do Exothermic Reactions Work?
Understanding how these heat-releasing reactions work starts with the opposite balance: the energy released when new bonds form in the products is greater than the energy required to break the bonds in the original reactants. Once that surplus of energy is freed up, it has to go somewhere, and it moves outward into the surrounding environment as heat, and sometimes light.
This is also why many exothermic reactions, once started, keep going on their own. The heat released in an early stage of the reaction can supply enough energy to trigger the next stage, which is part of why a fire spreads once it catches, and why some exothermic reactions can become dangerous if the released heat isn’t controlled or dissipated properly.
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Difference Between Endothermic and Exothermic Reactions
Here is a direct comparison of endothermic vs. exothermic reactions.
| Feature | Endothermic Reaction | Exothermic Reaction |
|---|---|---|
|
Energy flow |
Absorbs energy from surroundings |
Releases energy into surroundings |
|
Temperature change of surroundings |
Decreases |
Increases |
|
Enthalpy change (ΔH) |
Positive |
Negative |
|
Bond energy comparison |
Breaking bonds requires more energy than forming bonds releases |
Forming bonds releases more energy than breaking bonds requires |
|
Common feel |
Cold to the touch |
Warm or hot to the touch |
|
Common examples |
Photosynthesis, melting ice, cold packs |
Combustion, neutralization, rusting |
Endothermic and Exothermic Reaction Examples
Here are common examples that appear in coursework and everyday life:
Endothermic examples:
- Photosynthesis: plants absorb light energy to build glucose from carbon dioxide and water.
- Melting ice: solid water absorbs heat energy to become liquid water.
- Cooking an egg: heat energy is absorbed to break down proteins and change their structure.
- Thermal decomposition of calcium carbonate: heating limestone absorbs energy to produce calcium oxide and carbon dioxide.
Exothermic examples:
- Combustion: burning fuel releases heat and light energy.
- Neutralization: mixing an acid and a base releases heat as water and salt form.
- Rusting: the slow oxidation of iron releases a small amount of heat over time.
- Respiration: cells break down glucose and release energy that powers the body.
Test Your Understanding
Try answering these before checking the answers below, or browse more chemistry explanations in the Think10x.ai video library.
- If a reaction feels cold to the touch, is it endothermic or exothermic? Answer: Endothermic. A reaction that feels cold is absorbing heat from your hand and the surrounding air.
- Does an exothermic reaction have a positive or negative enthalpy change? Answer: Negative. Releasing energy corresponds to a decrease in the system’s enthalpy, so ΔH is negative for exothermic reactions.
- Photosynthesis absorbs light energy to build glucose. Is this endothermic or exothermic? Answer: Endothermic. The plant is absorbing energy from its surroundings (sunlight) to drive the reaction forward.
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Frequently Asked Questions
No, not for the overall reaction. Every reaction has an overall net enthalpy change that is either positive or negative, though individual steps within a multi-step reaction can absorb or release energy differently before the overall balance is reached.
No. Speed depends on activation energy and reaction conditions, not on whether the reaction is exothermic or endothermic. Some exothermic reactions, like rusting, are very slow, while some endothermic reactions can happen quickly once enough heat is supplied.
Often, yes. Many endothermic reactions stop if the external energy source, such as heat or light, is removed, since the reaction depends on that outside energy to keep breaking bonds. Photosynthesis stops in the dark for exactly this reason.
Even exothermic reactions usually require an initial amount of activation energy to begin breaking the first bonds, even though the overall reaction releases more energy than it takes in. Striking a match to start combustion is a common example of supplying that initial activation energy.
Enthalpy change is typically measured using calorimetry, where you record the temperature change of a known quantity of water or another substance before and after the reaction, then use that change to calculate how much heat was absorbed or released.