Endothermic vs Exothermic Reactions: What's the Difference?
The difference comes down to energy flow. In an exothermic reaction, energy escapes into the surroundings, so they warm up and the enthalpy change (ΔH) turns negative. An endothermic reaction runs the other way: it draws energy in from the surroundings, so they cool down and ΔH is positive. That single idea, energy in or energy out, explains almost everything else on this page.
What is an exothermic reaction?
An exothermic reaction is a chemical change that gives off energy, usually as heat. The energy stored in the products is lower than the energy stored in the reactants. That extra energy has to go somewhere, so it flows out into the surroundings. You can often feel this directly. The beaker, the room, or your hand warms up.
Because energy leaves the system, the enthalpy change is negative. We write this as ΔH < 0. A classic example is the combustion of methane. The reaction CH4 + 2 O2 → CO2 + 2 H2O releases a large amount of heat and light. That is why natural gas is used for cooking and heating.
Other everyday exothermic reactions include neutralization, when an acid reacts with a base, and cellular respiration, which powers your body. Hand warmers also work this way. The slow oxidation of iron inside the packet gives off steady heat for hours.
What is an endothermic reaction?
An endothermic reaction is a chemical change that takes in energy from its surroundings. The products store more energy than the reactants did. To make up that difference, the reaction pulls energy from nearby matter. As a result, the surroundings lose energy and cool down. A container can feel cold to the touch.
Because energy enters the system, the enthalpy change is positive. We write this as ΔH > 0. Photosynthesis is a well known example. Plants absorb light energy and use it to build glucose from carbon dioxide and water. Without a steady energy supply, the reaction simply stops.
Instant cold packs are another good example. Many of them contain ammonium nitrate, NH4NO3. When it dissolves in water, it absorbs heat and the pack turns cold. Melting ice and the thermal decomposition of calcium carbonate are endothermic too.
How bonds and enthalpy explain the difference
Every reaction has two energy steps. First, old bonds break in the reactants. Breaking bonds always absorbs energy. Second, new bonds form in the products. Forming bonds always releases energy. The type of reaction depends on which step wins. You can explore the numbers behind this on our bond energy page.
If forming the new bonds releases more energy than breaking the old bonds absorbs, the reaction is exothermic. Extra energy flows out and ΔH is negative. If breaking the old bonds costs more energy than forming the new ones gives back, the reaction is endothermic. Energy flows in and ΔH is positive.
The overall enthalpy change is simply the net of these two steps. A useful shortcut is this. ΔH equals the energy needed to break bonds minus the energy released when bonds form. A negative answer means exothermic. A positive answer means endothermic.
One more point often confuses students. Almost every reaction needs a small energy push to get started, called the activation energy. Even a very exothermic reaction like combustion needs a spark. Activation energy tells you how a reaction begins. The sign of ΔH tells you the net energy result once it is done.
Endothermic vs exothermic: side-by-side comparison
The table below sums up the key differences at a glance. Use it as a quick reference when you are labeling reactions in class or on a lab report. Notice how each row is a mirror image. If you know one column, you can predict the other.
| Feature | Endothermic | Exothermic |
|---|---|---|
| Energy flow | Absorbs energy from surroundings | Releases energy to surroundings |
| Sign of ΔH | Positive (ΔH > 0) | Negative (ΔH < 0) |
| Temperature of surroundings | Decreases (feels cold) | Increases (feels warm) |
| Energy of products | Higher than reactants | Lower than reactants |
| Common examples | Photosynthesis, melting ice, cold packs, thermal decomposition | Combustion, neutralization, respiration, hand warmers |
What are some less obvious examples?
Beyond the textbook classics, plenty of reactions catch students off guard. The examples below never appear in the table above, yet they follow the same energy rules. Learning a few of these makes it easier to label any reaction you meet.
Surprising exothermic examples
Adding water to quicklime, which is calcium oxide, releases so much heat the mixture can steam and boil. This slaking reaction has even started fires aboard cargo ships. The thermite reaction between aluminum and iron oxide gets hot enough to melt iron for welding. Curing concrete also warms up quietly for days as it hardens.
Surprising endothermic examples
Evaporating water pulls heat from your skin, which is why sweat cools you down. Mixing citric acid with baking soda, the fizz in some candies and bath bombs, absorbs heat and feels cool. The classic reaction of barium hydroxide with an ammonium salt turns so cold it can freeze a drop of water beneath the flask.
How can you tell them apart in the lab?
The simplest test is a thermometer. Measure the temperature before and after the reaction. If the temperature rises, energy was released and the reaction is exothermic. If the temperature drops, energy was absorbed and the reaction is endothermic. This works because the surroundings gain or lose the energy the system trades.
You can also feel the container, though a thermometer is more reliable. A warm beaker points to an exothermic change. A cold beaker points to an endothermic one. For a written clue, check the sign of ΔH. Negative means exothermic. Positive means endothermic.
Energy diagrams give another view. In an exothermic diagram, the products sit lower than the reactants. In an endothermic diagram, the products sit higher. The gap between the starting line and the peak is the activation energy, and the gap between reactants and products is ΔH.
Frequently asked questions
Is melting ice endothermic or exothermic?
Melting ice is endothermic. Ice absorbs heat from its surroundings to break the bonds holding the solid together. That is why a glass of ice water feels cold and why the room around it cools slightly. The reverse process, water freezing into ice, is exothermic.
Why is delta H negative for exothermic reactions?
ΔH measures the energy change of the system, not the surroundings. In an exothermic reaction, the system loses energy to its surroundings. Because the system ends with less energy than it started, the change is negative. So exothermic reactions always have ΔH less than zero.
Can a reaction be both endothermic and exothermic?
No, a single reaction is one or the other overall. Every reaction has both bond breaking, which absorbs energy, and bond forming, which releases energy. The net result decides the label. If the forward reaction is exothermic, the reverse reaction is endothermic by the same amount.
Is cooking an egg endothermic or exothermic?
Cooking an egg is endothermic. The egg must absorb heat from the pan or water before its proteins change shape and set. Since energy flows into the food to make the change happen, the process takes in energy overall. Remove the heat and cooking stops.
Does endothermic mean the reaction feels cold?
Usually, yes. An endothermic reaction pulls heat from its surroundings, so the container and nearby air cool down and feel cold. This is exactly how instant cold packs work. Keep in mind that feeling cold is a clue, but a thermometer or the sign of ΔH is more precise.