Reactions can either generate or consume energy in the form of heat. Read on to learn about how to distinguish endothermic and exothermic reactions, connect them to other chemistry concepts, and see practical examples!
Endothermic and Exothermic Definitions
What is an endothermic reaction? Endothermic reactions absorb energy from their surroundings, because the products are higher in energy than the reactants.
What is an exothermic reaction? Exothermic reactions release energy to their surroundings, because the products are lower in energy than the reactants. You can think about this visually using a reaction energy diagram, as seen below:
A reaction of each kind is shown, in which the reactants A and B undergo a transformation into a product C. If the energy of C is greater than the energy of A and B, then the reaction is endothermic, and there is net energy absorbed. If, on the other hand, C has lower energy than A and B, the reaction is exothermic, and there is net energy released.
To make the endothermic reaction occur, we must add heat to the system, otherwise there will not be enough energy available to form the product C. To make the exothermic reaction occur, we must take heat out of the system, otherwise it will be too high in energy to form C.
If we perform a reverse reaction, starting with C and doing the reverse reaction back to A and B, then the type of reaction would be opposite. If A + B → C is endothermic, its reverse reaction C → A + B will be exothermic, and vice versa.
Endothermic and Exothermic Reaction Examples
Endothermic and exothermic reactions are everywhere, even when we don’t notice the change in temperature they create. Below are some examples of each type of reaction.
Precipitation of Sodium Acetate
Some kinds of reusable hand warmers contain a supersaturated solution of sodium acetate. When you bend the pack, tiny pieces of metal allow the sodium acetate to “nucleate” or begin to form crystals. The formation and growth of those crystals is an exothermic process, so it heats up the hand warmers.
The oxidation of iron metal to form iron oxide or rust is also an exothermic process! Usually this is difficult to notice, since no one pays attention to the temperature of rusting metal. However, this reaction is used in a disposable version of the hand warmers mentioned above. In this version, taking the warmers out of their packaging exposes iron powder to the air, and the rusting reaction causes them to heat up. This is technically a form of combustion, which you can learn more about in the next section!
Combustion reactions are exothermic. This is why burning fuel like wood or gas gives off heat that keeps a house (or a group of campers) warm. What you feel when you sit near a campfire is the heat released by the combustion reaction.
Note: A common mistake is to think that because a small amount of heat is required to start a combustion reaction (think of striking a match) that this makes combustion endothermic. It requires heat input to overcome the activation energy of the reaction. Once this small amount of heat is provided, the reaction releases heat overall. This is why you don’t need to constantly add heat to your campfire to make it keep burning!
Dissolution of NaOH
Sodium hydroxide, a strong base, and sulfuric acid, a strong acid, both interact strongly with water and release lots of heat when they dissolve. This is the reason why chemists always add concentrated acids and bases to water and never the other way around. If you add water to one of these chemicals, they can release a dangerous amount of heat.
Dissolution of NaCl
The dissolution of sodium chloride, regular table salt, is an endothermic process. That means that if you dissolve it in water, the water will become slightly colder, due to the energy absorbed by the solvation of the ions. This is true for many other salts, including potassium chloride (KCl) and sodium bicarbonate (NaHCO3 or baking soda). It is not necessarily true for all salts, however. Dissolution can be endothermic and exothermic depending on the substance being dissolved!
Hydrolysis is a type of reaction that includes many biochemical and organic reactions. In a hydrolysis reaction, a water molecule reacts with a bond in a molecule to break the molecule into two pieces. This is an endothermic process, so in order to run a reaction like this in the lab, you might have to heat the reaction flask.
When you boil an egg or cook meat, you are causing the “denaturation” or unfolding of the protein molecules in these foods. This means the denaturation process is endothermic, because it must absorb heat energy rather than releasing it.
Le Chatelier’s Principle lets us favor the reactants or products of a reaction using temperature. If we increase the temperature, an endothermic reaction proceeds further towards completion, since it uses up the extra thermal energy we are providing. If the reaction is exothermic, we need to decrease the temperature to favor the products, because the reaction generates heat. Raising temperature for exothermic reactions, or lowering temperature for endothermic reactions, causes the reaction to go backwards, assuming the reactions are reversible.