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Endothermic vs. exothermic reactions

Let's see what Sam and Julie are up to in the chemistry lab.
Image titled chemistry lab: experiments are fun.
Excited but a bit confused, Sam and Julie run to their chemistry teacher. Sam asks, “Teacher, why did my flask turn cold after adding the salt to water, while Julie’s flask turned hot?”
The teacher replies: “That’s because you were given two different salts. One of your salts generated an endothermic reaction with water, while the other salt generated an exothermic reaction with water. Let me first reveal the identity of your salts: Salt A is ammonium nitrate (N, H, 4, N, O, 3) and Salt B is calcium chloride (C, a, C, l, 2)."
Now, Sam and Julie are curious about the difference between an endothermic and an exothermic reaction.
Consider the reaction mixture—salt plus water—as the system and the flask as the surrounding.
In Sam’s case, when ammonium nitrate was dissolved in water, the system absorbed heat from the surrounding, the flask, and thus the flask felt cold. This is an example of an endothermic reaction. In Julie’s case, when calcium chloride was dissolved in water, the system released heat into the surroundings, the flask, and thus the flask felt hot. This is an example of an exothermic reaction.
The reaction going on in Sam’s flask can be represented as:
NH4NO3 (s) + heat ---> NH4+ (aq) + NO3- (aq)
You can see, heat is absorbed during the above reaction, lowering the temperature of the reaction mixture, and thus the reaction flask feels cold.
The reaction going on in Julie’s flask can be represented as:
CaCl2 (s) + 2(H2O) ---> Ca(OH)2 (aq) + 2 HCl (g) + heat
In this case, heat is released during the reaction, elevating the temperature of the reaction mixture, and thus Julie’s reaction flask feels hot.
The teacher’s final comment to Sam and Julie about this experiment is, “When trying to classify a reaction as exothermic or endothermic, watch how the temperature of the surrounding—in this case, the flask—changes. An exothermic process releases heat, causing the temperature of the immediate surroundings to rise. An endothermic process absorbs heat and cools the surroundings.”
Based on the above definition, let's pick a few examples from our daily lives and categorize them as endothermic or exothermic.

Endothermic reactions: Heat is absorbed.

1) Photosynthesis: Plants absorb heat energy from sunlight to convert carbon dioxide and water into glucose and oxygen.
6CO2 + 6 H2O + heat ---> C6H12O6 + 6O2
2) Cooking an egg: Heat energy is absorbed from the pan to cook the egg.

Exothermic reactions: Heat is released.

1) Combustion: The burning of carbon-containing compounds uses oxygen, from air, and produces carbon dioxide, water, and lots of heat. For example, combustion of methane (C, H, start subscript, 4, end subscript) can be represented as follows:
CH4 + 2(O2) ---> CO2 + 2H2O + heat
2) Rain: Condensation of water vapor into rain releasing energy in the form of heat is an example of an exothermic process.

Why is heat released or absorbed in a chemical reaction?

In any chemical reaction, chemical bonds are either broken or formed. And the rule of thumb is "When chemical bonds are formed, heat is released, and when chemical bonds are broken, heat is absorbed." Molecules inherently want to stay together, so formation of chemical bonds between molecules requires less energy as compared to breaking bonds between molecules, which requires more energy and results in heat being absorbed from the surroundings.

What is enthalpy of a reaction?

Enthalpy of a reaction is defined as the heat energy change (ΔHΔH) that takes place when reactants go to products. If heat is absorbed during the reaction, ΔHΔH is positive; if heat is released, then ΔHΔH is negative.
ΔH value negative --> energy released --> exothermic reaction ΔH value positive --> energy absorbed --> endothermic reaction
H=H(bonds broken in reactants)H(bonds made in products)
Let's understand this through an example. We can calculate the enthalpy change (ΔHΔH) for the following reaction:
H, 2, left parenthesis, g, right parenthesis, plus, F, 2, left parenthesis, g, right parenthesis, equals, 2, H, F
We know that the bond energy—in kilojoules or kJ—for H, 2, F, 2, and H, F are 436, 158 and 568 kJ/mole respectively.
Let’s first figure out what’s happening in this particular reaction. Looking at the chemical reaction, it’s clear that one mole of H, minus, H and one mole of F, minus, F bonds are being broken to generate two moles of H, minus, F bonds. Breaking of bonds requires absorption of energy, while formation of bonds releases energy.
To break one mole of H, 2, energy absorbed is 436 kJ.
To break one mole of F, 2, energy absorbed is 158 kJ.
To form two moles of H, F, energy released is 2 X (568) kJ.
So applying the equation, H=H(bondsbrokeninreactants)H(bondsmadeinproducts)
ΔHreaction=(436+158)(2X568)=542kJ
The overall enthalpy of the reaction is negative, i.e., it’s an exothermic reaction where energy is released in the form of heat.

Depiction of an energy diagram

In a chemical reaction, some bonds are broken and some bonds are formed. During the course of the reaction, there exists an intermediate stage, where chemical bonds are partially broken and partially formed. This intermediate exists at a higher energy level than the starting reactants; it is very unstable and is referred to as the transition state. The energy required to reach this transition state is called activation energy. We can define activation energy as the minimum amount of energy required to initiate a reaction, and it is denoted by E, start subscript, a, c, t, end subscript.
An energy diagram can be defined as a diagram showing the relative potential energies of reactants, transition states, and products as a reaction progresses with time. One can calculate the E, start subscript, a, c, t, end subscript and ΔHΔH for any reaction from its energy diagram.
Let’s draw an energy diagram for the following reaction:
Activation energy graph for CO (g) + NO2 (g) ---> CO2 (g) + NO (g)
The activation energy is the difference in the energy between the transition state and the reactants. It’s depicted with a red arrow. The enthalpy change—ΔHΔH—of the reaction is depicted with a green arrow. So, now you should be able to clearly differentiate between E, start subscript, a, c, t, end subscript and ΔHΔH on an energy diagram.

Energy diagrams for endothermic and exothermic reactions

In the case of an endothermic reaction, the reactants are at a lower energy level compared to the products—as shown in the energy diagram below. In other words, the products are less stable than the reactants. Since we are forcing the reaction in the forward direction towards more unstable entities, overall ΔHΔH for the reaction is positive, i.e., energy is absorbed from the surroundings.
Image of a graph showing potential energy in relation to the process of a chemical reaction.
In the case of an exothermic reaction, the reactants are at a higher energy level as compared to the products, as shown below in the energy diagram. In other words, the products are more stable than the reactants. Overall ΔHΔH for the reaction is negative, i.e., energy is released in the form of heat.
Graph showing potential energy and progress of a reaction over time.

Attribution

This article is licensed under a CC-BY-NC-SA 4.0 license. https://creativecommons.org/licenses/by-nc-sa/4.0/
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  • In the section entitled, "Why is heat released or absorbed in a chemical reaction," I am confused as to why it says that when chemical bonds are formed, heat is released and when chemical bonds are broken, heat is absorbed. In my science class, I was taught that when heat is absorbed, something gets hotter. Yet, in an endothermic reaction, I was taught the substance gets colder. Please help my confusion!
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    • duskpin ultimate style avatar for user Davide Ghazal
      You need energy to break up bonds (heat absorbed)....where does this heat come from? It comes from the surroundings.....thus during this process the surrounding close to the substance "are deprived of energy"....As a consequence of that the surroundings get colder "not" the substance that is actually getting hotter....you perceive the hotness or coldness of the surroundings not the hotness or coldness of the substances involved in the reaction.........
  • male robot hal style avatar for user Samir1903
    Why does a frying pan absorb heat to cook an egg? Isn't it supposed to release heat to cook an egg or anything else?
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    • starky sapling style avatar for user Emma Chen
      The egg is gaining energy as it turns from an untrue liquid to a solid. If you look at it at the atomic level, its particles are slowing down. The frying pan absorbs heat, but since it is in contact with the egg, energy transfers to the egg as well.
  • aqualine ultimate style avatar for user hmorgan0813
    The only part that confused me was the section involving the enthalpy equation. In the video labeled "Hess's law and reaction enthalpy change", the equation states H(sum of products) - H(sum of reactants). However, this is backwards in this article. Could someone further explain this difference?

    This website also states the same equation as what's in the video http://chem.libretexts.org/Core/Physical_and_Theoretical_Chemistry/Thermodynamics/State_Functions/Enthalpy/Standard_Enthalpy_Of_Formation

    Thank you in advanced!
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    • The equation you gave (Hproducts - Hreactants) is also a valid equation, but the interpretation of delta H would just be the opposite of what was described above. Using your equation, a negative delta H would indicate an endothermic reaction and a positive delta H would indicate an exothermic reaction. Same concept, different interpretation.
  • Why do we need to learn this?
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    • primosaur ultimate style avatar for user Bryan52
      Because at some point you will have been kidnapped. To which your kidnappers will hold a gun to your head and make you recite what the enthalpy of photosynthesis is.
  • duskpin ultimate style avatar for user Eric Lee
    Why would rain or any type of water cycle step be considered a exo/endothermic reaction? Any reaction dictates that it must have a chemical change, which phase changes do not have. Therefore...wouldn't the water cycle just be an exo/endothermic PROCESS and not a REACTION?
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    • Not really an expert in thermochemistry (or really anything yet, for that matter), but drawing from what I know about biophysics and its analysis of the world, your interpretation would be correct. The change would be representative of a process; a scalar energy value converted into a vectorial physical change. But when looking at energy diagrams, the terms reaction and process are ambiguous until you describe the alteration of the physical matter, where they are not ambiuous terms.
  • starky sapling style avatar for user Sarah Lee
    In an exothermic reaction, why are the chemical bonds in the reactants weaker than the bonds in the products? Please help out :(
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    • Since in an exothermic reaction heat is released, the products will be more stable than the reactants. It's like saying that the products have less heat than the reactants. Since heat is needed to break bonds, the reactants are going to need less heat that the products-their bonds are weaker. :)
  • I'm not sure the changing states part of this article is correct. It mentions the breaking of bonds when water changes physical state (eg. solid ice to liquid). I understood that when a substance changes state that no bonds are formed or broken., generally speaking when a substance changes state there's no chemical reaction. Quite happy to stand corrected!
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    • starky sapling style avatar for user Emma Chen
      Think about it like this: When iron rusts, it is oxidizing. The surface of a piece of iron will begin to corrode first in the presence of oxygen and water. The process of rusting is a combustion reaction, similar to fire. Left in contact with oxygen, iron will react with the oxygen to form rust. In short, the bonds in the iron are breaking to form something new and in this case, it's rust.
  • why adding salt into water will form reactions?why acid base neutralisation reverse?
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  • When energy is absorbed in a reaction where does the heat energy go?
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  • for a reaction both delta s and delta h are positive under what condition the reaction will be spontanious
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