Exo vs Endothermic

[marlene 1K]

How do we know when a chemical reaction is exothermic or endothermic when we're not given the deltaH? For example I was working on a problem and it gave me the reaction : 2C(s)+O2(g)<->2CO(g) and asked if it was endothermic or exothermic then asked questions about Le Chatiers Principle, specifically about increasing temperature and decreasing temperature and which way the reaction would shift.

[MinjooPark_3I]

I'm not too sure but I think this reaction would be exothermic because it's the synthesis of CO2 gas. Synthesis reactions are exothermic because they release heat to be produced. Also when bonds are being formed, it is an exothermic reaction and when the bonds are being broken the reaction would be endothermic. hope this helps!

[Neal_Agarwal_3B]

We know a reaction is exothermic if its deltaH is negative as that means it is releasing heat into the surroundings. We know that a reaction is endothermic when the deltaH is positive as it is absorbing heat from the surroundings. For calculating a shift in equilibrium due to temperature its important to see whether the reaction is endothermic or exothermic. If heat is added to an endothermic reaction then it will produce more products. If heat is reduced in an exothermic reaction it will produce more products. I think this equation you present is likely a combustion reaction and is therefore exothermic as it releases heat, so increasing temperature would shift the reaction left and decreasing heat would shift the reaction right. I hope this helps!

[JonathanSung_2G]

I think another thing to note is that forming bonds releases energy and breaking bonds requires energy. If you were to use the bond enthalpies when figuring out the bonds that are broken and formed, you could calculate that the delta H will be negative. However, for this reaction specifically, I think that we are also supposed to know that heat is released as a result since this reaction is considered a combustion reaction. Otherwise, I think typically we will see a delta H given to us by the problem (I could be wrong). Hope this helped a bit!

[Namita Shyam 3G]

I agree with some of the previous comments---just remember that exothermic means forming bonds, and endothermic means breaking bonds. In your example, it looks like bonds are being formed, so it would be exothermic. I also feel like on the exam, the deltaH should be given to us, so I would suggest not worrying too much about this...

[Massimo_Capozza_1G]

I just think of exothermic (exo) and an explosion of energy while endothermic (end) is taking in energy.

[alette1a]

You would have to know some other information. Here are some things I can think of off of the top of my head:
- Stability of a certain side over another. The reaction will be endothermic if we are forming a more thermodynamically unstable product; it will be exothermic if we are forming a more thermodynamically stable product. (Just think of what state nature prefers a molecule to be in. For example, it prefers O2 over just O.)
- It tells you which bonds are stronger/weaker (like we saw in Sapling for this week). If the overall bond strengths are stronger on the reactants side, it is endothermic, because the energy required to break bonds > energy released from forming bonds. And vice versa for exo.

[Samantha Low 3D]

Combustion reactions reactions (chemical reactions that involve O2 and the formation of CO2 and most of the time H20) are almost always exothermic because they release heat. Hope this helps!

[Crystal Pan 2G]

When bonds are formed, energy will be released which would make it exothermic. When bonds are being broken, energy is required to break them, which means it is endothermic.

[Sharon Kim 2A]

We can tell if the reaction is exo or endo depending if the reaction is forming bonds or breaking. In an exothermic reaction, heat is being released and bonds are being formed. But in endothermic reactions, bonds are being broken because energy is being put in to break the bonds. The reaction that you wrote indicates that it is an exothermic reaction because from the elements 2C and O2, it is forming bonds to make 2CO.

[Violet Kwan 3H]

Sometimes in the book, you need to look at the chart for bond enthalpies and then solve to see if it is endothermic or exothermic. All the bonds broken would require energy, and the bonds formed will release energy.

[Morgan Gee 3B]

Something that's nice to know is that combustion reactions are always going to be exothermic. Synthesis reactions are usually going to be endothermic.

[apurva-3E]

In general the formation of bonds = exo
the breaking of bonds = endo

[Talia Dini - 3I]

Hi! The way I distinguish between endothermic and exothermic reactions is that if bonds are being formed, the reaction will be exothermic because energy is being released. However, if bonds are being broken, the reaction will be endothermic because energy is required.

[Danielle DIS2L]

Endothermic means that it absorbs energy and exothermic means a release of energy.

[Kandyce Lance 3E]

When bonds are formed, energy will be released which would make it exothermic. When bonds are being broken, energy is required to break them, which means it is endothermic.

Does that also mean more energy is needed to break vs form reactions or is it dependent on specific chemical reactions?

[Diana Aguilar 3H]

Exothermic is when energy is being released, like in the forming of bonds, and endothermic is when energy is required, like when you are breaking bonds. This is how I think of it, hope it helps!

[SophiaJenny3I]

Hi! Just remember that forming bonds is exothermic (the release of energy), while breaking bonds is endothermic (takes up/requires energy)! When thinking about the effect of a temperature change, I like to think about it like this: for exothermic reactions, the deltaH is negative. If the temperature increases, it does not want to release even more heat, so the reaction counteracts this by favoring the reactants. For endothermic reactions, on the other hand, the deltaH is positive as it is taking up energy. When temperature increases, the system wants to relieve itself of some of that heat so it favors the products. These are rather simplified explanations, but they help me remember quickly when I mix them up! Hope this helps!

[Britney Tran IJ]

endothermic means that the process is absorbing energy while exothermic means that the process releases energy.

[Mina Tadros 3L]

Endothermic is when heat is absorbed, so there is sufficient energy to break the bonds within a molecule. Exothermic, on the other hand, is when energy is released, which typically occurs when bonds are being formed. You would know it is exothermic for this reaction because it appears to be a combustion reaction (O2 in the reactants and CO2 is being formed).

[MariaCassol1L]

If a reaction is exothermic it releases heat, while if it is endothermic it absorbs/requires heat. When you are forming bonds the reaction is always exothermic because bonds form because they are in a lower energy form than the single atoms, so the element "want" to form bonds. Breaking bonds is also always endothermic because you need to bring the atoms to a higher energy form. Other things that are always exothermic/endothermic are phase changes, so for example going from a solid to a liquid is endothermic (think ice melting- needs heat), and a liquid to solid exothermic.

[Ashley Wagner 2A]

The reaction is exothermic when there is a bond formation and endothermic where there is a bond breaking.

[Ephrem Gerald 2A]

You could use the standard enthalpies of formation to see whether it is endo or exothermic. In this case it would be the standard enthalpy of formation for CO minus that of C and O2. However, since O2 and C both have a standard enthalpy of formation of zero (because they are in their most stable elemental forms), the delta H would be the standard enthalpy of formation of CO, and that happens to be a negative value, meaning the reaction is exothermic.

[Brianna Chen 3F]

When I am given an equation, I like to see if bonds are being broken or formed. For example, if a compound is forming, I think of the bonds forming, so it would release heat and be exothermic. This logic can also be applied vice versa.

[Shruti Kulkarni 2I]

Since forming bonds requires energy and breaking bonds loses energy, you can determine if it's exo or endothermic based on if bonds are being created or destroyed. If a bond is being created, it probably is endothermic because making a bond requires energy, and if a bond is being broken, it is probably exothermic as breaking bonds releases energy.

[MMorcus2E]

In terms of the relationship between being endothermic or exothermic and Le Chatelier's Principle, there were a few rules that were mentioned:
1. If you increase the temperature in an endothermic reaction (delta H>0), then the products will be favored. If you decrease the temperature in an endothermic reaction, then the reverse reaction will be favored, meaning reactants will be favored.
2. If you increased the temperature in an exothermic reaction (delta H<0), then the reactants will be favored (opposite of what happens in an endothermic reaction). If you decrease the temperature in an exothermic reaction, then the forward reaction will be favored.

[Jeremy Wei 2C]

A general rule is that breaking bonds takes energy while forming bonds release energy. So in this case, the reaction would be exothermic since bonds are formed.

[David Y]

When bonds break, that requires energy, making it endothermic. However, when bonds form, there is a release of energy, making it exothermic.

[Jacob Schwarz-Discussion 3I]

This equation seems very vague and I'm not totally sure how you would be able to tell if the equation is exo or endothermic. On the test the deltaH will likely be given but I like to think that exothermic is bonds being formed because forming bonds gives off heat whereas endothermic is breaking bonds because breaking bonds REQUIRES heat

[Ritika Prasad 1A]

Adding on to all of these replies, you would also be able to calculate or approximate deltaH if you had the temperature, change in Gibbs Free Energy, and change in entropy through the equation. So if you knew about the increase/decrease in G and S, you could probably estimate a value of H that might help in some situations through ΔG=ΔH−TΔS.

[VSU_3F]

For exothermic reactions, we're always going from reactants with higher free energy states to products with lower free energy states, which creates a net release of energy.

For endothermic reactions, we're always going from reactants with lower free energy states to products with higher free energy states, which causes a net absorption of energy.

To determine whether products or reactants have a higher energy state, draw out the Lewis structure and determine which arrangement is more stable. The more stable arrangements have lower free energy states.

[Bai Rong Lin 2K]

Typically think of exothermic as forming a bond and endothermic as breaking the bond when applying it to a question.

[Neel Sharma 3F]

You can use the enthalpy of formations to find the delta H values. You can also use your understanding of molecular stability and the fact that energy is released when bonds are formed and required when bonds are broken to understand the nature of the reaction. Hope this helps!

[kristinalaudis3e]

think about physically what is happening in the equation. exothermic means it releases energy while endothermic means it absorbs!

[Jose Miguel Conste 3H]

you look at phase changes if delta h is not given, in your example, one reactant was solid and the product was a gas, therefore you had to put in heat for that reaction to occur making it endothermic.

[Brian Acevedo 2E]

In the context of this question, I think the goal was to understand that the input of energy required to break the bond holding together the O₂ is less than the enrgy given off by the formation of the two C-O bonds in CO₂, therefore you can conclude the overall reaction would be exothermic.

[EnricoArambulo3H]

I think for this question we would have to comment on the relative stability of the molecules. The energy needed to break the bonds between C and O2 will be lower than the energy released when forming the C=O bonds. I hope this helped!

[Susan Chamling 1F]

In general we just think of endothermic reactions as ones which absorb energy, so if the question talks about an absorption of heat or other indication of energy absorption, the reaction is most likely endothermic. On the other hand, if the statement discusses a reaction which releases heat or energy, it’s most likely exothermic

[tholz11]

I think the question may be referring to how increasing T increases Pressure, (PV=nRT) and when pressure is increased the reaction will shift to the side with less mols of gas.

[Audrey Banzali-Marks 1A]

One way to tell if it's exothermic is if the reaction is forming bonds, which will release energy, likely in the form of heat. Also, if the description of the reaction in the directions mentions the reaction releasing heat, it's exothermic. If the reaction has to be heated to take place, I'd say it's probably endothermic.

[Talia Tam 3L]

I think that when bonds are formed, the reaction is exothermic. When bonds are broken, reactions are endothermic.