Bond breaking and bond forming confusion

[dancerdoc89]

Back in high school chemistry, I learned that breaking a bond releases energy and forming a bond requires energy input.

I'm studying with Kaplan books and they're telling me that breaking a bond requires energy input and forming a bond releases energy.

What's funny is that both make sense...i.e. you break bonds in an ATP molecule to release energy, however, forming bonds in water (to form ice) also releases energy.

Is one side of this more correct that the other? Thank you!

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[MD Odyssey]

Back in high school chemistry, I learned that breaking a bond releases energy and forming a bond requires energy input.

I'm studying with Kaplan books and they're telling me that breaking a bond requires energy input and forming a bond releases energy.

Is one side of this more correct that the other? Thank you!

Nope. Your high school chemistry class was wrong. Common misconception.

Bond cleavage always takes an input of energy. It's bond formation that releases energy.

If you look at ATP, one has to put energy into the system in order to break bonds. Energy is released when ADP and Pi are reformed.

 

[Donald Juan]

In Biology textbooks they often point out that catabolic reactions, breakdown of molecules, release energy and anabolic reactions require energy, which sort of incorrectly implies that breaking bonds releases energy. However, they are just trying to keep the thermodynamics simplified as possible.

Atoms prefer to form bonds with other atoms to get themselves to a more stable, lower energy state. Since they are going from a higher energy state to a lower energy state, energy is released when they form bonds.

 

[whiteshadodw]

In Biology textbooks they often point out that catabolic reactions, breakdown of molecules, release energy and anabolic reactions require energy, which sort of incorrectly implies that breaking bonds releases energy. However, they are just trying to keep the thermodynamics simplified as possible.

Atoms prefer to form bonds with other atoms to get themselves to a more stable, lower energy state. Since they are going from a higher energy state to a lower energy state, energy is released when they form bonds.

that doesn't necessarily mean that endothermic reactions are not possible. the only thing i'd worry about is the following

enthalpy of bond formation = sum of bonds broken - sum of bonds formed

you can form more or less stable bonds compared to the original bonds, and that will make the reaction endothermic or exothermic.

 

[plzNOCarribbean]

Nope. Your high school chemistry class was wrong. Common misconception.

Bond cleavage always takes an input of energy. It's bond formation that releases energy.

If you look at ATP, one has to put energy into the system in order to break bonds. Energy is released when ADP and Pi are reformed.

I dont think this is correct. Energy is not released when ADP and Pi are reformed, because it is unfavorable to have all of the negative charges on phosphate groups adjacent to one another. Thats why energy is released when ATP is cleaved to for ADP + Pi, and the energy that is released is couple to an non-spontaneous reaction to get it going.

Also, BREAKING a bond always requires the input of some energy. In the situation that you are referring with respect to energy being released when breaking a bond is because the energy that is released once the bond is broken is MORE than the energy that was put in to break the bond, so that is why sometimes they refer to it has energy being released after breaking a bond.

With respect to energy being released when bonds are formed, this is also true when the product bonds are stronger, and more stable than the reactant bonds. This added stability is a result of energy being released. In general, you can think of it like this. Bonds are made because electron clouds overlap and 2 electrons fill an orbital. In the classic gen chem example when there are 2 electrons, once they meet and form a bonding orbital, the overall energy of the electrons is lowered, since electrons prefer to be paired and in a bond. This is the simple bonding & anti-bonding picture , where the y-axis is labeled as energy. This illustrates that once a bond is formed, energy is released & the electrons assume a lower, more stable energetic state.


here a picture:

http://www.ch.ic.ac.uk/vchemlib/course/mo_theory/h2ed.gif

 

[MD Odyssey]

I dont think this is correct. Energy is not released when ADP and Pi are reformed, because it is unfavorable to have all of the negative charges on phosphate groups adjacent to one another. Thats why energy is released when ATP is cleaved to for ADP + Pi, and the energy that is released is couple to an non-spontaneous reaction to get it going.

No. Energy is released because the bonds formed after the reaction have a lower energy than the ones broken in the initial step. This is what determines reaction spontaneity, entropic concerns notwithstanding.

Also, BREAKING a bond always requires the input of some energy. In the situation that you are referring with respect to energy being released when breaking a bond is because the energy that is released once the bond is broken is MORE than the energy that was put in to break the bond, so that is why sometimes they refer to it has energy being released after breaking a bond.

That's a long sentence and I'm not sure I followed you, but I think you're confusing bond breakage and formation with reaction kinetics. Consider the case of a non-spontaneous reaction like the synthesis of glucose. Every single bond broken in such a synthesis would require an input of energy and every single bond formed would release energy. The fact that the net total energy input would be higher than the total energy released is what makes it non-spontaneous.

With respect to energy being released when bonds are formed, this is also true when the product bonds are stronger, and more stable than the reactant bonds. This added stability is a result of energy being released. In general, you can think of it like this. Bonds are made because electron clouds overlap and 2 electrons fill an orbital. In the classic gen chem example when there are 2 electrons, once they meet and form a bonding orbital, the overall energy of the electrons is lowered, since electrons prefer to be paired and in a bond. This is the simple bonding & anti-bonding picture , where the y-axis is labeled as energy. This illustrates that once a bond is formed, energy is released & the electrons assume a lower, more stable energetic state.

I'm aware of how covalent bonds are formed and why energy is released when a bond is formed - that was my argument earlier. That energy can be coupled to something to do work or released as heat. It sounds as if you agree with me, although I think you're mixing and matching terms a bit. I'm not sure why you disagreed with my assertion that energy was released when ADP and Pi were formed.

 

[Axon hillock]

No. Energy is released because the bonds formed after the reaction have a lower energy than the ones broken in the initial step. This is what determines reaction spontaneity, entropic concerns notwithstanding.

That's a long sentence and I'm not sure I followed you, but I think you're confusing bond breakage and formation with reaction kinetics. Consider the case of a non-spontaneous reaction like the synthesis of glucose. Every single bond broken in such a synthesis would require an input of energy and every single bond formed would release energy. The fact that the net total energy input would be higher than the total energy released is what makes it non-spontaneous.

I'm aware of how covalent bonds are formed and why energy is released when a bond is formed - that was my argument earlier. That energy can be coupled to something to do work or released as heat. It sounds as if you agree with me, although I think you're mixing and matching terms a bit. I'm not sure why you disagreed with my assertion that energy was released when ADP and Pi were formed.

Um. He was dead on correct. The bond being formed after the reaction is actually higher in energy. This is why ATP breakdown is coupled with an unfavorable reaction to make it favorable.

Edit: In fact his entire post is correct. Like he said breaking a bond ALWAYS requires an input of some energy.

Edit 2: Disregard everything I have said. Apparently I need sleep. I got confused by your "Energy is released when ADP and Pi are reformed". The "reformed" made me think you were saying energy is released when ATP is formed. I have no clue why I thought this. Sorry

 

[MD Odyssey]

Edit 2: Disregard everything I have said. Apparently I need sleep. I got confused by your "Energy is released when ADP and Pi are reformed". The "reformed" made me think you were saying energy is released when ATP is formed. I have no clue why I thought this. Sorry

Thanks. I was a little hurt I went back and re-read my post a couple times trying to find my mistake. Thanks for keeping me honest.

 

[plzNOCarribbean]

ugh, so basically in response to everything MD odyssey refuted...I was saying exactly what you were implying and I think you may be getting your terms mixed up.

In short, the argument was about energy being released when a bond is formed. If this occurs, it because the bonds formed in the product were stronger, and more stable than the reactant bonds. You should always think of energy being released when bonds are formed in this manner.

 

[hvairline]

Whenever a bond forms the participating atoms go to a lower energy state(if there was no energy pay off then the bond would not have formed). Therefore by conservation of energy, energy must be released.When you brake a bond you have to provide energy to overcome the attractive forces of the nuclei to the common pair of electrons.Energy is thus required. When you read in biology textbooks that glucose is broken down bla bla bla to make ATP you have to recognize that in order to break down glucose you have to put some initial energy. The energy payoff comes when much more stable bonds form resulting in a drop of potential energy and energy release that is used to produce ATP. ( Remember even in the initial stages of glycolysis 2 ATP/ 1 glucose are required before any molecules of ATP are produced. Hope this helps!

 

[whiteshadodw]

i think there seems to be a little confusion going on here between endergonic/exergonic and endothermic/exothermic. keep in mind that those are totally different factors.

you could have a reaction that is endothermic but its still exergonic for a given temperature. and i won't go into the numerous other scenarios, but i think its just best to be aware of this.

 

[MD Odyssey]

In short, the argument was about energy being released when a bond is formed. If this occurs, it because the bonds formed in the product were stronger, and more stable than the reactant bonds.

No. It always occurs, even if the product of the reaction is less stable than the reactant - energy is always released in the formation of bonds, regardless of whether the overall reaction is endergonic or exergonic. That's why bonds form in the first place. I think you're missing the intent of the original question. Several of the posters in this thread have remarked that energy is released in a reaction when the products are more stable than the reactants, which is true, but not what the OP was asking.

Whether an overall reaction releases or consumes energy is irrelevant to the question of whether bond breakage requires an input of energy or not. You seem to be focusing on the reaction energy, which is the finished product, while skipping over the parts in between. Let me try to simplify and maybe what I'm trying to say will make more sense:

1. The cleavage of any covalent bond always requires an input of energy
2. The formation of any covalent bond always releases energy
3. One can estimate the change in enthalpy for a reaction by subtracting the sum of the enthalpy changes for all the bonds formed from the sum of the enthalpy changes for all the bonds broken

If the change in enthalpy is negative, then the reaction is exothermic and if it is positive, than it is endothermic. Regardless, bond cleavage requires energy and bond formation always releases energy. The relative amounts for both quantities ultimately determines whether the overall reaction releases or requires energy.

Whether a reaction is favorable or not requires a knowledge of the entropy change of the reaction, which can be determined in a similar way. Once the changes in enthalpy and entropy are known, you can determine favorability by calculating the change in the Gibbs free energy of the reaction. It's entirely possible to have an exothermic reaction which is not favorable at normal temperatures.

 

[Axon hillock]

No. It always occurs, even if the product of the reaction is less stable than the reactant - energy is always released in the formation of bonds, regardless of whether the overall reaction is endergonic or exergonic. That's why bonds form in the first place. I think you're missing the intent of the original question. Several of the posters in this thread have remarked that energy is released in a reaction when the products are more stable than the reactants, which is true, but not what the OP was asking.

Whether an overall reaction releases or consumes energy is irrelevant to the question of whether bond breakage requires an input of energy or not. You seem to be focusing on the reaction energy, which is the finished product, while skipping over the parts in between. Let me try to simplify and maybe what I'm trying to say will make more sense:

1. The cleavage of any covalent bond always requires an input of energy
2. The formation of any covalent bond always releases energy
3. One can estimate the change in enthalpy for a reaction by subtracting the sum of the enthalpy changes for all the bonds formed from the sum of the enthalpy changes for all the bonds broken

If the change in enthalpy is negative, then the reaction is exothermic and if it is positive, than it is endothermic. Regardless, bond cleavage requires energy and bond formation always releases energy. The relative amounts for both quantities ultimately determines whether the overall reaction releases or requires energy.

Whether a reaction is favorable or not requires a knowledge of the entropy change of the reaction, which can be determined in a similar way. Once the changes in enthalpy and entropy are known, you can determine favorability by calculating the change in the Gibbs free energy of the reaction. It's entirely possible to have an exothermic reaction which is not favorable at normal temperatures.

To make up for my previous post. I will concur 100% with what you've written now

 

[plzNOCarribbean]

No. It always occurs, even if the product of the reaction is less stable than the reactant - energy is always released in the formation of bonds, regardless of whether the overall reaction is endergonic or exergonic. That's why bonds form in the first place. I think you're missing the intent of the original question. Several of the posters in this thread have remarked that energy is released in a reaction when the products are more stable than the reactants, which is true, but not what the OP was asking.

Whether an overall reaction releases or consumes energy is irrelevant to the question of whether bond breakage requires an input of energy or not. You seem to be focusing on the reaction energy, which is the finished product, while skipping over the parts in between. Let me try to simplify and maybe what I'm trying to say will make more sense:

1. The cleavage of any covalent bond always requires an input of energy
2. The formation of any covalent bond always releases energy
3. One can estimate the change in enthalpy for a reaction by subtracting the sum of the enthalpy changes for all the bonds formed from the sum of the enthalpy changes for all the bonds broken

If the change in enthalpy is negative, then the reaction is exothermic and if it is positive, than it is endothermic. Regardless, bond cleavage requires energy and bond formation always releases energy. The relative amounts for both quantities ultimately determines whether the overall reaction releases or requires energy.

Whether a reaction is favorable or not requires a knowledge of the entropy change of the reaction, which can be determined in a similar way. Once the changes in enthalpy and entropy are known, you can determine favorability by calculating the change in the Gibbs free energy of the reaction. It's entirely possible to have an exothermic reaction which is not favorable at normal temperatures.

Right, perfect. I agree with everything stated above. Thats what I was intending to say. And to the OP, the last part he is referring to is in regard to thermodynamics and the equation dG =dH - TdS where d= delta, G= gibbs free energy, H = enthalpy, and S= entropy. Keep these things in mind when trying to determine whether a reaction is spontaneous or not, because both the entropy component and enthalpy component play a role in rxn spontaneity.

 

[WTN]

No. It always occurs, even if the product of the reaction is less stable than the reactant - energy is always released in the formation of bonds, regardless of whether the overall reaction is endergonic or exergonic. That's why bonds form in the first place. I think you're missing the intent of the original question. Several of the posters in this thread have remarked that energy is released in a reaction when the products are more stable than the reactants, which is true, but not what the OP was asking.

Whether an overall reaction releases or consumes energy is irrelevant to the question of whether bond breakage requires an input of energy or not. You seem to be focusing on the reaction energy, which is the finished product, while skipping over the parts in between. Let me try to simplify and maybe what I'm trying to say will make more sense:

1. The cleavage of any covalent bond always requires an input of energy
2. The formation of any covalent bond always releases energy
3. One can estimate the change in enthalpy for a reaction by subtracting the sum of the enthalpy changes for all the bonds formed from the sum of the enthalpy changes for all the bonds broken

If the change in enthalpy is negative, then the reaction is exothermic and if it is positive, than it is endothermic. Regardless, bond cleavage requires energy and bond formation always releases energy. The relative amounts for both quantities ultimately determines whether the overall reaction releases or requires energy.

Whether a reaction is favorable or not requires a knowledge of the entropy change of the reaction, which can be determined in a similar way. Once the changes in enthalpy and entropy are known, you can determine favorability by calculating the change in the Gibbs free energy of the reaction. It's entirely possible to have an exothermic reaction which is not favorable at normal temperatures.

THANK YOU. People say this so wishy washy. So glad to find it straight up.