I was watching this Chemical Curiosities video which is full of some of the basic chemistry demonstrations that most of us probably saw in high school or college.
It started me thinking about exothermic reactions and trying to figure out what’s happening - energy wise - at the molecular level. I know that exothermic reactions have a net surplus of heat and endo- a net deficit of heat after all the chemical bond breaking and making is done with. But I have always thought that the chemical bonds themselves were the manifestation of the bond energy that is resident within the reactants before reaction begins. Therefore, breaking the bonds between atoms releases said energy and there’s the source of your heat in an exothermic reaction. But then I come across sites that say:
which contradicts my reasoning above. That makes me think I’ve misunderstood the concept of bond energy from the start. If bond energy isn’t really energy (that gets liberated as heat when the bond is broken) then what is it?
You’ve answered your own question with the quoted text. Bonds are what glue atoms together. It takes energy to pull them apart. Energy is released when you allow them to stick to each other. Think about two magnets that attract each other. When they are apart, you can harness the force of attraction to do some work. You have to do work to pull them apart.
If the new arrangement is lower energy than the old one then you’ll get energy released, whether by making bonds or breaking them. Don’t you generally have both going on at once anyway?
For instance, a car air-bag uses a pellet of sodium azide (NaN[sub]3[/sub]) to generate the necessary gas from solid really, really quickly. You’ll certainly end up with a bunch of N[sub]2[/sub] floating around so some nitrogen bonds have broken and others have formed, but energy is certainly released during the process, explosively so.
If breaking bonds released energy, the tendency in the universe would be towards no-bonds. We could theoretically get energy from taking apart the O[sub]2[/sub] and N[sub]2[/sub] molecules in the air, and they wouldn’t reform without using up some energy.
Your misunderstanding is in thinking bond energy is contained in the bonds, instead it’s somewhat opposite. Bond energy is what was released when the bond was formed, and is required to break it again. In an exothermic reaction like burning hydrogen gas in an oxygen atmosphere, the initial bonds between H and H, H and H, and O and O, require less energy to break than is released as bonds form between H, O and H, and H, O and H.
And for a given reaction, what happens is that some bonds get broken, other bonds get built, and the reaction is exo- or endo- thermic depending on the relative energies of the bonds on both sides of the equation.
Write
A + B = C + D + Energy
in the opposite direction, you get
C + D = A + B - Energy
There are reactions, such as the aforementioned formation of water from hydrogen and oxygen, which are not going to “go backwards”, but many others will, and one of the tools chemists have in order to push an equilibrium to one side of the equation or the other is precisely the addition or elimination of energy (heating vs cooling, in the simplest form).
I’m stuck here, and I think the reason I’m stuck is because I’m trying to apply this to photosynthesis. Sugar cane plants take in energy from the sun and that energy is used to make bonds between carbon, hydrogen and oxygen which become sucrose, starch and other saccharides. They’re using the energy coming into the system to form these chemical bonds, no?
Yes, but natural pathways begin by being a lot more complicated than “water plus carbon dioxyde equals sugar”. If that worked, there wouldn’t be carbonated water.
There are kynetic factors, activation energies, biomolecules acting as catalysts (usually enzimes), etc etc which are not present when our own bodies use the sugar plus oxygen to obtain water plus carbon dioxyde plus energy. The path is different. It’s like going from Los Angeles to Kansas City via Anchorage or via the Panama Canal - the energy required is different because the path is different.
I think you’re replying to the first version of my post.
The energy absorbed in the global reaction is what is released when we “burn” the sugars, but it’s not quite the same as the simple “A + B = C + D ± Energy” reactions because of the different pathways. I’m not explaining this well, my brain seems to be trying to think in way the wrong language today (thinking in pics and had a lousy day).
Making any individual bond from the two things it binds releases energy. Any bond. Making a bunch of bonds from other things that were bonded can absorb or release energy, depending on which bonds are being made (release energy) and which are being broken (absorb energy). The plant example is using some very complicated chemical pathways to make an impossible reaction possible - it absorbs so much energy and has such high energy barriers to surmount (it needs to go over the mountains on its knees, borrowing an image from my kinetics teacher) that it simply doesn’t happen without all those enzymes, chlorophyll, etc; the energy gets released when we burn sugar, or when we use it in our body. Plants are the battery chargers of animals.
Burning the sugar has two parts: breaking the bonds (which requires as much energy as making them) and making new ones. We always need energy to break the bonds, but the total reaction energy will be endo or exo depending on what gets made, what gets broken.
I’m wary of wandering too deeply into cellular metabolism, so instead let’s think about dehydration of sugar using acid. If I understand the replies above correctly, all the heat generated during this reaction is not coming from the acid ripping the hydrogen atoms off the sugar molecules. So where is it coming from?
It is coming from the difference between the energy released by the making of new bonds, and the energy spent in breaking the original bonds. If release > spend, exo. If release < spend, endo.
I guess the reason this seems so counterintuitive to me is that I think of the process of making bonds as work, and to do work you have to put energy into a system. I guess I need to stop looking at it that way.
Apparently not. My reply was based on an assumption that the products were correct, but the point still remains when it’s just sugar into carbon and water.
Well, it’s not any acid, it’s sulfuric acid usually. Concentrated sulfuric acid. And no, you’re not making sulfur dioxide (you do make water). The enormous energy comes from the strength of the bonds between the water and the sulfuric acid. It’s a different type of reaction than people usually think about at an elementary level, because if you draw out some lewis structures, you wouldn’t draw any bond between the water and the sulfuric acid. None the less, there is a very strong interaction between the two (basically a hydration energy)
You can think about it in terms of work but it gets complicated fast. In the simplest case of a covalent bond between two hydrogen atoms, there is a relationship between distance and force such that there is negligible attraction between the nuclei at long range, a strong attraction at short range, and a repulsion at very short range. The example of elemental hydrogen is a bit contrived, however, as you’ll never naturally see a cloud of mono-atomic hydrogen.
In more complicated interactions between molecules, there will be a large set of force-distance relationships between each nuclei and electron orbital. Some of those will include relatively long-distance repulsion, or a net interaction that is repulsive from most orientations. In a sense, the activation energy includes the work done to force two repelling molecules together, but once they get close enough they are attracted as a bond forms.
Even in a “simple” chemical reaction like spontaneous hydrolysis, you have to account for the breaking of two bonds (H-OH and RC-OR) and formation of two bonds (RC-OH and H-OR). The net work involving all of the force-distance relationships between all of the broken and formed bonds is what makes a reaction exothermic or endothermic. But that’s impractically difficult to calculate or even model except in the simplest cases.
(I hope I’m not getting in too much trouble with classical physics analogies here…)