# Chem 101 - exo & endothermic reactions **URL:** \ **Category:** Factual Questions\ **Created:** [March 27, 2015, 1:01pm UTC](https://boards.straightdope.com/t/chem-101-exo-endothermic-reactions/716107 "2015-03-27T13:01:44Z")\ **Posts on this page:** 14\ **Page:** 2
**Author:** ![johnpost](https://avatars.discourse-cdn.com/v4/letter/j/f17d59/32.png) [@johnpost](https://boards.straightdope.com/u/johnpost)\ **Post date:** [March 27, 2015, 5:05pm UTC](https://boards.straightdope.com/t/chem-101-exo-endothermic-reactions/716107/21 "2015-03-27T17:05:25Z")
explanations in science are hard, especially microscopic things like chemistry. you try to express reality you can’t see. you can try to offer an explanation of part of a phenomena or process but it has limits. in the end what exists spontaneously goes to lower energy, a more relaxed state overall. you can supply energy in a small area to affect things but overall it’s less energy where things end up. this is where it gets hard to wrap your head around things. you have to think microscopic about things you can’t see and think macroscopic about things bigger than you can see. in some ways much of a reaction process (which you would need the area of physical chemistry to understand) or a metabolic pathway like photosynthesis (which you would need the area of biochemistry to understand) are like a roller coaster ride. you might put some energy in at the start (or start at a high energy if it was previously stored), then gain or loose energy along the way and you end up no higher (in terms of chemical processes would be lower) than where you started. ---
**Author:** ![Learjeff](https://avatars.discourse-cdn.com/v4/letter/l/94ad74/32.png) [@Learjeff](https://boards.straightdope.com/u/Learjeff)\ **Post date:** [March 27, 2015, 5:35pm UTC](https://boards.straightdope.com/t/chem-101-exo-endothermic-reactions/716107/22 "2015-03-27T17:35:22Z")
Seems to me this is pretty simple. A bond is a lower energy state than no bond; otherwise it wouldn’t be stable. However, all bonds are not alike. The bonds in C6H12O6 + 6O2 aren’t nearly as stable (as low-energy) as the bonds in 6CO2 + 6H2O. Is the total energy state in C6H12O6 + 6O2 lower than 6C + 12H + 12O? I bet so. However, I wouldn’t want to guess about that versus C6 + 6H2 + 6O2. (Assume the C6 is actually graphite or some other common carbon molecule.) ---
**Author:** ![Patty\_O\_Furniture](https://avatars.discourse-cdn.com/v4/letter/p/96bed5/32.png) [@Patty\_O\_Furniture](https://boards.straightdope.com/u/Patty_O_Furniture)\ **Post date:** [March 28, 2015, 4:54am UTC](https://boards.straightdope.com/t/chem-101-exo-endothermic-reactions/716107/23 "2015-03-28T04:54:06Z")
> [@buddy431](#): > > 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). Is this incorrect: > [@](#): > > Summary > > Concentrated sulfuric acid is mixed with household sugar, which is attacked by the acid, a dehydrating agent. The final products are carbon, water vapor, and sulfur dioxide gas. > > [snip] > > The equation for this reaction is: > > C12H22O11(s) + H2SO4(aq) + 1/2 O2(g) → 11C(s) + CO2(g) + 12H2O(g) + SO2(g) From [here](http://www.chemeddl.org/alfresco/service/org/chemeddl/video.html?options=false&ID=vid:5892&guest=true). I think at this point I’m going to have to be satisfied with an explanation at the macroscopic level and simply accept that no energy is released when the sugar molecule gets ripped apart by the acid (indeed, energy must be added to the system from… [_somewhere?_], since bonds are being broken), and the heat energy that is demonstrated is actually produced by the formation of the new bonds as the products carbon, carbon dioxide, water (vapor) and sulfur dioxide are formed. It’s quite a challenge trying to unlearn something you thought you understood so well, but I appreciate all of your explanations. ---
**Author:** ![JWT\_Kottekoe](https://avatars.discourse-cdn.com/v4/letter/j/3ec8ea/32.png) [@JWT\_Kottekoe](https://boards.straightdope.com/u/JWT_Kottekoe)\ **Post date:** [March 28, 2015, 5:09am UTC](https://boards.straightdope.com/t/chem-101-exo-endothermic-reactions/716107/24 "2015-03-28T05:09:14Z")
> [@Patty\_O\_Furniture](#): > > 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? The key step in photosynthesis is the breaking of the bonds in H2O to release protons (Hydrogen) and produce diatomic Oxygen as a waste product. This takes so much energy that a single photon of visible light is not energetic enough to do the job. The remarkable chain of reactions in photosynthesis uses the energy of two photons to split the water molecule. Once that is done, the rest of the reactions to produce sugar go energetically down hill. ---
**Author:** ![Hector\_St\_Clare](https://avatars.discourse-cdn.com/v4/letter/h/b5ac83/32.png) [@Hector\_St\_Clare](https://boards.straightdope.com/u/Hector_St_Clare)\ **Post date:** [March 28, 2015, 2:12pm UTC](https://boards.straightdope.com/t/chem-101-exo-endothermic-reactions/716107/25 "2015-03-28T14:12:39Z")
> [@Patty\_O\_Furniture](#): > > 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? No, they’re using the energy coming into the system (primarily) to _break_ the chemical bonds in water. To incorporate carbon into organic materials it needs to be reduced (= have electrons added), and those electrons come ultimately from water, which requires breaking the hydrogen-oxygen bonds. ---
**Author:** ![LSLGuy](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/lslguy/32/5813_2.png) [@LSLGuy](https://boards.straightdope.com/u/LSLGuy)\ **Post date:** [March 28, 2015, 3:32pm UTC](https://boards.straightdope.com/t/chem-101-exo-endothermic-reactions/716107/26 "2015-03-28T15:32:44Z")
The only reason the universe isn’t a diffuse gas of monoatomic stuff is that atoms “like” to connect as molecules. Bonded together is (generally speaking) the preferred = low energy state. That is the basic rule & crude arm-waving explanation that belongs as the bottom layer of a non-specialists’ (like me) intuitive explanation of why the energy flows are the way they are. The next layer up is that any chemical conversion reaction involves both bond-breaking & bond-forming and we only see the net energy flow in or out. The next layer up is that biologically significant chemical reactions are vastly more complicated with intermediate states, catalysts, etc. So non-specialist intuition becomes inapplicable in the face of that level of complexity. So non-specialists just have to accept the results we see, be that photosynthesis or ATP /ADP respiration or … ---
**Author:** ![Isilder](https://avatars.discourse-cdn.com/v4/letter/i/cc9497/32.png) [@Isilder](https://boards.straightdope.com/u/Isilder)\ **Post date:** [March 29, 2015, 11:32am UTC](https://boards.straightdope.com/t/chem-101-exo-endothermic-reactions/716107/27 "2015-03-29T11:32:31Z")
> [@Patty\_O\_Furniture](#): > > a. Energy is absorbed to break bonds. b. Bond-breaking is an endothermic process. Well, that explains fire then… So you have a gaseous mix of H2 and O2… 1. its not burning 2. Apply a heat source to break some of those bonds… there’s no monoatomic H and O … 3. They join together. to form H2O… This releases more heat than absorbed at 2… 4. the temperature of the gases in the general area increases… so its a self sustaining process… the flame cooks the H2 and O2 nearby, causing them to react by process in 2… ---
**Author:** ![bizzwire](https://avatars.discourse-cdn.com/v4/letter/b/958977/32.png) [@bizzwire](https://boards.straightdope.com/u/bizzwire)\ **Post date:** [March 30, 2015, 9:10pm UTC](https://boards.straightdope.com/t/chem-101-exo-endothermic-reactions/716107/28 "2015-03-30T21:10:06Z")
> [@Nava](#): > > Yes, because of the difference in chemical pathways. Gotta take exception to this. The change in enthalpy in a chemical reaction depends only on the difference between the starting and ending energies of the reactants/products. it is completely independent of the pathway. ---
**Author:** ![md2000](https://avatars.discourse-cdn.com/v4/letter/m/73ab20/32.png) [@md2000](https://boards.straightdope.com/u/md2000)\ **Post date:** [March 31, 2015, 6:25pm UTC](https://boards.straightdope.com/t/chem-101-exo-endothermic-reactions/716107/29 "2015-03-31T18:25:32Z")
Think of some molecular bonds like pushing a spring-loaded door snapped shut. By exerting some force you push the atoms close enough that they link, thereby creating a bond that consists of stored energy, storing the energy absorbed when the atoms bonded. Now apply enough heat (energy) or other molecular attraction (i.e. acid) to rip those bonds apart, and they will release that stored energy. ---
**Author:** ![buddy431](https://avatars.discourse-cdn.com/v4/letter/b/6a8cbe/32.png) [@buddy431](https://boards.straightdope.com/u/buddy431)\ **Post date:** [March 31, 2015, 7:20pm UTC](https://boards.straightdope.com/t/chem-101-exo-endothermic-reactions/716107/30 "2015-03-31T19:20:54Z")
> [@md2000](#): > > Think of some molecular bonds like pushing a spring-loaded door snapped shut. By exerting some force you push the atoms close enough that they link, thereby creating a bond that consists of stored energy, storing the energy absorbed when the atoms bonded. > > Now apply enough heat (energy) or other molecular attraction (i.e. acid) to rip those bonds apart, and they will release that stored energy. That’s… backwards. Breaking bonds takes energy. Forming bonds releases energy. ---
**Author:** ![Patty\_O\_Furniture](https://avatars.discourse-cdn.com/v4/letter/p/96bed5/32.png) [@Patty\_O\_Furniture](https://boards.straightdope.com/u/Patty_O_Furniture)\ **Post date:** [April 1, 2015, 5:15am UTC](https://boards.straightdope.com/t/chem-101-exo-endothermic-reactions/716107/31 "2015-04-01T05:15:23Z")
Since this thread has come back to life, I’d like to take this and chew on it for a while: > [@buddy431](#): > > Breaking bonds takes energy. I can grasp this because it seems intuitive. Og sees two squirrels fighting over a stick, each stubbornly refusing to let go. Og picks up a rock and smashes the stick. Og has inputted energy into the system to break the bond between the two squirrels. But my analogy fails me because the Og-smash thought experiment above isn’t neatly reversible to help me understand why… > [@](#): > > Forming bonds releases energy. So forget about the squirrels. I’m picturing two atoms floating around, until they get close enough and realize they’d like to bond with each other. What energy do these individual particles possess in their solitary existence, that is then subsequently released when they bond with each other? Can this energy be measured, even in theory? Would _force_ be a better term for it? I get that there are forces of attraction and repulsion, but those forces aren’t energy. The reason I’m failing to understand this is because I’m leaning on a background of electrical engineering and we like to account for every watt or joule of energy as it moves from input to output. If some of it is lost in between, we can account for it and show that it has been converted to mechanical energy or heat energy. I understand that a generator doesn’t “release” energy, but it produces it from the combination of motion or a conductor through a magnetic force. That’s what I’m trying to picture regarding the energy that is released during chemical bonding. Even [this short chemistry video](https://youtu.be/EoHOXIvxNQc), which shows chemical element bonds actually forming and breaking, was not much help. ---
**Author:** ![buddy431](https://avatars.discourse-cdn.com/v4/letter/b/6a8cbe/32.png) [@buddy431](https://boards.straightdope.com/u/buddy431)\ **Post date:** [April 1, 2015, 7:35am UTC](https://boards.straightdope.com/t/chem-101-exo-endothermic-reactions/716107/32 "2015-04-01T07:35:20Z")
> [@Patty\_O\_Furniture](#): > > Since this thread has come back to life, I’d like to take this and chew on it for a while: > > I can grasp this because it seems intuitive. Og sees two squirrels fighting over a stick, each stubbornly refusing to let go. Og picks up a rock and smashes the stick. Og has inputted energy into the system to break the bond between the two squirrels. > > But my analogy fails me because the Og-smash thought experiment above isn’t neatly reversible to help me understand why… > > So forget about the squirrels. I’m picturing two atoms floating around, until they get close enough and realize they’d like to bond with each other. What energy do these individual particles possess in their solitary existence, that is then subsequently released when they bond with each other? Can this energy be measured, even in theory? Would _force_ be a better term for it? I get that there are forces of attraction and repulsion, but those forces aren’t energy. > > The reason I’m failing to understand this is because I’m leaning on a background of electrical engineering and we like to account for every watt or joule of energy as it moves from input to output. If some of it is lost in between, we can account for it and show that it has been converted to mechanical energy or heat energy. I understand that a generator doesn’t “release” energy, but it produces it from the combination of motion or a conductor through a magnetic force. > > That’s what I’m trying to picture regarding the energy that is released during chemical bonding. > > Even [this short chemistry video](https://youtu.be/EoHOXIvxNQc), which shows chemical element bonds actually forming and breaking, was not much help. Yes, two atoms floating in space have more energy then they do close together. Usually, we designate the energy at infinite distance as zero, so they have a negative energy as they approach. This can also be represented as an attractive force (force is the derivative of the potential). As two atoms get very close, a repulsive force develops, and the energy becomes positive again. Read the article about [interatomic potentiall](http://en.wikipedia.org/wiki/Interatomic_potential) Yes, the energy can be measured. It can also be computed, to a high degree of precision (if you’re willing to pay for lots of computing power) with various computational quantum mechanics methods. The physics of bonding between atoms is pretty well understood, and can be modeled very well, though it can be computationally expensive in more complicated systems. For example, if you have a hydrogen molecule, H-H, and you want to break the bond between the two hydrogens, you have to put in about 4.52 electron volts, or 7.24e-19 joules. That’s not very much energy to break just one bond, but realize a molecule is pretty small. If you want to break the bonds of a mole of hydrogen (about 2 grams), you need to put in 436 kilojoules, which is pretty significant. Another way to look at the strength of chemical bonds is to see what type of light you need to shine on it to break the bond. Light has energy, and shorter wavelength light has higher energy. To break an H-H bond, you would need to hit it with one photon of light with a wavelength of 274 nm: ultraviolet light. ---
**Author:** ![Patty\_O\_Furniture](https://avatars.discourse-cdn.com/v4/letter/p/96bed5/32.png) [@Patty\_O\_Furniture](https://boards.straightdope.com/u/Patty_O_Furniture)\ **Post date:** [April 1, 2015, 8:16am UTC](https://boards.straightdope.com/t/chem-101-exo-endothermic-reactions/716107/33 "2015-04-01T08:16:54Z")
Thanks that’s helpful, and that link will keep me busy for a while. ---
**Author:** ![naita](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/naita/32/5862_2.png) [@naita](https://boards.straightdope.com/u/naita)\ **Post date:** [April 1, 2015, 8:20am UTC](https://boards.straightdope.com/t/chem-101-exo-endothermic-reactions/716107/34 "2015-04-01T08:20:11Z")
> [@Patty\_O\_Furniture](#): > > So forget about the squirrels. I’m picturing two atoms floating around, until they get close enough and realize they’d like to bond with each other. What energy do these individual particles possess in their solitary existence, that is then subsequently released when they bond with each other? Can this energy be measured, even in theory? Would _force_ be a better term for it? I get that there are forces of attraction and repulsion, but those forces aren’t energy. Your two floating atoms have potential energy. Imagine lumps of sticky goo floating in space. They’ll attract each other gravitationally and their potential energy will convert more and more to kinetic until they hit each other and stick together, at which point the kinetic energy will convert to internal thermal energy. To get them apart again you need to use energy. Chemical bonds are more complex for several reasons, but the basic principle is the same. Now you asked if this energy could be measured. And the answer is yes, in the same way that the potential energy of a system of masses in space could be measured. Let the reaction happen and see how much energy is released. And force isn’t a better term for it. As you say, force isn’t energy. [Previous page](https://boards.straightdope.com/t/chem-101-exo-endothermic-reactions/716107.md?page=1)