How DO Molecules Store Energy?

how do molecules store energy is it really stored in the bonds why are some people confused about it and why should we care keep watching to find out even non-chemists instinctively know that some molecules contain more energy than others we fuel cars with petrol and not water we eat carbohydrates but not chalk so how did we work this out with food since the earliest emergence of animals we used a process of trial and error to find which combinations of chemicals allowed us to move around more after eating them these days we know that eating lots

of pasta is necessary before running a marathon whereas a diet of only celery will leave you unable to move within a couple of weeks then with the discovery of fire making we identified chemicals that consistently released heat in their reactions wood coal and animal fat were firm favorites and finally with the discovery stroke invention of thermodynamics we learned that chemicals that do work for example expanding gases or generating electrical current is another sign that a chemical contains movement stuff Rock oil which had previously been a relatively useless commodity then became a global necessity because with

it we can make things move these days chemists use a piece of equipment called a calorimeter to directly measure how much energy these fuels and foods contain for example it's measurements with calorimeters that tell you how many calories are in your packet of food but what is it that means some chemicals contain lots of energy energ and others don't how do molecules store energy is it stored in the bonds like the textbooks say well Derek Müller of YouTube channel veritasium emphatically said that it isn't he said that atoms lose energy by making bonds and therefore

the energy of a molecule is not stored there Nick Lucid from the science Asylum Channel followed this up by saying that things are a bit more complicated but basically said the energy is stored in the molecule as a whole and not in the bonds of the molecule so who's right have the chemists been wrong all this time to answer these questions we first have to understand what energy is what a bond is and what an abstract concept is so no big deal if you've already watched my video on activation energy I'm going to explain energy

so you can just skip straight to 5 minutes 12 seconds if you want to to get a bit of pedantry out of the way I'm specifically talking about how the chemical energy of a molecule relates to its electrons there are more complete explanations for talking about materials on the macro scale but that's a video for another day now if we're focusing on a single molecule hanging around in space all of its chemical energy comes from its veence electrons the electrons in the outside layers that control chemical bonding very simply the negatively charged electrons are attracted

to the positively charged nuclei and are repelled by each other when we separate the electrons from the nuclei they have the potential to move back together and that movement is kinetic energy so we can use some kinetic energy to separate them and then they have the potential to get that same amount of kinetic energy back when they move back together or put even more simply separating the electrons from the nucleus gives them the potential to have kinetic energy and even more simply separating the opposite charges gives them potential energy but if the electrons are attracted

to the nuclei why why don't they just stick to the nuclei and stay there well that's because of quantum mechanics that basically says the electrons can only occupy certain volumes of space called orbitals and no more than two electrons at a time can be in any one orbital that's a simplification but it'll do for this video quantum mechanics also tells us that certain interactions between electrons involve more potential energy than others but adding these details to our discussion won't help much so we'll focus on the distance of the electrons from the nuclei for this video

so that's it if we can separate the negatively charged electrons from the positively charged nuclei and hold them apart we have a store of potential energy ready to go when we release them now there's not really much we can do with atoms but with molecules there are lots of things we can do and what determines where the electrons are in a molecule it's time to talk about bonds the idea of chemical bonds came about even before the existence of molecules had been accepted by the scientific Community Check out my video on the history of the

avagadro number for a brief introduction but the concept of chemical bonds Rose as a means of explaining how and why elements kept turning up in certain integer ratios in different chemicals and to explain the different properties of those chemicals organic chemistry in particular supported the concept of chemical bonds because it turned out that arranging atoms like this gives you a different chemical to when you arrange them like this can't see the difference don't worry chemists need quite a lot of training to spot that difference and that's how important bonds are from about the 1920s onwards

the lines that join atoms in molecular diagrams were understood to represent pairs of electrons that pulled nuclei together into a definite geometry this theory of bonding is called veence Bond Theory and it has the distinct advantage of being easy to visualize and that's why schools typically teach only veence Bond Theory at roughly the same time however another theory of bonding emerged called molecular orbital theory in this idea when atoms make molecules the electrons are free to roam over the whole molecule in molecular orbitals it's much more complex than veence Bond Theory but it usually gives

more accurate results for detailed calculations of molecular energy and geometry since computers have become so widespread and cheap it's become the main theory for computational chemistry so if we have these two theories of how atoms make molecules and we can switch between theories as we need doesn't that mean that chemical bonds are just abstract Concepts no because we can see them Atomic Force spectroscopy is an analytical technique that can see areas of high electron density in molecules and guess where we see that density right where we said they would be using either veence Bond Theory

or molecular orbital Theory so yes our theories of how bonds work are abstract concepts with our line drawings and colored balloons but we always knew those are abstract representations of some something real in other words the bonds themselves are as real as atoms and molecules and the reality of molecules was confirmed in 1921 so yes bonds are real and they make a real difference to the energy of the molecule let's go back to our glucose example beta glucose the molecule on the left has a hydroxy group sticking out to the side alpha glucose the one

on the right has that same hydroxy group sticking straight up that small difference gives these two molecules a potential energy difference of about 12 KJ per mole now that's not a lot but it's enough for about a 3 minute walk and it shows that it's not just which atoms are in the molecule or even which atoms are stuck to which other atoms but also their precise orientation to each other now what is true is that the electrons are not bound tightly into little tubes like this in fact these lines are more like a map than

a picture a better representation is this one that shows where the electron density is but even then the majority of the electron density involved in chemical energy changes is between the nuclei right where those tubes are so does that mean the real energy in real molecules really is stored in the real bonds let's find out but before we do if you could just click the like button it'll help me make more videos thanks so let's begin by addressing the straw man that Derek Müller is so determinedly hacking to Pieces no one especially not chemists are

suggesting that chemical bonds are sort of bags filled with energy and that we somehow get that energy out of them when we break bonds those lines that we draw do not have any independent existence apart from the molecule but some of what Müller said was correct firstly he rightly says that making chemical bonds from atoms results in a loss of potential energy if we were to take a bunch of separate atoms and make them into molecules the resulting molecules would have less potential energy than the separate atoms started with whether you imagine them losing the

energy and then getting stuck in a molecule or making their bonding orbitals first and then losing their energy the final result is always the same the atoms get stuck in a molecule because they have less energy than when they were separate and now they don't have enough energy to get away from each other or to even change their Arrangement but and this is the key point that müller's missing not all bonds lose the same amount of energy in other words after making molecules some arrangements of atoms have still got more energy left than others a

classic example here is glucose and dioxygen if we take six carbon atoms 12 hydrogen atoms and 18 oxygen atoms we can make glucose and six molecules of dioxygen and when that happens those atoms will lose about 13,000 K per mole in other words making about this much glucose and oxygen would give out enough energy to boil a bath of water or drive an electric car about 20 km so yes those atoms have lost a lot of energy by making bones but if instead of making glucose and oxygen we used those same atoms to make carbon

dioxide and water we'd lose over 155,000 K of energy per mole in other words we lose more energy by making carbon dioxide and water than we lose by making glucose and dioxygen so if we make our glucose and dioxygen from free atoms first they'd have more energy than carbon dioxide and water because they've lost less energy than carbon dioxide and water lose it's a bit like this box on the top of this table it has about 2 two Jews of gravitational potential energy if I let it go some of that potential kinetic energy will become

real kinetic energy and it will move by itself down onto the chair now on the chair it is lost over a jewel of that potential energy but it still has almost a jewel of potential energy left and as long as friction prevents it from sliding to the end of the chair it will keep that potential en energy stored but if I add a little energy move it to the edge of the chair and Let It Drop the remaining potential kinetic energy becomes real kinetic energy and it moves again here on the floor it has lost

all of its gravitational potential energy and now it can't move by itself so if we can stop glucose and dioxygen from losing their energy and then when we want to turn them into carbon dioxide and water glucose and oxygen will be an actual store of energy so what is keeping glucose and dioxygen from turning into carbon dioxide and water that's right the bonds to rearrange these atoms into lower energy molecules we first have to break these bonds and then make new ones and if you've watched my video on activation energy you'll know that can't just

happen we need to get activation energy from somewhere just like we need energy to push this box off the chair until we get that activation energy from somewhere the atoms will stay in this relatively high energy State and that's why glucose is a store of energy and it's all down to those bonds so bonds are responsible for storing the energy but is it stored in the bonds very simply if one arrangement of chemical bonds holds electrons further from the nuclei than other arrangements then the electrons in those bonds will have more potential energy and even

if we put all the weird quantum mechanical bits of energy back in we are still doing that with electrons in the bonding orbitals the energy of electrons that are not involved in bonding do not change much and the energy of the nuclei themselves don't change at all so what's responsible for the big energy difference between glucose and dioxygen versus carbon dioxide and water the bonding electrons and where are they in the bonds but of course we almost never make molecules starting from atoms in chemistry so where does the energy stored in glucose and oxygen really

come from well you know that from school plants use some extraordinarily complex molecular Machinery to run the oxidation of glucose backwards and they get the energy for powering that Machinery from photons of sunlight where it is stored in the chemical bonds of glucose and when we eat that glucose and oxidize it using the dioxygen that the plants also made we get to use the energy that the molecules lose that's 2,700 K per mole which might not sound like much compared with the amount of energy emitted from making the molecules from atoms but it's huge on

the scale of living things and that's because we get all our chemical energy starting from molecules not atoms and one mole of glucose releases enough energy to run a marathon but having discussed all of that what about molecules that have bonds that have nowhere left to fall like carbon dioxide well yes it does not make much sense to say that carbon dioxide stores energy in its Bond but then no one would say that carbon dioxide is a store of chemical energy it's the chemistry equivalent of the Box on the floor so there we go most

molecules do store energy in bonds but at the end of the day why should we care who thinks what about where the energy is stored we all agree that it is stored somehow why not just say it's stored by the molecule and that it's all just an abstract concept it's important because our theoretical models are how we visualize scientific phenomena and that controls how we think about them if we get our model wrong or simply label a puzzle as an abstraction we limit our ability to find new Solutions and New Paths to Future discoveries and

even more more importantly if we don't have a good model in our heads we get confused and end up being disillusioned about a subject that is actually perfectly understandable so I hope you've enjoyed this video but I do want to emphasize that I'm a big fan of Derek müller's veritasium and particularly Nick lucid's science Asylum I'm sure you've come across their channels before but if you haven't I've put links in the the description so be sure to check them out and what do you think how do you think molecules store energy would you like to

know more about how energy works in chemistry let me know in the comments and don't forget that 326 is produced at kushu University it's one of Japan's top universities and we have courses in science and engineering in English and I'll see you next time