Energy changes in chemical reactions part 1 Middle school chemistry Khan Academy

consider these two processes one where plant is growing this is time lapse and another one where we are burning marshmallow both of these involve chemical changes how do we know well over here new stuff is being produced new leaves are being produced and over here we can see and feel the energy change light and heat but what's interesting is that this particular chemical reaction absorbs energy whereas this one releases it so why do certain chemical reactions absorb While others release energy and more importantly where does energy go or come from that's what we want to figure out in this video let's start with the plants growing the reaction in world is called photosynthesis it's where carbon dioxide reacts with water to produce glucose or sugars and oxygen now this equation is not really balanced but that's okay what's important for us is that look the energy is on the left side which means that energy is being absorbed over here and that energy comes from the sunlight the sun's energy is being absorbed to produce sugar and oxygen molecules but the big question is why why does this reaction require absorb energy well think about it this way chemical reaction are basically atoms being rearranged to form new Arrangements isn't it and it turns out that these arrangement of the atoms have higher chemical energy compared to these arrangement of atoms here's an analogy that really helps me understand this okay consider ball that is on lower level now compared to this ball that is at higher level has more energy right we say it has more potential energy compared to when it was over here so just like over here we could say the arrangement of the ball in the Earth when the ball was over here that has lower energy and the arrangement of the ball in the Earth when the ball is over here has higher energy and so now if you want to get ball from the lower level to higher level you need to supply that energy difference isn't it in similar way it turns out that carbon dioxide and water this arrangement of atoms has lower chemical energy and sugars and oxygen this arrangement of atoms has higher chemical energy so to go from here to here to rearrange the atoms from here to here you need to supply that difference in the energy to be more concrete here's how you can think about it okay to rearrange the atoms you need to first break these chemical bonds breaking bonds is kind of like pulling magnets apart it requires energy you need to supply energy and in this particular diagram you can imagine that the energy supplyed the amount of energy you need to supply is about this much the hill represents the amount of energy that you need to supply over here okay and when new bonds are being formed it's kind of like magnets sticking to each other they release energy when magnets stick to each other you hear sound that's because energy is being released similarly when new atoms new arrangements are formed they release energy and the amount of energy released in this particular case is about this much you can see energy supplied is way more than the energy that is released and so overall you need to supply energy for this reaction to happen make sense and such chemical reaction re where energy is absorbed we call that endothermic reactions the word Endo stands for the energy is going inwards it's being absorbed by the reaction and it's where is it going well it's going as and it's getting stored as chemical energy in these product molecules okay now quick question what do you think would happen if we were to run this chemical reaction backwards what if we started with this and we ended with this what would happen well it would now be the exact opposite right mean now the energy would be released because the product molecules have lower chemical energy than the reactant molecules again you can imagine that to break these bonds you require you need to supply this much energy you should always supply energy to break bonds but when the new bonds are formed they will release this much amount of energy so look lot more energy is being released compared to how much we are supplying therefore overall the energy is released in this reaction and that's why such Chemical Reactions where energy released are called exothermic reaction the word EXO kind of means outside the energy is going out of this reaction and where is this energy coming from well it was stored over here right that energy is being released and that's exactly what happens when you are burning this marshmallow by the way this particular reaction is called combustion reaction and then when you're burning marshmallow basically the oxygen and the sugar react to give you carbon dioxide and water and in the process lot of energy is released now of course you need to supply some initial energy to start the reaction because you know you need to you need energy to break the bonds but then once you get it started the energy released is so much more that part of that energy released can be used to further break more bonds and and the reaction keeps sustaining itself providing us with thermal energy and it'll keep on going until we run out of sugars or oxygen so what's interesting is that if you think about the energy flow it comes from the sun the plants store that energy from the Sun into the sugar molecules and the sugar finally Finds Its way into your marshmallow and when you you know when you burn that marshmallow that same from the Sun is released that's interesting right in fact most of the energy on the earth comes from the sun itself okay finally let's model how this energy moves around in little bit more detail by using another example hot packs and cold packs consider this reaction where ion reacts with oxygen to produce ION three oxide now it releases energy over here that's why we written as plus energy right so this is an exothermic reaction but the way we think about it where is the energy being released over here where is it going well we can think that the energy is going out out of our reaction system into the surrounding that's another way to think about exothermic reaction the energy is going into the surrounding and that energy can be used up to heat the surrounding and that's exactly how your hot Packs work this is how they can heat up the surrounding when the surrounding over here could be your hand for example and again where is this energy coming from well ion and oxygen this particular Arrangement had higher chemical energy when we went down to ion 3 oxide it has lower chemical energy and so this rearrangement causes some energy to be released now consider what happens when you are dissolving solid in water it's not really chemical reaction but they can break apart and that requires energy so in this particular case energy is being absorbed but from where well again from the surrounding and the surrounding could be anything like for example from your hand if you're holding this system then energy will move out from your hand and your hand will feel cold this is how cold Packs work notice that it's an endothermic process because energy is going from the surrounding to the system so putting it all together an exothermic reaction is where energy is released from the system to the surrounding where did this energy come from well the energy is released when the high energy reactant molecules is rearranged to form low energy product molecules and the endothermic reaction is the exact opposite in these reactions energy is absorbed and where does it go well the energy is stored when the low energy react molecules are ConEd into high energy product molecules and so fundamental thing we are seeing over here is that energy is is not created or destroyed it's only transferred from one entity to another we call this the law of conservation of energy it's one of the fundamental laws of our universe