energy and change in this video we're going to be speaking about important definitions and Concepts that you need to know for this topic you need to know these definitions and these Concepts in order to understand energy and energy change in chemical reactions we'll be looking at things like what does heat of reaction mean what does exothermic endothermic mean activation energy activated complex Catalyst all of those things let's Jump Right In but before we do please make sure you subscribed to my channel I do lots of math and science videos can't wait to see you in future videos let's Jump Right In first things first you need to learn your definition so here are some of the definitions that you can expect in this topic starting off with enthalpy change or heat of reaction otherwise known as Delta H Delta is the symbol for this triangle it means change in h h is enthalpy now enthalpy is the total internal energy of all the different substances that take part in a chemical reaction it's very difficult to measure enthalpy but we can measure change in enthalpy so enthalpy or energy changes when energy is transferred out of or into a system so if energy is released or if energy is absorbed enthalpy will change there will be a change in enthalpy and this is often accompanied by heat transfer that's why it's also called heat of reaction the unit for enthalpy change or heat of reaction is KJ per mole and we can calculate it using this formula enthalpy of products minus enthalpy of reactants this formula works when you are given a potential energy diagram and then they ask you to calculate the change in enthalpy for example in this reaction we can work out the change in enthalpy by saying the energy of the products or the heat of the products here's products over here corresponds to 100 minus the energy of the reactants reactants over here it corresponds to 400 100 - 4 400300 K per mole the second important definition or term is called activation energy now activation energy think about it it's energy needed to activate it's energy needed to start off a chemical reaction the official definition is it is the minimum energy needed for a chemical reaction to take place so we can always always identify activation energy by looking at a potential energy diagram and and it is from the reactants so it's from the energy of the reactants up to the energy of the activated complex which is by the peak of the curve now please take note grade 11s or grade 12s whoever's watching this sometimes they may indicate the arrow inside here like that sometimes they may indicate it on the outside like that they're indicating the same thing they're saying that it's the energy from the reactant so from here to the activated complex so if I were to ask you what is the activation energy for this reaction you would say Well it starts at 400 and it ends at 900 so it went up by 500 so the activation energy is 500 Jew 500 K 500 KJ per mole what they'll give you the unit on this axis over here so 500 is the activation energy and please note that we can get the activation energy for the forward reaction or for the reverse reaction now I did cover this in the previous video in this playlist but the forward reaction is read from left to right so for the forward reaction it is basically what you see on this diagram over here these are the reactants these are the products the activation energy is always from the reactants up to the activated complex easy so the activation energy for the forward reaction was what we just calculated 500 but if they want the activation energy for the reverse reaction we need to read this graph in Reverse okay so these will then be the reactants so instead of these being the products these will then turn into reactants and the products over here there we go they'll swap places so again activation energy is from the reactants up to the activated complex so the activation energy for the reverse reaction will be from here all the way up to the activ activated complex and what do you think the activation energy is therefore for this reverse reaction for the reverse reaction it's from the 100 up to the 900 so what's the difference between 100 and 900 800 okay so they have different activation energies the next important term is the activated complex and this is the unstable transition state so transition means it's going between two things from reactants to product in a chemical reaction basically if you look at the potential energy diagram of a reaction it is the peak of the curve over there essentially what happens is we have our reactants they have a certain energy or enthalpy they absorb energy activation energy and that energy what that energy does the activation energy that is absorbed is it causes the bonds of the reactants to break think about it if we break compounds or molecules we break the bonds they're going to form individual atoms this is the unstable transition state it's a transition between reactants and products it's unstable because it's not very good for react for the atoms to just be existing by themselves it's not stable and things tend towards stability so they will want to rejoin reconfigure in a new way to form products the next two terms that are very important for you to understand is exothermic and endothermic so for this video I'm going to keep it brief I will do a video where I go over exothermic reactions versus endothermic reactions but to keep it brief EXO means that heat is exiting heat is leaving think of EXO exit endothermic means that heat is entering we're taking in energy or heat when we break bonds we're taking in energy it's an endothermic process when we're making bonds we are releasing energy it is an exothermic process what this ends up resulting in is two different looking potential different um potential energy curves so if you look at the exothermic curve you can see that the energy of the products is lower than the energy of the reactants the reason why is because exothermic means that there's a net exit or net release of energy more energy is released than what's taken in so we take in a little bit we release a lot that's why the energy of the products is much lower than the energy of the reactants the graph looks like this endothermic is a net intake of energy so what it means is we take in a lot of energy and we release a little bit that's why the energy of the products is much higher than the energy of the reactants we only release a little bit of what we took in another term that you need to know is catalyst you need to know what a catalyst is you need to know what a catalyst does so now a catalyst is a substance that increases the rate of a chemical reaction without itself undergoing a permanent change in the reaction so what this means is that the Catalyst will increase the rates of the reaction it'll make the reaction go quicker it'll make the reaction go faster it'll make the reaction reach finish it'll finish quicker that's basically it but the Catalyst never changes itself so basically think of a catalyst as something that comes along it helps speed up the reaction increases is the rate and then the Catalyst goes off again completely unchanged it doesn't react with the other reactants so why do we need a catalyst if the activation energy is very high the reaction will take place slowly the reason why let's look at this graph if the activation energy which would be over here as you can see they also represented with this Arrow over here it's the same thing if the activation energy is very big it's a very high activation energy which this one is the reaction is going to take a while to happen because a lot of energy needs to be absorbed in order for the bonds to be fully broken so what a catalyst does is the Catalyst lowers the activation energy this is super super super important you need to know that catalysts lower the activation energy and what this does is it gives the reaction an alternative pathway or mechanism basically it allows the reaction to continue with a decrease activation energy and remember the Catalyst remains unchanged after the reaction occurs so this is a reaction curve or a potential energy diagram and this is what it looks like without a catalyst the solid line is without a catalyst and the dotted line is with a catalyst I want you to be observant of things on this diagram the first thing that I see is that the dotted line and the solid line start and end at the same place what that means is that the energy of the reactants and the energy of the products remain unchanged however take a look at these curves the peak of this curve is much lower than the peak of this curve what it means is that if I had to draw an arrow indicating the activation energy for the reaction without a catalyst it would look like this if I had to draw an arrow for the activation energy of the reaction with a catalyst it would start here and end here remember you can also draw it inside the curve like that compared to the other one which would go like that can you see that when we have a catalyst the activation energy has been lowered it's much smaller those are all the definitions that you need to know for this lesson I hope it's been helpful remember if you want more details on certain video or certain topics like exothermic versus endothermic or maybe heat of reaction check out the other video videos in this playlist also past paper practice I can't wait to see you all in more videos in the future Please Subscribe if you haven't done yet give the video a thumbs up I'll see you in another one very soon