in this lesson we'll start looking at chemical equilibrium please don't forget to subscribe if you have not joined my classroom officially yet let's jump right in so as we've learned in previous sections like rates of reaction or sections in grade 11 when we have a reaction equation a chemical equation we have reactants on the left hand side we have products on the right hand side we can refer to these sorts of reactions with a single arrow like this as a normal reaction and this is a reaction that proceeds to completion so what I mean by that is the reactants gets used up either one of the reactants gets fully used up the limiting reagent the other one is an excess or both of them get completely used up and then the reaction stops so most reactions or a lot of reactions are single arrow reactions they proceed to completion what's important to note about these reactions is that the products remember these are the reactants and these are the products these products do not break down and form reactants again no not at all all that's happening is that the bonds of the reactants are broken the atoms recombine and they form products and this continues to happen until one or both of these reactants runs out however not all reactions go to completion so for example I might have a reaction of A plus B and it forms let's say AB okay so reactants forming products however sometimes AB which is the product can break down to form A + B again so it goes forwards so when I say it goes forwards I mean that A plus B produces AB that's called the forward reaction but it can also go backwards we call this the reverse reaction ab can break down to again form A + B what happens is in the beginning the forward reaction will be a lot bigger than the reverse reaction do you see how big I've drawn that for forward arrow and how small I've drawn the reverse arrow the reason why the forward reaction is so big so the reaction that goes this way the reaction that makes AB so it uses A plus B to make AB the reason why the forward reaction would be bigger is because in the beginning there would be a lot more reactants then as the reactants gets used up and the product gets formed the reverse reaction will start to slowly increase so over time it will be like this will get a bit bigger and then eventually the forward reaction the rate of the forward reaction will equal the rate of the reverse reaction and once this has happened we have said that this system has reached dynamic chemical equilibrium and the definition of dynamic chemical equilibrium which we often refer to as just chemical equilibrium is the stage in a chemical reaction when the rate of the forward reaction equals the rate of the reverse reaction and now what people don't understand is that the reaction is still continuing so A and B they are reacting to produce AB and then AB is breaking down to produce A plus B so both forward and reverse reactions are still going on the reaction never reaches completion it's never complete so basically this is another situation i've just called the reactants A and B and the products C plus D so this arrow over here would represent the forward reaction so just think of the one that points to the right as the forward reaction and how you can think of the forward reaction is A and B get used up in order to produce C plus D the reverse reaction is this one over here that goes the other way this is the reverse reaction the one that points to the left essentially and in the reverse reaction C plus D they are used to form A + B so it's the backwards it's the reverse of what's happening for the forward reaction and remember a reaction is reversible when products can be converted back to reactants and vice versa so in this example over here my forward reaction would be the nitrogen plus hydrogen forming NH3 the reverse reaction would be the NH3 basically breaking back down into nitrogen and hydrogen so if I had to ask you the following question what compound or compounds or molecules does the ford reaction produce so you'll look at it and you'll say "Well the forward reaction goes this way it uses these to produce NH3." So the forward reaction produces NH3 by using or by reacting N2 and H2 the reverse reaction that's the one that points this way the reverse reaction produces so what does it make it produces N2 and H2 by using NH3 now remember not all reactions are reversible you need to see the double arrow we use these half arrows over here to illustrate it and there are certain criteria that need to take place in order for a reaction to be reversible and for a system to reach chemical equilibrium so these are the requirements number one we need a closed system and we need a reversible reaction so remember I said not all reactions will have the double arrow remember this one over here had a single arrow so this one can never reach chemical equilibrium so just to show you what's the difference between an open and a closed system a closed system is isolated from its surroundings an open system continuously interacts with its environment so that would be the difference so you'll often see them saying that we have a closed beaker like that but chemistry is an interesting thing and I just want you to note that a beaker or test tube or container doesn't have to be physically closed with a lid for the for the system to be considered closed an acid base neutralization so if I add acid to a base taking place in an open test tube that can still represent a closed system because hardly any evaporation is taking place there's no interaction with the environment and that is essentially a definition of the open versus closed system okay but generally we speak about a closed beaker another simple representation of an equilibrium situation or an equilibrium reaction is something like when a phase change is taking place so over here I have water and we have an evaporation rate we add heat as you know we go from liquid particles to gas particles the gas is the water vapor initially the evaporation rate is greater than the condensation rate but after a while when the water particles build up inside my conical flask then the condensation rate starts to increase until the evaporation rate so the evaporation of particles is equal to the condensation rate the rate of the forward is equal to the rate of the reverse so you can see initially this arrow is bigger indicating the rate of evaporation is bigger than the rate of condensation so this is not yet in equilibrium but eventually the rate of the forward reaction equals the rate of the reverse reaction it's also interesting to think of it like a reversible change it never goes to completion it occurs both in the forward and the reverse direction so there's the double arrows indicating that it's reversible and in this case I'm showing you a phase change equilibrium so liquid going to gas and eventually that reaches an equilibrium so if we take a look at this in terms of a simple graph let's plot rate on my y ais and let's plot time on my x-axis i'm going to ignore the units for now i'm going to ignore a heading for now i've got A plus B gives me C plus D now I hope that you can understand that initially we will have more reactants in my container and therefore we will start off with a very high rate of my forward reaction so we're going to draw two lines on the graph one representing the forward reaction and one representing the reverse reaction initially we're going to have a very high rate of my forward reaction let's do that in red okay high rate of my forward reaction and I hope you can understand that the rate of my reverse reaction is initially going to be zero because I have no products yet so there will be no reverse reaction as time goes on I use up these reactants so the rate of the forward reaction is going to decrease like this okay but I'm using up my reactants to create my products and the more products I get then the reverse reaction starts to increase so we'll see an increase in the reverse reaction and eventually what's going to happen is these rates are going to be equal to one another and we know that when the rate of the forward reaction equals the rate of the reverse reaction my system has reached chemical equilibrium and over here you can see a T1 where the rates become equal that's when dynamic chemical equilibrium has been reached we'll look at graphs in more detail later on so we will be looking at other things coming up in this playlist that has to do with equilibrium for example we'll be looking at the factors that affect the position of equilibrium so just very very briefly if we take my A plus B that produces C plus D but then C plus D can go backwards again and produce A plus B once it's reached equilibrium I can make changes to one of these three i usually remember it as TCP temperature concentration or pressure and that affects the position of equilibrium so for example I can decide hm as A + B is making C plus D I am going to let's say I'm going to add I'm going to keep adding more of B so I'm going to increase the concentration of B square brackets means concentration now we're going to be learning about something called Lhatalius principle and lately principle basically says that when I disturb equilibrium the system is going to act in a way to oppose to do the opposite of the disturbance so what I did is I made more I made more B the system's going to say uhoh you made more B i want to do the opposite of that i want to get rid of B and if you think carefully about what I said earlier which reaction forward or reverse which reaction is going to use up B the forward reaction will use up B so we will favor we call it favoring the forward reaction but again I will cover all of this in its own separate video but it should make sense if I make more if I add more B the system says "Oh no I want to get rid of B." The forward reaction will use up B because remember we said if you have a forward reaction this forward reaction this one over here this uses these to produce these so if I'm adding more and more and more and more of B then it's going to say okay make this forward reaction increase that rate of that forward reaction increase that the forward reaction favor the forward reaction favor the forward reaction because I want to get rid of all this extra B okay we're going to be discussing Lhata's principle we're also going to be discussing something called KC which allows me to determine the position of equilibrium in terms of a calculation it's called the equilibrium constant and we're going to be looking at graphs but I really hope that that was a good introduction to chemical equilibrium for you please subscribe if you haven't done so yet and comment down below what you'd like to see next i love helping all of you i love my YouTube students very much and yep I'll see you in the next video bye everybody