hello everyone welcome to one of the most important lessons for grade 11 stoichiometry this is an area where a lot of students that i've tutored in the past they get a little bit confused with this in the beginning so i'm going to do my best to explain it as best as i can we're going to be talking about limiting reagents now those are some interesting words so let's look at it more in terms of everyday life okay now you can clearly see from this recipe that i've got on my screen that i am not someone who enjoys baking or i don't even know how to bake if you ask me two eggs and a teaspoon of flour that's giving you a cake i've obviously left out a whole lot of ingredients i wouldn't know where to start without with baking cakes all right so what the rest so let's say we look in a recipe book and the recipe book tells us two eggs plus one teaspoon of flour gives us a cake so everything looks good here we've got one teaspoon of flour and we've got two eggs fantastic okay but now we're going to look at a couple of situations and i want you to try and tell me whether the flour is whether we don't have enough flour or whether we don't have enough eggs okay are you ready let's do it all right then let's say in someone else's fridge or in their cupboard we go look at what they have and we are able to find two teaspoons of flour and five eggs now you can't just go put all of that together your cake is going to taste very strange remember you have to always follow the recipe the recipe says that you must use two eggs for every teaspoon of flour okay so if we wanted to use all of the flour that would mean we would need four eggs and look we have enough but now what are we gonna do with this guy so maybe we could make a third cake no that's not gonna work then in somebody else's cupboard or fridge we look through what they have and they have two teaspoons of flour and six eggs so now we're not gonna just go throw all of that together that cake is gonna taste very uh eggy so it sounds like such a baby word eggy so so um what we need to look at is if we look at the number of eggs there's one two three four five six so if we have six eggs then we could technically try and make three cakes but for every two eggs you need one teaspoon of flour so if you have six eggs then you would need three teaspoons of flour whoops three oh that is going bad three teaspoons of flour so we can see now that the flour is limiting us so we cannot make three cakes because we have enough egg but we don't have enough flour so how many cakes can we make well we're gonna have to look at the flour we know that for every cake you need one teaspoon of flour so we can make one cake with that one and for that you'd use two eggs then for this teaspoon of flour you'd cross it out and you could use two eggs and there we go we will only be able to make two cakes and then we'll have two eggs left over so we can go put them back in the fridge or in the cupboard okay so in this scenario the flour is the limiting reagent because it limits how many cakes we can make okay kevin now i feel like i'm in a consumer's class yes okay guys so we're gonna stop with this now and we're gonna return back to normal science guys can you remember in the previous lessons whenever i gave you an equation i always gave you one of the starting material and then i always said that the other one was in excess do you remember that i always gave you one of them and then i said that the other one was an excess in this scenario and this is how limiting reagent works they give you both of the starting material can you see that they give you the nitrogen and they give you the hydrogen so now this is where you have to use limiting reagent whereas in the previous ones i only gave you one of them and so you didn't have to worry about limiting reagents i really hope that you understand that what i've just said that is very important so they've given us nitrogen and they've given us hydrogen now if we look at the recipe remember just like when we're baking cakes here's your recipe it tells you that for every one nitrogen you need three hydrogens okay so for every one nitrogen you need three hydrogens so what we can do to try see which one's limiting is just block one of them off okay so i'm just going to block this one off completely and it doesn't matter which one you choose just block that off now have a look at this one if we have three moles of nitrogen then how many hydrogens would we need have um think about your recipe your recipe says for every um for every one nitrogen you always need three hydrogens so if i have three nitrogens then how many hydrogens would i need well you would need nine so we need nine hydrogens if we're going to use all of the nitrogen now let's go see how much hydrogen we actually have whoops hydrogen come on you only have seven you were supposed to have nine so hydrogen there is not enough hydrogen and so hydrogen is the limiting reactant because there's not enough of it okay so i hope that that makes sense i'm not going to go any further with that for now we're just going to practice working out the limiting reagent so hydrogen is limiting let's keep practicing so here we are given uh the balanced equation which is like our recipe and now we are given six moles what do i mean by you are given remember when we looked at the recipe with the cake in the with the eggs and the flour there was a recipe that we were following but then when you go look in your cupboard or your fridge you might have more than what the recipe says okay so to work out the limiting reagent just block one of them off i'm just going to block the bottom one off again and so what the recipe tells us is that for every one nitrogen you need three hydrogens okay so i'm going to say nitrogen hydrogen for every one nitrogen i should have three now they've given me six nitrogens and so you times by three and you times by three and that's going to give us 18. so to use up six nitrogens you will need 18 hydrogens so let's see if we have enough hydrogen oh look at that hydrogen there's more than enough of you there's 19 hydrogens and so this means that there is more than enough hydrogen and then that what that means is that nitrogen is going to be limiting think about it you're going to be able to use all of the nitrogen and then when the nitrogen is finished there'll still be an extra hydrogen left over so the one that finishes first is the limiting reagent now let's do the same question but let's do it the other way around let's block off the nitrogen just remember that nitrogen was limiting in the previous one nitrogen was limiting so now we can block that so now we are given in this example we are well it's the same example but we crossing out the hydrogen i mean the nitrogen this time so now we have 19 moles of hydrogen okay so the balanced equation tells us all the recipe tells us that the nitrogen to hydrogen must always be in a 1 to 3 ratio now they've given us 19 moles of hydrogen so to work out how much nitrogen you would need you divide by 3 and 19 divided by 3 is going to be an awkward little number 6.33 so if you would like to use all of your hydrogen then you're going to have to use 6.33 moles of nitrogen let's see if we have enough ah nitrogen come on man there's not enough nitrogen there's only six moles we needed 6.33 so once again nitrogen is limiting so can you see that no matter which way we do this nitrogen is always going to be the limiting reagent why not try this one yourself pause the video and see if you can work out which one is limiting all right so what i would do is i'll always cross one of them out so i'm just going to cross off the nitrogen i'm then going to say that the balanced recipe is in a 1 to 3 ratio i'm then going to fill in what i have 11 moles of hydrogen so to go backwards you would divide by 3 so you'd say 11 divided by 3 and that's 3.67 so we need if we want to use all of our hydrogen then we would need to use 3.67 nitrogen let's see if we have enough yes nitrogen there is enough of you so that means there if we use all of our hydrogen which is the 11 moles that we used here then you only need to use 3.67 nitrogen and there'll be a little bit of nitrogen left over but all of the hydrogen will be used so therefore hydrogen is limiting if we do it the other way around where we cross off the hydrogen then let's see if we still get the same type of answer so let's say we use all of we first have to say one two three then we use all of our nitrogen so four now to get the hydrogen you were times by three and that's going to give us 12. so if we would like to use all of the nitrogen then we would need to have 12 hydrogens let's see if we have enough ah hydrogen really you only have 11. so you are going to stop us from being able to react so you are the limiting reagent because there is not enough of you so hydrogen once again is limiting so it doesn't matter which way you do this it'll still give you the same answer whether you cross out the top one or you cross out the bottom one it's still going to point to the same answer let's practice a little bit more because sometimes the ratios in the balanced recipe are not very nice for example a 3 5 ratio that's not a very nice ratio so let me show you how to handle that so once again we block one of them off i'm just going to block with b and so i i say a and then i say b the balanced recipe says that it should be in a three to five ratio now how many a's do we have oh we have four okay now how do we go from three to five usually we can just like times by two or times by three the way to do this one is you times by five over three so we'll times this one by 5 over 3 and let's see what we get 6.67 6.67 for b so that is how much b we would need let's go see if we have enough b yes more than enough so b thank you very much there's more than enough of you a you are our limiting reagent that you you are going to stop the reaction from happening so a is limiting because there's not enough of it let's do it the other way around so now we'll cross out a and we'll write out our ratio so a to b is a three to five ratio we have nine moles of b now how do we go back this way you times by 3 over 5 times by 3 over 5. there are different ways to do that sometimes you can say you can divide by what you have and then times by what you don't have so you can say nine divided by five times by three okay there's different ways your teacher would most likely have told you that or showed you a different way but it's just working with ratios okay so we need to get we need times about 3 over 5 and that gives us 5.4 so if we use all of b so if we use 9 moles of b then we'll need 5.4 for a let's see if we have 5.4 for a ah a you needed 5.4 but you only have four so a is limiting once again a is limiting okay once more let's try another example and let's practice using awkward ratios so we cross one of them off so let's say now we go fill in our equation so for a and b it's two to seven then we have four moles of a now guys here's the way it works we know that if you go from a to b the number becomes bigger so we want to make this 4 become bigger so you could either times it by 2 over 7 or you could times it by 7 over 2. always put the number that you are going to at the top always put the number that you are going to at the top so we are going from a 2b so we going to b so you're going to times by 7 over 2. always put the number that you are going to at the top and that's going to give us 14 okay so it makes sense that the numbers becoming bigger 14. now let's go see if we have enough of b b you need to be 14. ah b come on you're only 11 so b you are limiting there's not enough of you and then let's do it the other way around so now we cross off the a you only have to do one of these in the test you don't have to show both of them i'm just showing you that it works both ways so now we go fill in our ratio 2 to 7 you go fill in what you have which is 11 for b so now we're gonna multiply by always multiply always put the number that you are going to on the top so we're going to a so we put that on the top like that we're going to a so you put that on the top and so that's going to be 11 times by 2 over 7 and that's going to be 3.14 so a you need to be at least 3.14 or more oh very nice a you need it to be 3.14 but you are already four so you are there's more than enough of you b you are limiting us once again b is limiting okay so the limiting one is the one where there is not enough i hope that that makes sense guys in the next lesson we're gonna take this limiting reagent to a different level thank you for watching