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. 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 with baking cakes.
All right, 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 gonna 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.
Alright 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.
Now what are we going to do with this guy? So maybe we could make a third cake. No, that's not going to 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 going to just go throw all of that together. That cake is going to taste very eggy. So it sounds like such a baby word, eggy. So 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. Whoa, 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 going to 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 would 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 going to stop with this now and we're going to 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 in 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 to 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 a think about your recipe. Your recipe says for every 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 9. 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 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 and 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 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're 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, or 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 6 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 would 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 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... 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 1, 2, 3, then we use all of our nitrogen so 4 now to get the hydrogen you would times by 3 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 off B.
And so I say A and then I say B. The balanced recipe says that it should be in a 3-5 ratio. Now, how many A's do we have?
Oh, we have 4. Okay. Now how do we go from 3 to 5? Usually we can just like times by 2 or times by 3. The way to do this one is you times by 5 over 3. 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. 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 3 to 5 ratio. We have 9 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 9 divided by 5 times by 3. 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.
So we need to get, we need to 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 4. 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 2 to 7. Then we have 4 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 to B. So we're 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 number is becoming bigger. 14. Now let's go see if we have enough of B. B, you need to be 14. Oh, 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 going to 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. Ah, very nice, A. You needed to be 3.14, but you are already 4. 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 going to take this limiting reagent to a different level. Thank you for watching.