Introduction to Limiting Reactant and Excess Reactant

In this video we are going to talk about the

limiting

reactant

, which can also be called the

limiting

reactant

. Along the way, we will also discuss a related topic called

excess

reactant now limiting reagent

excess

reactant this all has to do with chemical reactions, but to explain from the beginning I don't really want to start talking about chemistry, instead I want to start talk about some analogies with cooking. We will compare a chemical reaction with a cooking recipe. Let's say you invite some friends over for a barbecue. I want to make some cheeseburgers, this is what you will need to make them, you will need a bun, you will need a slice of cheese and you will need a piece of meat now, since we are comparing chemistry and cooking, we can write the recipe for this as a chemical equation, here's a button plus a slice of cheese plus a piece of meat, all together and gives us a cheeseburger, the ingredients here. what we start with we can think of those kind of similar reactants in a chemical reaction and the products here, the cheeseburger is what we end up with, okay, now that we have our recipe to cook a little bit, we have to get some ingredients further. rub it in the fridge and see what we can find, here are our ingredients and you'll notice we have different amounts of each.

This happens a lot in cooking and it also happens a lot in chemical reactions, it often starts with different amounts of each reactant, so here we are talking about limiting reactant and the limitation of the reactant comes into play when we ask the question, given this different amount of ingredients, what is the largest number of cheeseburgers we can make. Okay, we have four muffins here. nine slices of cheese, but if we only have three pieces of meat, we can really only make three cheeseburgers, look at this, here's a first cheeseburger, here's a second and here's the third and then we're out of meat and can.

I won't make any more cheeseburgers. If this were a chemical reaction, we would say that meat is the limiting reactant. Well, here's a definition. The limiting reactant is the first reactant to be used up in a reaction when the limiting reactant is used up. No more product. can be formed and the reaction stops, so clearly the meat was the first of these ingredients that ran out while we were making the cheeseburgers and as soon as we used it all up, there is no more product or form, we can't make any more burgers with it. cheese and everything.

The process has to stop correctly. We call it a limiting reactant because it limits the amount of the product we can correctly produce. It doesn't matter if we have a thousand rolls or 100 slices of cheese, once we run out of those three pieces of meat. Once finished, we can't make any more cheeseburgers, so the total number of cheeseburgers we can make depends entirely on the amount of meat, the limiting reactant we have, so after we run out of meat we still We have some additional ingredients left. Up here and these are what we call excess reactants.

Excess reactant is what remains after the reaction stops because the limiting reactant is used up. Okay, so we have excess reagent. We have the bun here. We have a leftover muffin and then some more. excess reagent, we have six pieces of cheese left over after the meat is used up. Okay, now that we've talked about the concepts behind reagent limitation and reagent excess, let's use this information to solve some problems that are a little more complicated. advanced this is more like a chemical equation but we're still cooking here. I'm pretending this is a muffin recipe, in reality it would be a terrible recipe, it's just flour and water.

I would definitely like to eat these muffins, but this is going to be great practice for limiting the reagent and the excess reagent, so we will use this recipe or this equation to solve the following problem: what is the largest number of muffins that can be make with six cups of flour and three cups of water If this were a chemical reaction, what would be the limiting reagent, which reagent is an excess and how much is left over, then we want to make as many muffins with six cups of flour and three cups of water, let's take these ingredients Here here are now to determine the largest number of muffins that we can make.

First we have to identify which of these ingredients is a limiting reactant. That's a little more complicated than the cheeseburger problem we just solved. We can not. just look at these and find out which one is a limiting reactant. Instead, we will have to solve this problem this way. We will start by saying the flower and ask if we want to use the maximum amount. Of this flour, if we want to use six cups, how much water will we need? Do we have enough or is it going to run out? So we're going to change and we're going to do the opposite.

Let's ask if we want to use the maximum amount of water. We want to use all three cups of this. How much flour do we need? Do we have enough or is it going to run out? Well, let me show you what I mean, let's get started. with a flower, okay, we have six cups here now, this recipe is only for three cups, so what if you were in the kitchen and you really wanted to make this recipe with six cups of flour instead of three? What would you do well? I would multiply everything by two, okay, so we can multiply this by two to make the recipe with six cups of flour.

Now, if we use six cups of flour, how much water do we need? Well, we can multiply this by two, which will tell us. that we need two cups of water to accompany the six cups of flour and, finally, how many rolls are we going to make? Well, we're just doubling the recipe, so instead of five we're going to end up with ten, okay, so that's it. To try to use the maximum six cups of flour, what are we going to need in terms of flour if we want to use the maximum amount of water? Okay, we have three cups of water.

Well, the recipe is for one cup of water, so that's nice. We have to triple the recipe. We are multiplying one by three. How about flour? How much flour will we need for three cups of water? Well, we triple this to get three times three. We will need nine cups of flour. How many muffins are we going to get? Triple the recipe five times three fifteen muffins. Okay, here's all our information. What is the largest number of muffins we can make? Well, one of these situations is not going to work because we are going to run out of one of the ingredients, okay, let's take a look at this.

If we start with six cups of flour, we need two cups of water. Do we have that? Yes, we have three cups. In reality we have more than enough. This would work, we could make ten muffins. What happens if we start with three cups of water? So we need nine cups of flour, but that's not going to work because we only have six cups of flour, so we can't make 15 muffins because we don't have enough flour, the flour will be the first ingredient or the first reagent to run out, that means Since flour is the limiting reactant, it will limit the number of muffins we can make, so here's the flour. limiting reactant now we can also show this visually well imagine we are cooking this recipe well three cups of flour and one cup of water combine well so here are three cups of flour one cup of water we take them we put them in the oven or whatever and we get five muffins one two three four five okay, let's do that again another three cups of flour a cup of water combine them and we get five muffins plus one two three four five and now you can see the flower is gone the flower is the first thing that runs out so it's the limiting reactant we still have a cup of water here and that's excess that's what we would call our excess reactant a cup of water that's left after we run out of limiting reactant okay that's right How to solve this type of problem, you say: I want to use a maximum amount of flour, how much water do I need and I run out and then you say: I want to try to use the maximum amount of water, how much?

I need flour and I run out? There is something very important to mention here. There is a common misconception that the limiting reactant is the least thing you have, but that is not true. Look at the example we just had. more flour than water, but the flour was the limiting reactant and that is because the limiting reactant is not what has the least amount, but what runs out first, so although we had more flour than water when we followed this recipe, flour runs out before water, that's why flour is a limiting reactant even though we have more of it, so keep that in mind anyway, now that we've solved this problem, let's finally look at an example that uses a real chemical. equation here is a chemical equation and we are going to use it to answer this question what is the largest amount of NH3 in moles that can be prepared with 3.2 moles of N2 and 5.4 moles of H2 what is the limiting reagent what reactant is an excess and how many moles are left over, okay, these numbers may seem a little scary and this may seem a little confusing because it's a chemical equation, but the way we're going to solve this problem is almost exactly the same as how we solve it. the last example with a flower and water to find the largest amount of NH3 that we can produce, first we have to find out which of these two things is the limiting reactant and we will do it exactly the way We did the previous example, we are going to take the maximum amount of N2 here, we'll see how much H2 it would take to use all of that and then we'll turn around, we'll take the maximum amount of H2 and We'll see how much N2 we would need to use all of that and then we'll ask which one we run out of first and that's the limiting reactant.

Well, we'll start with N2. Now I have 3.2 moles of N2. In the previous example we solved this by multiplying the recipe by a number, doubling or tripling it, it's good to know how to do it, but I want to show you another strategy here, okay, I want to use some conversion factors to do it. Let's look at the coefficients, these numbers before each of the compounds tell us how many moles of each one come together. Well, there is nothing in front of the N2 so I want to add a 1. Well, if there is nothing in front it just means that it is one, okay so you can see here the equation says we have one mole of N2 here I have 3.2 moles of N2 so it's like we're multiplying this by 3.2 to calculate how much H2 we have.

I need, I'm going to use a conversion factor that shows the relationship between N2 or moles of N2 to moles of H2. Well, I start with 3.2 moles of N2 and then I'm going to multiply this by a conversion factor that has three. moles of H2 over one mole of N2 that will tell us how much H2 we need, so moles of N2 cancel out, moles of N2 cancel out and we end up with 9.6 moles of H2, so this is how much H2 we need to use up all of this N2. and again, this is like we're multiplying by 3.2 or should I say it's like we're multiplying by 3.2, it's just another way to get this answer, so this is the amount of H2 that we need for this amount of N2.

Now let's do it. Otherwise, okay, we have 5.4 moles of H2. I want to calculate how much N2 we are going to need to use all that. Well, in this case we're going to start here, it's 5.4 moles of H2. get rid of the H2 so I'm going to put H2 at the bottom here 3 moles of H2 one mole of N2 moles of H2 cancels out and we do these calculations 5.4 times 1 divided by 3 will give us 1.8 moles of N2 here it is and you can see we go from 1 to 1.8, so although we use conversion factors to solve this, it's as if we were multiplying both by 1.8, so now we have all the information we need to calculate.

Which of these is the limiting reactant? Is it N2 or is it H2? Well, if we start with 3.2 moles of N2, we need 9.6 moles of H2, but we don't have that much, we only have 5.4 moles of H2. H2 will be the first thing to be used up, on the other hand if we start with 5.4 moles of H2 we only need 1.8 moles of N2 we have enough so this shows that we have enough N2 and H2 will be the first thing to be used up , which means that H2 is our limiting reactant. Now we want to know what is the greatest amount of NH3 that we can produce.

We have to start with this amount of H2 because that is our limiting amount. reactive, so I'm going to multiply this by I need the multiplication signs, there it is. I'm going to multiply this by 2 moles of NH3 over 3 moles of H2 moles of H2 moles of H2 cancels out and I get 3.6 moles of NH3, which is the maximum I can do right and there's 3.6 moles of NH3, that's as if we multiply this by 1.8, so this number is the maximum we can produce without running out of any of the reactants. Now, finally, let's talk about the excess reactant if it is H2. is the limiting reactant, which means that there will be XS in that is our excess reactant.

Well, let's find out how much is left. Well, we start with 3.2 3.2 moles of N2 and then we subtract how much we used. just use 1.8 moles, okay, so minus 1.8 moles and that will give us 1.4 moles of N2 for our excess, that's how much we have left over after the H2 runs out and we can't do this reaction anymore, okay, like this This is how the limitation is resolved. reactant problems first find out which of your reactants is the limiting reactant and then, using the quantitymaximum of the limiting reactant, calculate how much product you can produce for the excess reactant, you are going to calculate how much you have left Done after the reaction. stops because you have used all the limiting reactant, now the live reactant can be one of the most difficult concepts in chemistry, so I really urge you to watch other videos and do a lot of practice problems so that this becomes very, very comfortable.