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Limiting Reactants - Concept
M.Ed., Columbia Teachers College
Kendal founded an academic coaching company in Washington D.C. and teaches in local area schools. In her spare time she loves to explore new places.
Limiting reactants are reactants in chemical reactions which limit how much product can be formed. The limiting reactant is the reactant we run out of first, leaving the other reactants in excess. When predicting how much product we anticipate producing, we have to take the limiting reactant into account.
Alright so in the stoichiometry you'll probably going to come come across something called "limiting reactants" often known as limiting reagents and that limits the extent of reaction the amount of product formed so what exactly does that mean? Let's break it into something that makes more tangible sense to us. Let's say you worked in a bike shop and you're making bicycles and I gave you 20 wheels 20 bicycle wheels and 15 bicycle seats along with with many other things that are required to make a bike. How many bikes with these pieces of equipment, how many bikes can you actually make? Well we know that each bike requires 2 wheels so I can make a total of 10 bikes okay I'm limited by the amount of wheels I have, I have an excess or I'm left over with 5 bikes so I'm sorry 5 seats so I'm I when I'm finished I have 10 bikes and 5 seats leftover. I have 5 seats in excess this is going to be my excess reactant. I'm limited by the amount of wheels I make my product is dependent on the main limiting reactant in this case my wheels so I'm going to call this my limiting reactant okay so let's actually break this down into something that's it's Chemistry based.
Let's say you worked in a plant that's making ammonium ammonia particles. We know ammonia has a reaction of nitrogen parti- nitrogen gas plus 3 mols of hydrogen gas yielding 2 mols of nitrogen gas I'm sorry ammonia gas okay so let's say we gave you 3 mols of each nitrogen gas and hydrogen gas how much ammonia can you produce? Well I know right away looking at my mol ratios 1 to 3 I'm going to use my 3 mols of hydrogen molecules right away with 1 mol of nitrogen molecule so I'm limited by the amount of hydrogen this is my limiting reactant I'm actually going to make 2 I'm only able to make 2 mols of ammonia okay? I'm limited by my ammonia by my hydrogen and my excess is my nitrogen. How much nitrogen do I have leftover after the reaction took place? Well I have I used 1 when it reacted with those 3 so I have 2 leftover. I'm going to say 2 mols in excess leftover let's actually do a problem that actually incorporates all these concepts.
Okay solid sodium and iron 3 oxide is one in a series of reactions that inflates the car airbag. If 100 grams of sodium and a 100 grams of iron 3 oxi- iron 3 oxide are used, determine; a, limiting reactant, b the excess reactant, c the mass of product and d the mass of the excess whatever is leftover of the reactant. I'd actually put this reaction on the board for experience and I gave the products and this is the single placement reaction so it's easy for me to figure out I just swap the metals and I look at my the information I have for my problem and I know that I have a 100 grams of sodium I'm going to write that down here to make it easier for me to see and I'm also given a 100 grams of iron to react by okay now right away before even looking at the problem of what they're asking me I know right away this is the limiting reacting problem. How do I know that? Well I gi- I'm given two the masses of two reactants in my chemical equation that right away is indicator that it's a limiting reacting problem and you have to go about the procedure this way and figure out which one is limiting reactant because sometimes the problems aren't so clear in that.
So the first thing you have to do is to figure out which one is the limiting reactant so I'm going to figure out if I'm given a 100 grams of sodium, how much iron 3 oxide I'm going to need to react with this fully so I'm going to start doing mass to mass problem so I'm going to say 100 grams of sodium and the only way to compare this is going through mols so I'm going to say for every in 1 mol of sodium I know from my periodic table that it's 23 grams of sodium I also know from my reaction I require 6 mols of sodium to react with one mol of iron 3 oxide okay so now how many mols of iron 3 oxide I want to get to grams of iron 3 oxide so when I compare the grams I get from here to this that I have in my hand or have in my posession so for every 1 mol of iron 3 oxide looking at the periodic table its 159 I'm pretty sure 159.6 grams of iron 3 oxide so doing this Math 100 times 1 times 1 times 159.6 times 23 sorry divided by 23 divided by 6 divided by 1 its going to give me 115.7 grams of Fe2O3 required, required for a 100 grams do I have 115.7 grams? No I only have a 100 so I'm actually limited the products are limited by the amount of Fe2O3 and we have in excess of sodium so I'm going to say that the limiting reactant in this case is Fe2O3 my excess is sodium okay this I'm going to write that here also limiting reactant excess reactant.
Okay so I did a and b great what's the mass of solid iron produced? Well in order to figure out how much products I have produced, I have to go by limiting reactant my products are limited by my limiting reactant so I'm going to say, is this number here I'm going to say 100 grams I have to do a mass to mass relationship to figure out how much iron I need so 100 grams of Fe2O3. I know the mass of the Fe2O3 1 mol is looking at the periodic table is 159.6 grams. I know that so I look at my periodic table I know that 1 mol of Fe2O3 I require 2 mols of iron and looking at periodic table I'm looking for grams how much I'm looking for the mass of solid iron produced so to make sure that's in grams so 1 mol of Fe looking at the periodic table is 55.8 grams of Fe so doing this Math 100 times 1 times 2 times 55.8 divided by 159.6 divide by 1 divided by 1 gives me 69.99 grams of ironoxide so I'm actually with a 100 grams of Fe2O3 I'm able to produce 69.9 grams of iron.
Great lastly I'm going to do d on how much of the excess reactant is left on un-reacted so I know that I started out with 100 grams of sodium how much actually reacted? So in order to actually get the information about everything I have to deal with my limiting reactant so actually let's do the Math over here. I have a 100 grams of iron, iron 3 oxide Fe2O3 I know I might have to do mass to mass ratio so get to mols so I'm going to have to get 100 grams it is 159.6 grams it's every one mol of Fe2O3 and looking at the reaction I have 1 mol of Fe2O3 to 6 mols of sodium and I'm going to get the mass sodium so I'm going to say 1 mol of sodium is 23 looking at periodic table grams of sodium sorry -- alright so how did the mass of this 100 times 1 times 6 times 23 times 159.6 I'm sorry divided by 159.6 divided by 1 divided, divided by 1 it's actually going to give me 86.47grams of sodium that's how much I actually use in my reaction a hundred I'm sorry 86.47 grams of sodium but I gave you 100 grams of sodium how much was left un-reacted? We're going to subtract these two and we get 13.53 grams of sodium leftover okay so these are all the ty- different types of questions you're going to see when dealing with limiting reactive problems and hopefully this helped you out.
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