HOW to IDENTIFY the LIMITING REAGENT for A Level Chemistry
Online A Level Chemistry Tutor Paul Morgan with fellow tutor Atul Rana demonstrates molar calculations involving limiting and excess reagents using the online whiteboard bitpaper
_______________________________________
(00:00)
Hi, I'm Atul and I am in London. Hi, I'm Paul and I'm in Burnley Lancashire.
So today we look at limiting reagents and molar calculations.
So the question here is what mass of Iron is formed when 8 g Iron oxide reacts with 2.16 g of Aluminium.
For limiting reagents we look at the moles of the reactants. First of all for Iron oxide and Aluminium, we write down the mass, the Mr and the moles.
We have 8g of Iron Oxide and 2.16g of Aluminium.
27 is the atomic mass of Aluminium. For Iron Oxide there are 2 iron atoms and three oxygen atoms. So 2(55.8) + 3(16) =159.6
(03:12)
We've been asked for the mass of Iron so Mr (Ar) = 55.8.
Moles = Mass / Mr so Iron Oxide is 8 ÷ 159.6 = 0.05 moles and Aluminium is 2.16 ÷ 27 = 0.08 moles.
(04:13)
We've got to work out which of these is the limiting reagent. So if this was a one to one ratio, then our Iron oxide would be the limiting reagent because it's less moles.
But it's not a one to one. It's a one to two so if you look at the Aluminium we can say that 0.08 moles of Aliminium, would react 0.04 moles of Iron Oxide and we've more than that. So this is in excess.
Therefore the Aluminium is our limiting reagent
(05:13)
To work out the mass of iron that's formed we use the limiting reagent moles
(05:27)
The Aluminium and the Iron have a one to one ratio. So we would form exactly the same number of moles.
Mass = Moles x Mr so 55.8 x 0.08 = 4.464g = 4.46 g of iron (3sf)
(06:31)
So we workout the limiting reagent and then we work at the maximum product formed.
(06:37)
I'm guessing that if they assumed that iron oxide is the correct one, just hypothetically, they'll soon find out that the Aluminium doesn't compute because that'd be like 0.1 or something.
(07:21)
Yes. They could work out the other the other way round as well. They could look at the, the iron oxide is 0.05 moles so therefore that would react with double the aluminium 0.1 moles.
And therefore you can see that the aluminium is the limiting reagent there because 0.08 moles is less than 0.1 moles.
(07:42)
So you can choose either reactant and work out which is the excess and which is the limiting reagent.
(07:48)
Yeah. And it's great because in real life one is going to be in excess and one is not just for practical reasons.
And it's something students might've seen in titrations for example. So they can extend that idea.
(08:05)
Yeah. And sometimes the question will tell you that a certain reactant is in excess, so then you just use the other reactant but where it doesn't then you have to work out which is the limiting reagent.
_______________________________________
More Information:
Paul Morgan | A Level Chemistry Tutor
https://www.alevelchemistrytuition.com
Atul Rana | Maths and Science Tutor
https://www.atulranatutors.co.uk
#LimitingReagents #LimitingReactants #OnlineALevelChemistryTutor
-~-~~-~~~-~~-~-
Please watch: "HOW I TUTOR A LEVEL CHEMISTRY ONLINE "
https://www.youtube.com/watch?v=Z6vq7x368gU
-~-~~-~~~-~~-~-
