In this video we are going to learn what a theoretical yield is, how to calculate a theoretical
yield and how we account for any losses of desired product from a chemical preparation
In a chemical reaction, you react two or more reactants together so you can make a desired
product.
The amount of desired product you get is controlled by something called, the theoretical yield.
The theoretical yield is the maximum possible amount of product that can be produced.
Think of it like a test.
You can never score higher than the maximum possible mark.
You can calculate the theoretical yield from any reaction because the mass of the reactants
used must equal the mass of the products produced.
For example, if you react magnesium with hydrochloric acid, you get magnesium chloride, a salt;
and hydrogen, a gas: Mg (s) +2HCl(aq) → MgCl2 + H2(g)
Let's say you want to calculate the maximum possible amount of magnesium chloride (also
known as the theoretical yield) that can be made from using 12.15g of magnesium.
First, you have to calculate the formula mass of magnesium chloride from the relative atomic
atomic masses of its elements -- magnesium and chlorine.
These are displayed on the screen.
Can you calculate the formula mass of magnesium chloride?
Pause and continue when ready.
The answer is that you have to add 24.3, the relative atomic mass of magnesium, to the
relative atomic mass of two chlorine atoms, each at 35.5.
In total, the formula mass of magnesium chloride is 95.3g.
This is also the theoretical yield for one mole.
Did you get it right?
If you use half the formula mass of magnesium atoms (12.15g), what do you think the theoretical
mass of magnesium chloride is now?
Pause the video and continue when ready.
The answer is that you expect half of the theoretical yield, as well.
This means that 12.15g of magnesium would theoretically yield 47.65g magnesium chloride.
This use of ratio can be applied to any chemical reaction.
There is a video on reacting masses on this channel.
The idea is explored there, in further detail.
In reality when you do any reaction rarely do you achieve the theoretical yield.
There are a number of reasons for the mass of a product being lost and lowering your
theoretical yield.
Here's a challenge.
Can you think of any reasons why the theoretical yield may not be achieved?
Pause and continue, when ready.
Some of the reasons are: The reaction might not be 100% complete because
if it is reversible, the products made can turn back into the reactants, until an equilibrium
is established.
You always get losses of product when it is separated from a reaction mixture by filtration,
distillation, crystallisation or any purification technique
Some of the reactants may react in another way to give a different product to the one
you wanted.
You always lose some mass whenever you pour a liquid from one place to another, or if
you transfer a solid from one place to another.
So, in summary, a theoretical yield is the maximum possible mass of a desired product
that resulting from a chemical reaction.
Losses in product mass may result from purification processes and/or from the transfer of reactants
from one place to another.
Losses also result if the reaction is in equilibrium (that is goes backwards and forwards between
reactants and products).
Finally, the theoretical yield is lowered if the reaction proceeds to give a different
product to the one you desired.