Leah here from leah4sci.com/MCAT and in this video we're going to look at redox pairs and
common oxidizing and reducing reagents.
When it comes to redox my favorite mnemonic is LEO the lion says GER which tells us that
the loss of electrons is equal to oxidization and the gain of electrons is equal to reduction.
I like this better than OIL RIG which stands for oxidation is loss and reduction is gain
because it doesn't tell us what we're gaining and what we're losing and a mnemonic that's
confusing is not worth the time, LEO the lion says GER clearly tells us loss of electrons
is oxidation, gain of electrons is reduction but we have to remember that they are connected
to each other.
Say I walk into a deli and try to order a sandwich, the sandwich costs five dollars
so I hand five dollars to the cashier, the cashier hands me a sandwich.
I want the sandwich but in a process of acquiring a sandwich I have lost five dollars that means
the deli in the process of gaining five dollars has cost me to lose five dollars.
They can't gain my money without me having a loss, with oxidation and reduction, if there
is a transfer of electrons something can only gain electrons if something else loses the
electrons, that means redox reactions have to be paired with the same number of electrons
are gained from one and loss from the other where the total electrons loss in one half
of the reaction have to be gain in the other half of the reaction.
That's why they're called redox pairs; Red, Ox, Reduction and Oxidation.
What are the consumers and cashiers of redox reactions?
In this reaction as we'll learn in the next video, Sn or Tin is oxidized and Iron is reduced.
When Sn2+ is getting oxidized in the process of getting oxidized it is causing the iron
to get reduced and at the same time when the Fe3+ is getting reduced it is causing the
Sn to get oxidized this means that when Sn2+ gets oxidized it acts as the reducing agent
or the reductant and when Fe3+ gets reduced it acts as the oxidizing agent or the oxidant.
We'll look at redox reactions in more detail in the next video but first let’s look at
some common oxidizing reagents.
From the mnemonic LEO says GER, we know that something that has a loss of electrons is
going to be oxidation and a gain of electrons is reduction, what I want you to take that
a step further.
Electrons, when they're added to a proton gives us a hydrogen atom so you can think
of oxidation as the loss of a hydrogen atom.
If reduction is the gain of an electron and electrons and protons together gives us hydrogen
atoms then we can assume that reduction is the gain of a hydrogen atom, in general chemistry
you typically look at just electrons but in organic chemistry and biochemistry you've
seen redox that involve oxygen atoms.
Think about your alkene reduction where you added hydrogens, that's the hydrogen atom,
that's a reduction reaction.
Think about the oxidation of an alcohol, we've lost a hydrogen atom but we also gained a
bond to the oxygen atom, oxygen and hydrogen tend to be the opposite of each other so another
way to think of oxidation is loss of hydrogen or the gain of oxygen and reduction would
be the gain of hydrogen or the loss of oxygen if I turn that carbonyl into an alcohol, that's
a reduction because I lost a bond to oxygen.
What does this have to do with reducing agents?
When you're looking at the molecules, instead of just memorizing what they are, try to pay
attention to what we would expect on an agent of oxidation or an agent of reduction.
You can download the common oxidizing and reducing reagents cheat sheet on my website
link below but let’s look at some of them first.
In general chemistry, you've seen a lot of oxidizing and reducing agents as simple metals
and I wouldn't memorize any of them because a metal will act as an oxidizing agent under
a certain condition but that same metal can act as a reducing reagent under other conditions,
think back to what you covered in electrochemistry but in organic chemistry, you've learned a
list like lithium aluminum hydride also called LAH, sodium boron hydride, sodium hydride,
H2 gas, H2 and Lindlar's catalyst, what do you notice with each of these reducing agents,
if a reducing agent is going to cause something to be reduced that means that something has
to gain electrons or gain hydrogens, shouldn't the agent that causes something to gain electrons
or hydrogens have plenty of hydrogens to provide?
Reducing agents with four hydrogens, four hydrogen, one hydrogen, two hydrogen, that's
an easy way to remember it and let’s not forget from biochemistry NaDH and FADH2, once
more, hydrogen and lots of hydrogen.
We mentioned metals can act as oxidizing or reducing agents, if oxidizing agents are going
to cause a loss of electrons, they're also going to cause a gain of oxygen and the way
to remember that is oxidizing has oxygen in the name, it causes something else to gain
oxygen so what do you expect to see in your oxidizing agents?
Oxygen.
In general chemistry you would've seen O2 peroxides like H2O2 and some strong acids
like H2SO4.
In organic chemistry you would've seen KMnO4, H2CR2 of seven or some version of chromic
acid. more O2, Pcc which if you think about is pyrodinium chlorochromate more chromic
acid variations and in biochem, NAD+ and FADH.
Once again, what do we see?
Oxygen, oxygen, oxygen, oxygen, more oxygen, there's oxygen hiding in there and this one’s
a little tricky because we don't appear to see any oxygens in the NAD+ or FADH, so instead
look at the hydrogen balance.
The reducing agent has more hydrogen than the oxidizing agent which in NAD just has
a positive charge.
In FAD, the reducing agent has two hydrogen atoms, the oxidizing agent only has one.
Be sure to join me on the next video where we look at oxidation and reduction reactions
in more detail.
First make sure to give this video a thumbs up and subscribe to my channel so you don't
miss when I post new videos and visit my website leah4sci.com/reactions for the entire MCAT
Stoichiometry series along with the practice quiz and cheat sheet.