Redox Reactions in Terms of Electron Transfer Reactions Video Lecture | Chemistry for JEE Main & Advanced

this video is an introduction to
oxidation reduction reactions we often
abbreviate oxidation and reduction so we
just call these redox reactions for
short so what is an oxidation reduction
or redox reaction here's a quick
definition in oxidation reduction
reactions electrons move between atoms
check out this diagram here we got atom
a and atom B and there's an electron
moving out of atom a into atom B so this
is definitely an oxidation reduction
reaction because there's an electron
moving between the atoms now we can
describe the movement of this electron
using the words oxidation and reduction
so let's learn specifically what both of
these mean reduction is a gain of
electrons I know this seems kind of
confusing because usually when you have
a reduction of something you have less
of it we'll talk later about why a gain
of electrons is called a reduction but
anyway
reduction gain of electrons and then
oxidation which is a loss of electrons
so we got reduction and oxidation what's
happening to atoms a and B over here
well atom a is giving up one of its
electrons it's losing an electron so
atom a oxidation is taking place atom B
is receiving an electron it's gaining an
electron so it atom B reduction is
taking place now we can modify these
words slightly to come up with terms we
use to describe what's happening to the
ABS atom a is undergoing oxidation so we
can say that atom a is being oxidized
atom B is undergoing reduction so we can
say that atom B is being reduced so we
got oxidation oxidized reduction reduced
the big picture here is that reduction
is a gain of electrons and oxidation is
a loss so I don't know about you but
when I was first learning this stuff I
found it so confusing why should a
reduction be a gain of electrons usually
when you're reducing something you're
gay
less of it so here's one thing that
might be helpful to think about that's
that electrons have negative charge so
as you add electrons to something its
charge goes down the more electrons you
add the lower the charge so as you add
electrons as electrons are gained the
charge reduces so that might be helpful
to keep in mind to have this make a
little bit more sense still these things
can be really confusing to remember so
here are a couple really great mnemonics
first is the term oil-rig this is the
name one of those big structures you use
to drill for oil
Oh il stands for oxidation is loss and
RI G stands for reduction is gains it's
a really good mnemonic here's another
one you might like if you like animals
Leo the Lion says ger all right le o
stands for loss of electrons is
oxidation and ger stands for gain of
electrons is reduction so learn one or
both of these will be a great way you
can always remember what reduction is
and what oxidation is okay oxidation and
reduction happen in all sorts of
chemical reactions there like the super
important thing that makes those
reactions happen that makes them go so
to learn more about this stuff we're
going to have to look at an example of a
chemical reaction where oxidation and
reduction are taking place here's a
chemical reaction that we're going to be
talking about sodium and chlorine coming
together to make sodium chloride table
salt now sodium chloride is an ionic
compound which means it's made of ions
specifically the sodium one plus and the
chloride 1 minus ions these two ions
stick together because I have opposite
charges and opposite charges attract
now Na and Cl they didn't always have
charges though over here before they
combined they have no charge they are
two electrically neutral atoms so in
order to come together and make sodium
chloride these two atoms have to end up
getting charges and then they stick
together so let's talk about how this
happens how they end up getting charges
so we start out with Na and Cl here as
electrically neutral atoms with no
charge and they're on their own
they're separate the next thing that
happens is sodium gives one of its
electrons to chlorine so this is where
the charges come in sodium loses one of
its electrons so it becomes in a plus
one and chlorine gains an electron so it
becomes CL minus one now we have two
ions with opposite charges what an
oppositely charged things do they stick
together so these two ions come together
and they stick making sodium chloride
okay so this is the process where we
start from electrically neutral atoms we
have an electron transfer we get ions
that then stick together so here's the
equation that I showed you earlier that
describes this process there are a
couple things that I could add to this
though to make stuff a little clearer
check out these numbers that I've
written it here in the red these are
called
oxidation numbers and they give me a
sense of what's happening with the
electrons and with the charges so above
Na and Cl I've written zeros that
indicates that these two atoms start out
being electrically neutral they have no
charge that's what the zero means then
on this side of the equation once the
sodium chloride have taken on charges I
put those charges above each element
plus one for the sodium and minus one
for the chloride these oxidation numbers
help us keep track of how charges chain
during a reaction most importantly for
this reaction they remind us that sodium
and chlorine start out neutral and only
end up getting a charge later after this
electron transfer takes place so that is
oxidation numbers and we'll talk more
about those in coming lessons so
obviously this is an oxidation reduction
reaction because there is a transfer of
an electron between these two atoms I
now want to describe what's happening to
sodium and chlorine using these terms
reduction and oxidation so just to
refresh your memory reduction here is a
gain of electrons and oxidation is a
loss of electrons so for sodium what's
happening is it being reduced or is it
being oxidized well sodium is giving up
one of its electrons it's losing an
electron so it's being oxidized here's a
diagram of sodium giving up that
electron now chlorine on the other hand
is receiving an electron it's a gaining
an electron so chlorine is being reduced
so there's oxidation and reduction going
on here we can sort of show this
oxidation and reduction with this
picture that I drew or we can describe
it in words the next thing that I want
to do is I want to show you how we can
write chemical equations that show
oxidation and reduction so let's start
out here with let's start here with
chlorine I want to write a chemical
equation that shows the reduction of
chlorine so I'm going to start out with
CL here this is neutral chlorine it
doesn't have any charge it's this thing
here then it receives an electron so to
write an equation for that I'm going to
write plus e - e - is a symbol we use
for electrons so Cl neutral chlorine
plus one electron gives me this guy
right here CL one - so this is the
equation for the reduction of chlorine
for chlorine gaining this electron here
okay now for sodium for sodium things
are a little bit trickier for
oxidation let me show you why so what I
really want to write is na - one
electron gives na one plus but
unfortunately I can't write this because
in chemical equations we can't use minus
signs we can only use plus signs so I
can't write the oxidation like this
instead I'm going to write it like this
I'm going to start out with na here and
then I'm going to put my arrow so na
turns into na one plus plus one electron
writing the electron on this side of the
equation indicates that it's being spit
out that na is losing it it's no longer
part of the atom so to write reduction
we put the electron on this side of the
equation because it's coming together
with the atom to write oxidation we put
the electron on this side of the
equation because it's getting spit out
so these are the reactions for oxidation
and reduction of sodium and chloride now
I want to talk a little bit more about
these two reactions and how we can
combine them to make a larger bigger
picture equation here's our big picture
reaction for sodium and chlorine coming
together to make sodium chloride now
this reaction here really has two
processes going on within it one of
these processes is the oxidation of
sodium it starts out at zero loses an
electron and becomes sodium one plus and
here is the equation for just the
oxidation the other process that's going
on in this reaction here is the
reduction of chlorine chlorine starts
neutral at zero it gains an electron and
it ends up is CL one minus and here is
the equation for the reduction of
chlorine now these two equations down
here are called half
reactions they show either oxidation or
reduction and they're called half
reactions because they each kind of tell
half the story you put them together and
you get the big picture reaction that
shows both oxidation and reduction so
for every oxidation reduction reaction
like this one you can write two half
reactions one for the oxidation that
takes place and one for the reduction
that takes place now it's super
important to keep in mind that oxidation
and reduction always have to happen
together they always have to happen at
the same time because if one atom is
gaining an electron that electron had to
come from somewhere
so another atom had to give up that
electron you can see that right here the
electron here that chlorine is gaining
is the same electron that sodium was
releasing so oxidation and reduction
always have to happen at the same time
they always have to happen in parallel
you can't have reduction without
oxidation you can't have oxidation
without reduction so that's an
introduction to oxidation reduction or
redox reactions we learned that
reduction is a gain of electrons and
oxidation is a loss of electrons we
looked at a chemical reaction where
sodium and chlorine come together to
make sodium chloride sodium starts off
as neutral it gets oxidized by losing an
electron and becomes sodium one plus
chlorine starts off as neutral it gets
reduced by gaining an electron and
becomes chloride one minus we also learn
that for every big picture oxidation
reduction reaction like this we can
break it down into two half reactions
one of which shows the oxidation that's
going on and the other of which shows
the reduction that's going on finally we
learned that oxidation and reduction
always have to happen at the same time
because when one atom gains an electron
and gets reduced
it had to get that electron from
somewhere else which means that another
atom had to lose an electron and get
oxidized