Oxidation-reduction (redox) reactions involve the transfer of electrons between substances, characterized by changes in oxidation states. Understanding these changes is key to identifying oxidized and reduced species, as well as oxidizing and reducing agents.
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in this video we're going to talk about
oxidation and reduction reactions
and here's the example we're going to
start with magnesium plus oxygen gas produces
produces
magnesium oxide
an oxidation reduction reaction or
simply a redox reaction electrons are
being transferred from one element to another
now perhaps you found or watched my
video on oxidation numbers if you have
then you know that the oxidation number
of any pure element is zero
now in magnesium oxide magnesium being
an alkaline earth metal has a charge of
plus two
oxygen has a charge of minus two
notice that the oxidation state of
magnesium went from zero to plus two
so the oxidation state increased
whenever the oxidation state goes up the
substance is set to be oxidized
in the case of oxygen the oxidation
number decreased from zero to negative two
two
whenever the oxidation number decreases
the substance
now you need to know that oxidation
always occurs with
a loss
of electrons reduction
reduction
is associated with a gain
of electrons
metals they like to give away electrons
they like to form
metal cations as they give away
electrons they will acquire a positive charge
charge
non-metals like oxygen
they like to acquire electrons and so
they will develop a negative charge
the substance that is oxidized
is known as the reducing agent
and the substance that is reduced
is known as
metals are reducing agents because
they will cause the other substance to
be reduced
not metals like oxygen gas fluorine
they're oxidizing agents because they
cause another substance to be oxidized
and that's where you got to see it
so these are some terms that you want to
be familiar with when dealing with redox reactions
reactions
the reaction between magnesium and
now magnesium
is changing into the magnesium ion
this magnesium ion has a plus two charge
and in order to become
a plus two cation it has to lose two electrons
electrons
now oxygen
i'm gonna write atomic oxygen because
this reaction is not balanced
individually oxygen acquires two electrons
electrons
now because magnesium lost electrons
this reaction it's which is known as a
half reaction
that's the oxidation part of the reaction
reaction
anytime a substance loses electrons it's
being oxidized
this second half reaction is the
reduction part
since oxygen is acquiring electrons
whenever a substance gain electrons or
if the oxidation number decreases
decreases
its reduction
now for half reactions anytime you have
electrons on the right side it's going
to be an oxidation half reaction
whenever the electrons are on the left
side it's a reduction half reaction
that's another way in which you can see
it too
consider this reaction
zinc metal reacts with hydrochloric acid
to produce hydrogen gas
identify the substance that is oxidized
and the substance that is reduced
and also identify the oxidizing agent
and reduce an agent
now if you get a question like this on
the test to find a substance that is
oxidized or reduced always look at the
reactants it's one of these two
now let's find the oxidation states of
everything the oxidation state of any
pure element is always zero
now whenever hydrogen is bonded to
a non-metal
it's going to have a positive one
oxidation state
which means chlorine has to be minus one
now in this one two chlorine is still
negative one which means zinc has to
have a plus two charge
now the oxidation of zinc changes from
zero to positive two
so zinc
is being oxidized
hydrogen changes from one to zero
so hcl as a substance is being reduced
even though only the hydrogen portion of
that substance is being reduced
so typically they actually for the
substance even though it's only the hydrogen
hydrogen
element that's being reduced
you would say the whole substance is
being reduced if you have to choose an
answer on a multiple choice test
now the substance that is oxidized
is to reduce an agent
and typically metals tend to be reducing agents
agents
the substance that is reduced
and this is a common question that you
might see on a typical chemistry exam
now let's try one more example for the
sake of practice methane
methane
reacts with oxygen gas
and it produces carbon dioxide and water
so feel free to pause the video identify
the oxidation states of every element in
this reaction
and then find a substance that is
oxidized reduce and identify the
oxidizing agent and reduce an
agent so let's identify the pure elements
elements
which is only oxygen gas that's going to
be zero
now we said that whenever hydrogen is
bonded to a non-metal
it's going to have a plus one oxidation state
and typically when oxygen is found in a
compound the oxidation state is negative two
two
except when it's bonded to fluorine or
except when it's in the form of peroxide
or superoxide it's different
whenever you hear the word oxide
the oxidation state of oxygen is
negative two if you hear the word
peroxide it's negative one if you hear
the word superoxide
it's negative one-half
now we gotta find the oxidation state of carbon
carbon
so let's start with methane
so we have one carbon atom and four
hydrogen atoms
which has to add up to zero because
methane is neutral in charge
now each hydrogen atom has an oxidation
state of one
so therefore c plus four
must equal zero therefore carbon has to
have an oxidation state of negative four
in methane
now let's calculate the oxidation state
of carbon in co2
so it's c
plus two oxygen atoms
which equals a net charge of zero
and each oxygen has a charge of negative two
so in this case carbon is going to have
so carbon changes from negative four to
positive four
therefore the oxidation number of carbon
is increasing which means carbon
is oxidized or technically methane is oxidized
oxidized
but oxygen
changes from zero to negative two
so therefore oxygen gas
the oxidation number is decreasing
so it's being reduced
which means that methane
is the reducing agent
and oxygen gas
it's being reduced which makes it the
so keep in mind even though the element
carbon is being oxidized you still
describe the whole substance methane as
being oxidized because carbon is part of methane
methane
now the next thing that you need to be
able to do
is you need to take a look at a reaction
and tell if it's an oxidation reduction reaction
reaction
so consider this reaction aluminum metal
reacts with copper chloride
to produce aluminum chloride
and copper metal is this reaction a
redox reaction
so this reaction is a single replacement reaction
reaction
and if there's a transfer of electrons
then it's going to be a redox reaction
aluminum has an oxidation state of zero
but in alcl3 it has an oxidation state
of clustering
so if the oxidation number changes
then it's a redox reaction
all single replacement reactions
are redox reactions
now if you don't want to look for the
oxidation numbers there's a quick way to
tell if something is going to be a redox reaction
reaction
if you see a pure element on one side
and then that element being part of a compound
compound
on the other side it's always going to
be a redox reaction there's a transfer
of electrons
and i want you to determine
so both of these examples
are synthesis reactions also known as
combination reactions
sometimes a combination reaction can be
a redox reaction and sometimes it won't be
be
now if you look at the first example
there are no pure elements all we have
is compounds when you see that chances
are it's not a redox reaction
so if you look at the second example we
have a pure element zinc
and then that same element is in a
compound so the second example is
is
a redox reaction and let's prove it so
let's start with the first example
oxygen has a negative two charge
magnesium in magnesium oxide has a plus
two charge
hydrogen has a positive one charge
now magnesium hydroxide
is composed of mg plus two
and two hydroxide ions
but as you can see magnesium still has
an oxidation state
of positive two
now let's focus on hydroxide
hydroxide is oxide
if you want to solve it you know that
an oxygen
is going to have a negative two charge
when you add up negative two and one
you're going to get the net charge of a
hydroxyl ion which is negative one
so in magnesium hydroxide oxygen is
still negative two and hydrogen is plus one
one
so notice that the oxidation state of
magnesium did not change
it's still positive two the oxidation
state of hydrogen is still the same
it's positive one
and the oxidation state of oxygen has
not changed
if the oxidation state does not change
that means there was no transfer of
electrons and so this is not
now let's analyze the second example
zinc and chlorine has an oxidation state
of zero
in zinc chloride
chlorine has a negative one oxidation
state zinc has a positive two oxidation
state so notice that zinc is being oxidized
oxidized
therefore there's a transfer of
electrons which makes this reaction a
redox reaction
now consider these two
decomposition reactions
when mercury oxide is heated it
decomposes into mercury and oxygen gas
and if you were to heat calcium
carbonate it will decompose into calcium
oxide plus carbon dioxide gas
so which of these decomposition
reactions is the redox reaction
looking at the first example
we have oxygen as a pure element on the
right side and then that same element is
within a compound so the second example
i mean the first example is a redox reaction
reaction
in the second example there are no pure elements
elements
all we have are just compounds
so the second example is not a redox
reaction that's a quick way to tell if
it's a redox or not
so now i'm going to give you a list of
reactions and determine which ones are
so feel free to pause the video if you
so the first example
is it a redox reaction
combustion reactions are always redox reactions
reactions
here we have a pure element on one side
and then that same element is inside a
compound so the first example is a redox reaction
reaction
what about the second
example notice that there are no pure
elements in this example
acid-base neutralization reactions are
not redox anytime you have a double
replacement reaction is never going to
be a redox reaction but all single
replacement reactions will be a redox reaction
reaction
the third example is a combination reaction
reaction
and there are no pure elements so
therefore this is not going to be a
redox reaction the last one is a double
replacement reaction specifically a
precipitation reaction
and as we can see
all we have are just four compounds in
this reaction there are no pure elements
so it's not a redox reaction
so to review
all combustion reactions
are redox reactions
all single replacement reactions are
redox reactions
now synthesis and combination reactions
sometimes the redox sometimes are not
and the same is true for decomposition reactions
reactions
double replacement reactions are never
redox reactions so that includes
acid-base reactions
precipitation reactions and other types
of double replacement reactions
and a quick way to tell
is if you see a pure element on one side
and then the same element within a
compound on the other side
it's going to be a redox reaction that's
the easiest way to tell
so that's it for this video if you want
to find more chemistry videos just check
out my channel
and you can find other videos on like
physics calculus algebra trig
trig precal
precal
and other stuff like that so thanks for watching
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