YouTube Transcript: Chapter 7.2d Types of Redox Reactions
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This content explains the concept of redox reactions by defining them as reactions where oxidation numbers change, and then categorizes and illustrates different types of redox reactions, including combustion and single displacement, along with examples of non-redox reactions.
hi everyone Welcome to our next video on
redox reactions we're going to be
talking about different types of redox
reactions and so many of our learning
outcomes are the same as what you've
seen in a previous video
so to remind you we determined that a
redox reaction is one in which one or
more elements is going to change its
oxidation number and so in order to
determine if a reaction is a redox
reaction we would want to First assign
the oxidation numbers to all the species
involved and then see if one of them has
changed in the process
so last time we talked about the
mnemonic device Leo the Lion says gur we
can use Leo to remember that the loss of
electrons is oxidation and gur to
remember that the gain of electrons is
reduction another way that helps me to
remember what oxidation and reduction
are is that oxidation is an increase in
oxidation number and both oxidation and
increase start with vowels
and then a reduction is when we see a
decrease in oxidation number and both of
those start with consonants so my vowels
together tell me oxidation is increase
and my consonants together tell me
reduction is a decrease in oxidation number
number
so as we said earlier we're going to
look at different types of redox
reactions and
um we'll see what makes them redox
reaction so first we have combustion
reaction a combustion reaction is one
and where we have some sort of fuel
which is going to be burned in the
presence of an oxidant and usually this
generates a good amount of heat and
often a flame right so often we think
about burning a fuel in the presence of
oxygen so here's an example of such a
reaction the combustion of Ethylene
so ethylene is c2h4 and we see that it
is burned in the presence of oxygen gas
and in this type of combustion reaction
and in the ones that you'll mainly be
dealing with in general chemistry the
products are always going to be carbon
dioxide gas and water so this is always
true when our
fuel that is being burned is organic
meaning that it has carbon hydrogen and
usually some oxygen present are products
when we burn something like that are
always going to be CO2 gas and water
so we know that this is a redox reaction
it's a type of redox reaction so let's
look at the oxidation numbers in order
to determine what is changing
so I'm going to fill those in for you
and then I'll restart the video once I
all right so I've added in all of my
oxidation numbers and I found these by
looking at the individual molecules and
assigning the oxidation number for
example in c2h4 I assigned the oxidation
number of plus one for hydrogen I
recognized that there were four
hydrogens so the total contribution to
charge would be plus four
and then recognizing that this is a
neutral molecule the sum of my total
charges has to equal zero so the carbon
would have to have a minus four total
contribution to charge since there are
two carbons each one would have an
oxidation number of minus two and so
that's the strategy I used when
assigning all of these oxidation numbers
for the different elements and compounds involved
involved
so now what we want to do is look to see
which species are changing their
oxidation number
so the first one that I see is that
oxygen is going from zero right to minus
two in the carbon dioxide we also see
oxygen as minus two in the water so we
can see that oxygen is having its
oxidation number decreased it goes from
so we see a reduction in our oxidation
number which tells me that this is a the
we also can see that the carbon goes
from being minus two to plus 4 in the
carbon dioxide gas
so the C
of c2h4
goes from -2
Plus 4.
right so this the carbon is oxidized so
this is a redox reaction a combustion reaction
reaction
our next example is a single
displacement or replacement reaction so
these are redox reactions in which an
ion in solution is displaced or replaced
via oxidation of a metallic element so
here's an example
we see that the ion in solution when I
look at my reactants would be the silver
nitrate right the agno3 so the cation
that's present in that would be the
silver right the silver ions
are present in solution but when this is
reacted with the copper the copper
replaces the silver as the ion in
solution so here we have copper two plus
ions that now are in solution and we see
then as a product the silver is now our
metallic element so this is a
replacement or single displacement
reaction so I'm going to fill in my
oxidation numbers so we can look at what
is changing in this reaction
so I filled in our oxidation numbers for
this reaction so let's look to see which
numbers have changed
first I can see that copper starts off
as the element so it has an oxidation
number of zero and then that changes it
increases to plus two in the copper
nitrate so we can see that copper goes
from zero to plus two so there is an
increase in its oxidation number and an
increase means that we're seeing oxidation
when I look to see what else has changed
I notice that my silver ions start out
as plus one and these decrease in
oxidation number down to zero so my
silver ions go from a plus one oxidation
number down to zero so the decrease
so copper is oxidized and the silver
ions are being reduced
and the pictures at the bottom of this
slide show this reaction so inside the
test tube is silver nitrate solution the
wire is made of copper once these are
once you stir the solution with the
copper wire then the displacement
reaction occurs and what's happening is
the copper is going into solution so
that's why we see the characteristic
blue color and then the silver is
actually going to be deposited onto the
wire so that's the Gray colored metal
so we have a few more examples so in
these we want to identify whether these
equations represent redox reactions and
if they do then we're going to name the
oxidant or the oxidizing agent and the
reducing agent so let's fill in our
oxidation numbers
all right with those oxidation numbers
filled in let's look to see if our first
example is a redox reaction
so we're looking to see if any of our
oxidation numbers have changed when I
look at zinc as a reactant it's plus two
and as a product it's still plus two
when I look at carbon as a reactant it's
plus four and as a product it's also
plus four
and similarly for oxygen as a reactant
it's minus two and its oxidation state
is -2 as a product so here I see no
change in terms of my oxidation numbers
which tells me that this is not a redox reaction
reaction
let's look at our second example here we
have gallium reacting with bromine to
make gallium bromide let's look to see
if we have a change in oxidation number
I do see that gallium starts as zero and
increases to plus three
and my bromine starts at zero and
decreases down to negative one
so gallium goes from zero to plus three
so this is increasing oxidation
whereas my bromine
goes from zero down to negative one so
now this is asking us to name the
oxidant and reductant it's just another
way for calling the oxidizing agent and
the reducing agent
so remember the species that is oxidized
it essentially provides the electrons
that can be given to the bromine for its reduction
reduction
the species that is reduced is my
oxidizing agent so here the bromine is
accepting the electrons from the gallium
and so it oxidizes the gallium and
serves as our oxidizing agent all right
let's look at our last two examples
in this first reaction we have barium
chloride reacting with potassium
sulfates to give us barium sulfate and
potassium chloride
so let's look to see if any of our
oxidation numbers have changed we see
that barium starts at plus two as a
reactant and then the products it's plus two
two
the chloride starts at -1 and is minus 1
as the product
potassium is plus one as a reactant plus
one is a product
sulfur plus six plus six as a product
and oxygen stays at -2 as a reactant end
product so here I see no change in terms
of my oxidation numbers so this means
that this is not a redox reaction
let's look at our last example here we
have hydrogen peroxide that is
decomposing to give water plus oxygen
gas when we look at the oxidation number
for oxygen in the peroxide remember that
this is one of our exceptions normally
oxygen has a minus two oxidation number
but when it's in a peroxide it would
have a minus one the reason is that we
want to First assign a plus one
oxidation number for hydrogen and here
we have two hydrogens so that gives us a
plus two total contribution to charge to
balance that and have this equal to zero
for the neutral molecule the oxygens
have to have a minus two overall charge
and since we have two oxygens each one
would carry a minus one charge right for
their oxidation number
so here we have oxygen at a minus one
oxidation number and we look to see
oxygen in our products has a minus two
oxidation number so we go from -1 down
to minus two right so oxygen in the peroxide
peroxide
goes from minus one
to minus 2 in water
but we also see that oxygen goes from -1
to zero when we look at the oxygen gas
so the oxygen in the peroxide goes from
minus one
to zero in the oxygen gas because this
is an increase in our oxidation number
so let's assign the oxidizing agent and
the reducing agent we know the species
that is reduced is our oxidizing agent
right so the peroxide is our oxidizing agent
and we know that the species that is
oxidized is our reducing agent so it
just happens to be that the peroxide is
both the oxidizing agent and the
reducing agent so this is a little
strange it's actually a specific type of
redox reaction it's called a
disproportionation reaction so we have a
reaction in which the same element
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