in this video we're going to talk about

electrochemistry and in particular the

two types of electrochemical cells

there's the galvanic or voltaic cell and

then the electrolytic cell so you might

think what does chemistry have to do

with electricity

well what these cells do is they convert

between chemical energy and electrical

energy and you might think how does that

happen well remember when we learned

about redox reactions and how they

transfer electrons that's all

electricity is it's the transfer or

movement of electrons so if we have a

redox reaction we have the potential to

harness electricity from it so this is

actually really cool because this is how

batteries work so starting with the

galvanic or voltaic cell the most

important thing you need to know about

this is that it is a spontaneous process

and it produces electrical energy and we

can harness this electrical energy and

where is the electrical energy coming

from it's coming from the chemical

energy from the redox reaction that's

occurring here so the redox reaction is

occurring between iron and copper and

the setup here looks fairly complicated

but the good news is there's just a few

important things you have to know about

it so over here we have a molten

solution of iron and then a solid block

of iron and then over here we have a

molten solution of copper and then a

solid block of copper and these blocks

are connected by a wire here along which

electrons can travel and this battery

here is facilitating the transfer of

electrons another important thing you

should know is that these solutions are

connected by what's known as a salt

bridge and this essentially completes

the electrical circuit so you should

remember from redox reactions oil-rig

oxidation is losing electrons and

reduction is gaining electrons and that

should remind you that over here since

we're losing electrons from this

solution the iron solution is being

oxidized and then over here since we're

gaining electrons in the copper solution

the copper solution is being reduced

another cool thing you can remember is

an ox red cat and this reminds you that

where oxidation is occurring this is

always where the anode is and where

reduction is occurring that's always

the cathode is so how did we know that

this was going to be spontaneous well it

turns out each element has a natural

tendency to want to either gain

electrons or donate electrons and we can

quantify that with what's called the

standard reduction potential or II with

the little degree symbol or sometimes

known as Enoch so you can see here that

I've written out the standard reduction

potentials for copper and iron and they

have a certain voltage associated with

them and you can see here that I wrote

when a naught is positive Delta G is

negative and when we know Delta G is

negative we know it must be a

spontaneous reaction so since the II not

for copper is positive that means copper

must spontaneously want to accept

electrons and get reduced

but since the e nought for for iron is

negative that means this is a non

spontaneous reaction Delta G would be

greater than zero so iron does not want

to gain electrons it does not want to

become reduced it actually wants to do

the opposite and wants to get oxidized

it wants to give off electrons so in

this case when the iron gets to give off

electrons like it wants and the copper

gets to accept electrons like it wants

then everyone's happy the reaction is

spontaneous it happens without any

intervention and it actually produces

energy so again this converts chemical

energy into electrical energy in other

words it collects electrical energy from

a spontaneous chemical reaction and this

is like using a battery to power a

device like having your phone in your

pocket a mobile phone that has a mobile

battery that powers your phone wherever

you go so the important thing you need

to know about these calculations is that

II not is always the standard reduction

so notice even though I've written out

an oxidation reaction here because

that's what's happening you have to use

the e nought for ions reduction and

similarly we use the e nought for

coppers reduction and that's actually

what happens copper is getting reduced

so the equation

to determine e-cell or the overall

reduction potential of this entire

system is right here ESL equals the e

not of the cathode minus the II not of

the anode so here the cathode was 0.34

volts positive 0.34 volts and you can

just find this on a standard reduction

table and the e nought for iron was

negative point for 4 volts and again you

can find that on a table so the cathode

minus the anode minus a negative means

this is going to be plus point 4 4 we

get the overall es L is positive 0.78

volts and again when es L is positive or

greater than zero it is spontaneous

Delta G is less than zero okay moving on

to the electrolytic cell so this is a

non spontaneous process it needs

electrical energy to occur so this redox

reaction doesn't naturally want to

happen we have to power it this is

analogous to charging up a rechargeable

battery from a wall outlet you know your

phone is not going to charge itself you

have to plug it in and obtain the

electrical energy to drive this non

spontaneous chemical reaction so notice

how we have the same setup we had before

except there's one important difference

the flow of electrons is reversed now

it's going from the copper to the iron

and remember iron has a natural tendency

to not want to be reduced it wants to be

oxidized it wants to go in the opposite

direction because it has a negative V

naught copper on the other hand has a

positive voltage associated with its

reduction so it loves to get electrons

it loves to be reduced this is positive

that means it's negative Delta G and

thus it's spontaneous

so naturally the electrons want to go to

the copper iron wants to give them up

and the copper wants to accept them

that's why this is non spontaneous so

notice how reduction is still at the

cathode and oxidation is still at the

anode that's never going to change

but now the copper is the anode and the

iron is the cathode so using the same

process we did to obtain this e cell we

can do that over here so

again II not 4-iron is going to be

negative 0.44 volts because we always

use the standard reduction potential

table to find that negative 0.4 4 volts

and again the e nought for copper even

though it's being oxidized this time we

still use its reduction potential

standard reduction potential for its

anat and we get positive 0.34 volts so

for our es el equation e cathode - anode

this time our cathode is 0.44 volts and

negative 0.44 and then we subtract e

anode so minus 0.3 4 and we get negative

0.78 volts that means e cell is negative

it's not greater than 0 it's negative

therefore Delta G is greater than zero

so this is non spontaneous this does not

want to happen that's why now this

battery is doing work it's actually

powering this entire process so this

process converts electrical energy into

chemical energy in other words it uses

electrical energy to drive a non

spontaneous chemical reaction to occur

ok since I talked a little bit about

standard reduction potentials I want to

go through this chart a little bit

because it can be confusing so this is

an abbreviated chart I don't have all of

the elements written up here but I just

want to illustrate the concepts and how

you read this chart so standard

reduction potentials essentially tell

you the e naught or the voltage that's

given off when a certain element is

reduced at 25 degrees Celsius and this

entire chart is based around the

reduction of H+ or protons chemists

arbitrarily chose the reduction of H+ to

have a voltage of zero or an e nought of

zero and this is in particular a 1 molar

solution of H+ and of course that means

the pH is zero so everything is based

around this benchmark we know that the

reduction of protons is associated with

a zero e naught or zero voltage and then

everything is built off of that so as

you go higher you can see that the e

naught gets higher it becomes more

positive and that means it be

comes more favorable remember II not

greater than zero means Delta G less

than 0 which means it's spontaneous this

means the reaction is spontaneous as

written in the forward direction and if

these reactions are spontaneous these

are all reduction potentials so that

means these things are very good at

getting reduced so I wrote that here

they're better at being reduced as you

go higher and that means they're better

oxidizing agents or stronger oxidants

remember an oxidant is an oxidizing

agent it accepts electrons in other

words it gets reduced and it oxidizes

something else oxidation is losing so it

it takes the electrons from something

else and uses it to become reduced so as

you go down the e nought gets negative

and it becomes more and more negative

the further you go down so this means

that II not is less than zero the

reaction is spontaneous in the reverse

direction so when e naught is less than

zero Delta G is greater than zero in

other words this stuff is non

spontaneous this doesn't want to happen

iron doesn't like to accept electrons

lithium doesn't like to accept electrons

they would rather go the reverse

direction in the spontaneous direction

and become oxidized or lose electrons so

that's why I wrote that here these are

better at being oxidized the lower you

go because they want to go the reverse

direction and thus they are weaker

oxidants they're weaker oxidizing agents

they don't want to accept electrons and

get reduced they would rather be

reductants they're stronger reductants

because they're better at donating their

electrons to something else they donate

their electrons and want to become

oxidized so as you go higher up here

again these are weaker reductants these

are up here above the hydrogen these are

better at accepting electrons these are

better at donating electrons and these

definitions here are sort of confusing

but just try to keep in mind if

something gets oxidized

it's a reducing agent if something gets

reduced it's an oxidizing agent an

oxidant means oxidizing agent reductant

means reducing agent and there's also

rare case

we're not equals zero the reaction is at

equilibrium at standard conditions and

this is exactly the case with hydrogen

and remember our equation here in order

to find the e naught or the standard

reduction potential of an entire

electrochemical cell you do a cathode

minus e a node so another important

thing you should know is that voltage is

in units joules per Coulomb and a

Coulomb is amps times seconds which is

essentially a measure of how much charge

is going through a wire okay so let me

ask you a question what's a better

reductant lithium solid or h2 gas well

of course lithium right because as we go

down the reductants gets stronger we get

better at donating our electrons

remember this is negative so it's non

spontaneous in the forward direction but

it is spontaneous in the reverse

direction thus lithium loves to donate

electrons it loves to become oxidized

and reduce something else thus it's a

strong reductant okay what's a better

oxidant F to gas or h plus 2 h plus well

of course the F 2 right it's a stronger

oxidant because of this positive een on

it it is spontaneous in the forward

direction it loves to become reduced and

to gain electrons and thus to oxidize

something else it's taking the electrons

from something else and oxidizing it so

it can become reduced so it's confusing

but I promise if you go through it

enough times in your head it'll

eventually sink in please keep looking

at this chart and memorize these

definitions down here and really make

sure you've got it straight so I really

hope this video helped you guys out and

I'll see you in the next one