- [Voiceover] When you're dealing with a voltaic cell,
it's important to relate the potential of the cell
to the free energy of the redox reaction,
and here's the equation that relates free energy
to the cell potential.
So, delta G,
delta G is the change in free energy
and we know, for a spontaneous reaction,
delta G is negative,
so let me go ahead and write that down here,
so delta G is negative for our spontaneous
redox reaction that we have
in our voltaic cell,
and we're going to calculate the value for delta G
at the end of this video.
E, over here, this represents our cell potential
or our cell voltage,
and this is easy to measure,
just hook up a volt meter,
and for this cell, the cell potential is
positive 1.10 volts,
so very easy to measure the voltage
associated with a voltaic cell.
Next, the n,
the n represents the moles of electrons
that are transferred in the redox reaction.
In this example, we're talking about two moles
of electrons are transferred in our redox reaction,
and finally, let's talk about F,
which represents Faraday's constant
and Faraday's constant is the magnitude of charge
that's carried by one mole of electrons,
so we can calculate Faraday's constant,
let's go ahead and do that up here,
so one electron has a charge of
1.6 times 10 to the negative 19 coulombs,
so coulombs is the unit for charge,
so let's go ahead and write that.
1.6,
actually it's 1.602 times 10 to the negative 19
coulombs for every one electron.
So one electron has that charge.
If we want to find
the magnitutde of charge carried by one mole
of electrons, we would need to multiply
by Avogadro's number,
so if we multiply by Avogadro's number,
which we know is 6.022
times 10 to the 23rd.
That's how many electrons there are in one mole
of electrons.
So that's how many electrons in one mole of electrons.
So if you multiply these out,
you're going to end up with charge per one mole of electron.
This is 96,472.
The units would be, this is coulombs
per mole of electrons.
So coulombs per mole.
And if you do a more careful calculation,
you'll get
96,485
coulombs per mole,
and so sometimes you'll see textbooks use this number
for different calculations.
Most of the time, you can just round this
to 96,500,
so 96,500 coulombs per mole,
and that's good enough for almost all the calculations
that you will do.
So that explains each term of our equation here.
All right, so this cell potential that we talked about,
1.10 volts,
this is actually the standard cell potential,
which is the voltage measured
when the cell operates under standard conditions,
which is defined as all of your solids
are in pure form,
and that's the case for us here,
because we're dealing with zinc metal,
so that's a solid in its pure form,
and copper metal, a solid in its pure form.
Also defined as the solutions are at one molar concentration
so we have a one molar concentration solution
of zinc sulfate, which gives us one molar concentration
of zinc two plus ions in solution,
and a one molar concentration
of copper sulfate, which gives us
one molar concentration of copper two plus
ions in solution,
and our temperature is 25 degrees,
so this is 25 degrees C,
so if those are your conditions,
and you hook up a volt meter,
this is the voltage that you will get.
So we can actually modify the equation that we've talked
about, by adding in a superscript here.
So we could write E zero
instead of just E, and E zero is our standard
cell potential.
It just means that everything is under the conditions,
under standard conditions,
and then this would be delta G zero,
so that's delta G zero now,
the standard change in free energy.
So let's go ahead and solve for delta G zero.
We already know it's going to negative
because this is a spontaneous redox reaction,
but let's go ahead and plug in everything,
so let's get a little bit more room down here.
Let's plug in our numbers,
so we're trying to solve for delta G zero,
the standard change
in free energy,
so we have a negative sign here.
I'll talk about what that means a little bit later.
So, next, we have n.
Remember what n represented here,
so n is the moles of electrons
that are transferred in our redox reaction
which would be two,
so this would be two moles of electrons
and put these in parentheses here.
Next, we have Faraday's constant, F,
and so we've already seen that we can use
96,500 coulombs per mole
for Faraday's constant,
so this is 96,500
coulombs per mole.
Next, we plug in the value for our standard cell
potential, so E zero is 1.10 volts
for this voltaic cell,
and when we're thinking about units,
so a volt is how many joules per coulomb?
So, instead of plugging in volts here,
we're going to plug in joules over coulomb,
so our units will work out properly,
so this is positive 1.10
joules per coulomb.
All right, let's do this math.
So let's look at our units first,
and let's see what cancels out here,
so we, our moles cancels out,
our charge, coulombs, cancels out
to give us joules for our answer.
So let's go ahead and calculate how many joules,
so let's do the calculation,
so we have two times 96,500
and then we're going to multiply that by
1.10,
so that would give us,
this would give us
212,300 joules.
So, remember this was in joules,
and let's go ahead and convert that to kilojoules.
So 212,000 joules would be 212 kilojoules,
and remember we have a negative sign here,
so don't forget about this negative sign.
This would be negative 212
kilojoules,
for our value for delta G zero.
So, the standard change in free energy
for this voltaic cell
is negative 212 kilojoules,
and we already know, right,
that when delta G is negative,
that's a spontaneous reaction,
so this is a spontaneous redox reaction here.
Notice that delta G zero and E zero
have opposite signs, right?
We had a positive sign for our voltage,
and we ended up with a negative sign for delta G,
and that's why we have this negative in our equation here,
so a spontaneous reaction in a voltaic cell
has a positive cell potential,
so a positive value for the voltage,
but a negative change in free energy
because that indicates a spontaneous reaction.