Right, so I'm going to give you, well we're going to start out with an example,

of how you use these equations to do some calculations,

to do some manipulations that you'll need to do.

So, we have an example, here, of a UV photon of light.

So that, as we know, damages DNA, and it has a wavelength of 234 nanometers.

[COUGH] And you might be asked to calculate the frequency

of the photon in hertz and its energy in kilojoules per mol.

And electron volts, you also see [INAUDIBLE], but

you'll find that energy term clear enough, as well.

So, one thing we've introduced here that I haven't told you is hertz.

Does anybody know what hertz?

What's another unit for hertz?

>> [INAUDIBLE] >> Sorry?

>> [INAUDIBLE] >> Didn't hear that.

>> [INAUDIBLE] >> Yeah.

What's the [INAUDIBLE] Just press seconds.

Yeah. Okay, so hertz, I should write in here,

is sometime is written like that Hz.

I'm sure it was someone famous.

Some famous physicist.

So, they gave this unit his name.

All right, so I'm gonna do that for you.

So here, I've written out here, on this it was the same question.

So, to do this kind of question you want, you're given the wavelength and

you want to calculate the frequency and you want to calculate the energy.

So let's do, I can do it all.

We'll do the best of it so you have that

use this fundamental equation from Planck that E is equal to h nu and

my nus aren't that good, sometimes they look like vs but that should be a nu and

we also know that that's equal to h c overlap.

These are equations that you should have it in your head.

So, we know from that that nu therefore I wrote this down already is c over lambda.

So, therefore for our question there for nu is equal to c, which we all know.

If you don't remember it, you'll always be given it in an exam or a textbook.

It's a 2.998 by 10 to the 8.

And you should always, when you are doing a question like this,

you should always write down your units.

So, that's meters per second and we'll try and stick always to SI,

what we call SI units.

So, it's meters per second, and then you divide that by the wavelength.

[COUGH] And we're told the wavelength is 234 nanometers.

Well, we want our SI unit is meters, so we have to multiply that.

Does anybody know what nano is?

[INAUDIBLE] it's [INAUDIBLE] right, yeah.

So, that's 10 to the minus 9.

And that's meters, okay?

So, as you can see, if you always put down the units question of this so

now I cancel out the meters and I'm left with seconds minus one.

So, that's my frequency.

So if you plug that in and work it out,

you should get something like 1.28 by 10 to

the 15 seconds minus 1.

Or, I could have said set down this the same as obviously I've written,

instead of 15 seconds minus one, you put down hertz.

Okay, so there's 1.2, there's quite a lot of waves per second, isn't there?

10 to the 15 in that photon.

Next question, we're asked once we've worked out the frequency, and

now we also ask the energy in kilojoules per mole and in electron volts.

So, how would you do that?

So, now you use the Planck relation E is equal to h new.

Cause we've worked out the frequency and we know Planck's constant but

we don't know it.

It's in your textbook, or it's in the back of the exam paper.

[COUGH] So we're going to have 6.626 [COUGH]

by 10 to the minus 34, and the SI units for

blank's constant is joules seconds.

[COUGH] And let's put a bracket around that.

And you need to multiply that by the frequency and

we worked out the frequency up here as 1.28 by 10 to the 15,

so it's 1.28 by 10 to the 15 and that's seconds minus 1.

So, the seconds cancel so we're left with J or Joules.

And you put that in your calculator, and you're going to get 8.48 x 10 to the -19.

Joules.

Now, we were asked.

The question asked us.

We wanted our question, it was asked.

We wanted it in [COUGH] kilojoules per mole.

[COUGH] So, that's just for.

One units, so an E in kilojoules, well E in kilojoules,

let's do that first, is going to be 8.48.

By 10 to the minus 19,

by 10 to the minus 3.

Okay?

So, its a thousand Joules in a kilojoules.

So, that would be in kilojoules and then,

E in kilojoules per mol, how would you do that?

Anybody know?

There's some constant you multiply by.

We have a number of them in a mol.

So, it's 8.48 by 10 to minus 19 by 10

to minus 3 and all that's in kilojoules.

Always keep your units together.

And then you multiply that by, again you find this,

you don't have to remember Avogadro's number.

It's 10 to the 23.

And Avogadro's number, the units is mol to the minus one, it's per mole.

The number of atoms per mole, the number of molecules per mole.

So, it's mole to the minus one.

So.

If you worked that out, plug all these values into your calculator you get or

at least I get 510.8 kj per mol [INAUDIBLE] [COUGH] so

that's the way I want you to, you'll get loads of practice

in the workshop, you'll get practice doing this.

But, you should always write them out clearly, [INAUDIBLE] exam,

when you're doing a calculation like this, you write everything out.

And you write in your units.

Cause the easiest way to go along in a calculation is maybe in a calculator in at

the end if that's the wrong button or something.

But, if you've worked it out like this, then you're gonna get full marks.

Cause the marker can see whether he or she made a mistake in the end.

And it's also a good practice cause the key thing is to know how to quote units.

You should never give an answer without the units, unless this is dimensionous.

We'll be coming across in a few minutes something that is dimensionous, but you

should never supply an answer without the units because it doesn't mean anything.

Okay.

If it's units, it has to be supply.

Right the last bit then [COUGH] was the electron volts.

We haven't come across that, or maybe you've come across it already.

But you, again, if you look up your tables,

what you find is that one electron volt is equal to

1.602 by 10 to the -19 Joules.

[INAUDIBLE] Our one Joule, because

we want to convert from Joules above, is equal to 1 over that.

So it's 1 over 1.602 by 10 to the -19.

Electron volts.

Okay, so again that's the conversion that you have in your tables.

So, what we've worked out, we worked it out in Joules up here.

It's 8.48 by 10 to the minus.

19 Joules.

So, we need to divide that then by this figure

here to convert it to electron volts.

So therefore, E electron volts is equal to this 8.48 we got up here first.

8.48 by 10 to the minus 19.

You divide that by 1.602.

Times 10 to the minus 19 Joules,

and that's electron volts to minus one.

So, work out that in your calculator and

it's 5.3 electron volts.

Electron volts come across it quite honestly.

The energy required to move electrons through potentially one volt.

But, again you'll find that like to use electron volts so

you need to know what it is and

you need to know how to be able to convert from another energy value.

Okay, so that was just a short interlude.

On doing proper calculations.

Example of energy calculation

Introduction to Molecular Spectroscopy

Meet the Instructors