CHEMISTRY

CHEMISTRY SKILLS• Organised

• Proactive

• Independent

• Efficient

• Abstract-thinker

• Resilience

• Problem-solver

• Logical

• Meticulous

• Conscientious

• Open-minded

• Critical thinker

• Methodical

• Mathematical

OCR Chemistry B (Salters)

•Developing Fuels

• Elements of Life

• Elements of the Sea

•Ozone Story

•What’s in a Medicine?

• The Chemical Industry

•Oceans

•Polymers and Life

•Developing Metals

•Colour by Design

6:4 split

Practical Endorsement - PAGS • Pass or Fail for PAGs

• 12 skill areas –complete one practical for each one

PAG EXPECTATION•Write up each practical

•Analysis/calculations/evaluation for each one

•Extension questions for each one

ASSESSMENT

•End of unit test for each unit (so x10!)

•Mocks regularly throughout the year (this year Y12/Y13 had 3 each)

•PAGs

JOBS Geneticist

Biochemist

Medical engineering

Chemical engineering

Law

Medicine and nursing

Volcanologist

Toxicologist

Try A (Very Condensed) Lesson!

Lesson Topic:Identifying Compounds Using Mass

Spectrometry and Infrared Spectroscopy

To note…•We usually do these topics (mass spectrometry and

infrared spectroscopy) near the end of Y12…

• Students would get 2 lessons on each of them, at least!

• This is a crash course in them… it is tricky stuff! But really good for problem solving skills!

Technique 1 - Mass Spectrometry You have a sample of an organic substance. You have no idea what is it. How do you go about identifying it?Answer: Mass spectroscopy!Mass spectroscopy is a technique used to identify unknown substances.

How does it work? You put your unknown sample in a machine called a mass spectrometer. The machine sends the sample molecules flying through it at speed, and in the process breaks your molecule down into pieces called fragments. The fragments whizz through a long tube, and hit a detector at the end. The detector records how heavy the fragments are, and their abundance/intensity.

The Graph: It then produces a graph like the one on the right. This graph is called a mass spectrum. The mass of your pieces is on the x-axis (m/z) and the abundance/intensity of them (how much there was) is recorded on the y-axis. We are interested in knowing the MASSES of them!

Some of your sample will probably reach the end completely in tact, but lots of it will be broken down into pieces!

Ethanoic Acid

Mass Spectroscopy Example – Ethanoic Acid

Ethanoic Acid

THE MOLECULAR ION PEAKSome of your molecule usually makes it through the machine in tact. This is the peak furthest to the right – the heaviest mass you will get, called the MOLECULAR ION PEAK.

This means that it’s mass detected by the detector is just its relative formula mass (RFM). Remember this from GCSE C3? Ethanoic acid has the formula C2H4O2 - 2 x carbon atoms, 4 x hydrogen atoms and 2 x oxygen atoms. C has a mass of 12, H a mass of 1, and O a mass of 16.Therefore ethanoic acid has an RFM of 60.12 + 12 + 1 + 1 + 1 + 1 + 16 + 16 = 60

If you look on the spectrum for ethanoic acid, we can clearly see that heaviest mass is 60 – this is where whole molecules of ethanoic acid have passed through without being broken into fragments!All peaks to the left of this are fragments of ethanoic acid – bits of the molecule that have broken off in the machine.

Think I have forgotten about this sneaky fella here at 61?

Usually, the molecular ion peak has a tiny peak +1 next to it. This is just where some of the ethanoic acid molecules will have had the isoptope carbon-13, rather than carbon-12 in them. Remember an isotope is the same element, just with an extra neutron! 1% of all carbon atoms are carbon-13, so it is really common to get this tiny peak next door, just where some of the molecules had a carbon-13 instead of a carbon-12! You can ignore it! The important molecular ion peak, that tells you the RFM of the molecule, will always be the big fella to the left!

So what about those fragment peaks?

Ethanoic Acid

As have said on the previous slide, the peak at 60 is for whole ethanoic acid molecules that went through the machine, without breaking up into fragments. Remember this is called the MOLECULAR ION PEAK.

The peak at 61 is also for whole ethanoic acid molecules, which just incorporated some C-13 isotope, rather than C-12. Ignore it!

All the peaks to the left are fragments of ethanoic acid that have broken off. Here are some examples!

Fragment with a mass of 15 = CH3

broken off the end.CH3 = 12 + 1 + 1 + 1

Fragment with a mass of 17 = OH broken off the end.OH = 1 + 16

Fragment with a mass of 45 = COOH COOH = 12 + 16 + 16 + 1

Fragment mass at 43 = CH3CO CH3CO = 12 + 1 + 1 + 1 + 12 + 16

You don’t need to identify EVERY peak – you’d be there all day! But we can see enough proof

from these examples that there are bits of ethanoic acid there, judging by the masses we

have present! Lots of the tiny peaks next to the big peaks are just where some C-13 is present, rather than

C-12.

Try it… Is this mass spectrum for butene or butanol?

Butanol

Butene

Answer…Butene!How can we tell it is for butene and not butanol…?• Butanol has a mass of 74. Butene has a mass of 56.• The molecular ion peak here is for 56 – it doesn’t go

up to 74! This suggests it is probably butene…• We can’t just go by the molecular ion peak though –

what if none of the whole molecule made it through and it was all fragmented (This can happen!)?! We need fragment proof too…

• Fragment at 15 is for CH3 – but they both have a CH3

that can break off so this doesn’t help!• The fragment 29 is for CH3CH2 – but they both have

a CH3CH2 that can break off, so this doesn’t help either!

• What DOES help is that butanol would likely have a peak for 17 for OH. But there is NO peak at 17, as you would expect.

• There is also NOT a fragment at 31 for CH2OH from the end of butanol, as we would expect. So we can deduce it is butene, because peaks we would expect for butanol are NOT there, AND the highest mass of a peak we have, is 56, the RFM of butene.

Butanol Butene

Technique 2 – Infrared Spectroscopy

Infrared spectroscopy is a different technique, also used to help identify unknown molecules. It tells you which type of BONDS are present. E.g. do you have an O-H bond, a C-O bond, an O-N bond, etc.! It will NOT tell you anything about mass, like mass spectrometry!

How does it work? Your sample molecule (unknown!) goes into the infrared spectrometer. Infrared light is passed through it. Certain bonds in your molecule will absorb certain frequencies of infrared light. The rest of the IR light will pass through. The detector at the end detects which frequencies of light were passed through (transmitted) and which frequencies of light were absorbed.

Every time light is absorbed by a certain type of bond, it causes a big downwards peak on the infrared spectrum graph (see right!).

If the line is up here, it means that that wavenumber of IR light passed straight through your

sample and was NOT absorbed.

Every time there is a downwards dip (called a peak or trough) this means a bond absorbed some of that IR light at

that particular wavenumber! Hence the transmittance is below 100% - some of the light got absorbed by your sample

instead!

These downward peaks are the bit we are interested in!

The x-axis shows you the wavenumber of IR light. (Don’t worry about what wavenumber is – just use

the number!)

The y-axis tells you the transmittance – how much

light passed through.

Any peaks below 1500cm-

1 generally aren’t useful. This is called

the fingerprint region, if you wish to look this up. We

tend to ignore it, most of the

time!

An example with ethanoic acid again!How can I identify that this molecule is ethanoic acid from its infrared spectrum?There is a broad (wide) peak/dip at around 3000cm-1. We use our data (which you get in an A level exam!) to find out which bond this is for. See the table below!

A broad peak around 3000cm-1 suggests an O-H in a carboxylic acid is present. This means that the O-H bond has absorbed IR with a wavenumber of 3000cm-1!

A peak at just over 1700cm-1 suggests a C=O bond in a carboxylic acid too.

Try it… is this infrared spectrum for butene or butanol?

Butanol Butene

Butanol Butene

Absorbance at approx. 2850-2950cm-1 – suggests C-H bonds – not useful to us as they both have these!

Broad (wide) absorbance at approx. 3200-3500cm-1 – suggests O-H bond present – BINGO! Must be butanol! There would be no absorbance here if it were for butene, as butene does not have an O-H bond!

Answer…Butanol!

Summary

Mass Spectrometry

• Tells you about MASS! (In general, don’t worry about the height of the peak/abundance of the fragment – worry about the mass on the x-axis!)

• Molecular ion peak = RFM of molecule = heaviest mass = furthest to the right (not always present, but usually is!)

• Molecular ion peak + 1 (sometimes there) = Just some carbon-13 present, ignore it = possible tiny peak furthest to the right, +1 over the big molecular ion peak

• Fragment peaks = pieces of molecule that have broken off. Work out possible bits of molecule using the masses!

Infrared Spectroscopy

• Tells you about BONDS/FUNCTIONAL GROUPS we have present!

• Peaks = where certain bonds present have absorbed IR light of that particular wavenumber.

• Use data table to read which bonds are present in our unknown molecule. Identify is an alcohol, alkene, etc.!

Ideally…

• Use both techniques TOGETHER – put your unknown sample through both machines, and get a mass spectrum AND an infrared spectrum.

• Infrared will tell you the type of molecule you have, by the bonds present – alcohol, alkene, alkane, carboxylic acid, etc.

• Mass spectrum will tell you the RFM of the molecule, and confirm bits of molecule with fragment masses.

Feel free to have a little read about each of them here…Mass spectrometry –Infrared -

Up for a tough challenge?...

Match the molecule to its mass spectrum and its infrared spectrum. (File: Mass spec and infrared molecule match – Muddled images). • There are 8 molecules to match with their mass spectra and infrared spectra.

• The images are NOT correctly matched on the file!

• You can either cut out the images to move them round to match them (what I would have given you in class), or look at them on the file and write down the name of the molecule with the two matching letters.

You will need to use the table of data for infrared spectra absorptions. You can either use the table on the previous slide, or find a copy on the datasheet (File: Datasheet).

The answers are also attached to see if you are correct! (File: Y12 Induction - Chemistry - Mass spec and infrared molecule match – Answers).

Hints: What to look out for?

•The mass peak•The fragment

masses (can the fragments of that mass be made?)•The bonds

absorbing IR –what functional groups are there?

For mass spectrometry -masses of common atoms• Carbon = 12• Hydrogen = 1• Oxygen = 16• Nitrogen = 14

Summer Work! Yay!

Lots of questionsWrite on lined

paperLook up answers!

Due in for September