Hey everyone and welcome back to another HSC Chemistry blog!
This week’s blog will provide insights into module 8 particularly how to analyse mass spectrometry, infrared spectroscopy, proton, and carbon-13 NMR.
Analysing and interpreting all the different spectrums have become very common in the new syllabus. They can appear in multiple choice, short answer, and long response.
Did you know in the 2020 HSC paper, 11 marks were dedicated to analysis of mass spectrometry, infrared spectroscopy, proton, and carbon-13 NMR. This means to maximise your marks you should make sure you know how to analyse each spectrum and be able to write correctly structured responses as this is often where students normally lose their marks.
So, let’s get started with Mass Spectrometry!
When you analyse each peak, it is important that you write the following points in your response:
1. The specific peak and what it represents.
2. What the peak tells us about the organic compound.
For example, if I was to analyse two peaks from this mass spectrum of a carboxylic acid, I would write:
The peak at m/z = 74 is the molecular ion which represents the molar mass of the organic compound. The peak at m/z = 45 represents COOH+. This means that there is a carboxyl group present in the organic compound. Since there is a carboxyl group present and the molar mass is 74, the organic compound is propanoic acid.
Now let’s move onto Infrared Spectroscopy!
Again, it’s very important to structure your responses correctly. To do this, I would highly recommend including both the following points for each absorption band:
1. The intensity of the absorption band (strong, medium, weak, broad, or narrow)
2. The wave number of the absorption band and its corresponding bond (refer to the data in your formula sheet)
For example, if I was given an infrared spectrum for propanoic acid, this is how I would describe the absorption bands:
There is a strong, narrow absorption band at approximately 1700cm-1 which represents the carbonyl bond (C = O). There is also a broad absorption band at approximately 2500 – 3000 cm-1 which represents the hydroxyl group (O – H) for acids.
Time for Carbon-13 NMR!
To structure your response correctly, I would recommend including the following two points:
1. The number of peaks present and hence the number of carbon environments in the organic compound.
2. The type of carbon represented at a specific chemical shift (ppm) – refer to the data in your formula sheet.
For example, if I was given a carbon-13 NMR for propanoic acid, this is how I would incorporate these two points into a response:
Since there are three peaks present, there are three carbon environments in the organic compound. The peak at 180ppm represents the carbonyl bond for acids.
Lastly, we have the Proton NMR:
I always found the Proton NMR the most difficult to analyse because it had so many components! However, once I was able to break down each of the components using a table, and understood how to structure the responses correctly, it became one of my strengths!
I would definitely recommend creating a table like the one I have below if you find it hard to analyse proton NMR spectrums!
Now just so it’s super clear as to how each characteristic is analysed here is an example for ethanol:
[number of peaks] The proton NMR has three peaks which means there are three proton environments.
[chemical shift] The hydrogen atoms attached to the -CH3 are furthest away from the electronegative oxygen which means that it would experience the most shielding and hence be closest to TMS. The hydrogen atom on the -OH is unshielded by the oxygen and hence is further away from TMS.
[intensity] There are 3 hydrogen atoms that share the same -CH3 environment resulting in the highest intensity, followed by 2 hydrogens in CH2 and the single hydrogen in -OH.
[splitting] The neighbouring atom of the CH3 contains 2 hydrogens. Hence, by the n + 1 rule, there will be a triplet (split of 3) for that environment. The neighbouring atom of the CH2 contains 3 hydrogens so by the n + 1 rule there will be a quartet (split of 4) for that environment. The neighbouring atom of the OH contains 0 hydrogens and hence by the n + 1 rule there will be a singlet (1 split) signal.
What should you do this week?
Start the week off with an hourly timetable for each day in the week. Make sure to include regular breaks!
Practise analysing and writing responses on questions which cover mass spectrometry, infrared spectroscopy, proton, and carbon-13 NMR.
I highly recommend going through the mistakes that you may have made in past practise papers, assessment tasks, homework, or your trail exam. Re-attempt these questions and find questions of similar nature to ensure you are comfortable with these types of questions just in case they appear in the HSC. I did this and found it very helpful as it made me feel more prepared because I was able to target my weaknesses in Chemistry and convert them into my strengths.
Complete at least 2 HSC chemistry papers this week in timed conditions.
If there is any content that you are unsure about, spend some time revising it and complete questions from past HSC papers which target those concepts.
Quote of the week:
“Never let the fear of striking out keep you from playing the game” – Babe Ruth
Well, that’s it for this week’s blog!
Good luck studying!