VCE Chemistry Question Thread

[alchemy]

How could I draw the H NMR structure for Hexane indicating peak splitting?
Just has to be approximate (i.e. peak heights don't matter as we aren't actually analysing a sample).
The middle CH2 is bothering me...

[sunshine98]

Can someone help me with spectroscopy? I am just struggling with the definition as a whole, specifically like what is electromagnetic radiation and how does it involve chemistry?

Thanks

The electromagnetic spectrum is a spectrum which contains all possible types of electromagnetic radiation ordered by wavelength. But what is electromagnetic radiation ?It is basically waves that are produced by the vibration of charged particles ( electrons and protons and ions), which have both electrical and magnetic properties. Each type of radiation has a specific wavelength , frequency and energy.
Wavelength measures the distance in meters, between two adjacent crests.
Whereas , frequency measures how many waves per second that pass a particular point.
Their is a relationship between wavelength and frequency - they are inversely proportional. This is because the  longer the wavelength the smaller the frequency , the shorter the wavelength , the higher the frequency.
And their is a relationship between frequency and energy- they are directly proportional.
So this is all you need to know (well at least how much I've been told to know). Now as for spectroscopy.
Spectroscopy is simply  the interaction between electromagnetic radiation and matter. Electromagnetic radiation has varying effects on a substance (this is b/c of the different energies). Basically in spectroscopy , chemists place atoms and molecules in the presence of a specific form of electromagnetic radiation and see what happens. Obviously these atoms and molecules undergo a change. For atoms this change is the movement of electrons from lower energy level to higher energy levels , whilst for molecules we observe the movement of to higher vibrational  , rotational and nuclear spin energy levels.

Hope this is helpful

[jessss0407]

hey guys!

i just had a quick question - why are some hydrocarbons gases whereas others are liquids? is there a specific reason in relation to the structure?

thanks

[IndefatigableLover]

hey guys!

i just had a quick question - why are some hydrocarbons gases whereas others are liquids? is there a specific reason in relation to the structure?

thanks

It's to do with the length of the overall structure! As you increase the length of your hydrocarbon, you also increase the affinity to attract the molecules between each other due to dispersion forces acting upon it! These forces accumulate as you lengthen out your hydrocarbon which results in an increase in boiling temperature (the energy needed to break the bonds between the individual molecules). As a result, the hydrocarbons which are shorter in length will generally be gaseous, medium length hydrocarbons with 5 carbons to 12 carbons are generally liquid and above are solid (this is at room temperature)

[warya]

How could I draw the H NMR structure for Hexane indicating peak splitting?
Just has to be approximate (i.e. peak heights don't matter as we aren't actually analysing a sample).
The middle CH2 is bothering me...

add all adjacent H's and +1

[just_jordan]

I just have a few questions because we have a SAC on carbohydrates tomorrow:

Why does Starch turn blue/black when iodine is added to it unlike reduced sugars and glucose?
Why does the addition of benedicts solution in glucose and maltose cause it to turn red but no change in sucrose?

[Redoxify]

How do you work out the initial concentration of a fertiliser solution that has been diluted 10 times in the form of mgL^-1,

[Sundal]

Are 'Low-resolution NMR' and 'High-resolution NMR' subsets of all types of NMR, or do they only exist for proton NMR?
Lastly, why does the number of nucleons have to be odd, for NMR to work?

Thanks guys.

[alchemy]

add all adjacent H's and +1

Yes, but you see this is kind of a curve ball case....attempt the question yourself and you'll see why after a while....
Anyways, I asked the teacher today and figured out it was a case of overlapping peaks (or just peaks very close to each other, if that's how to describe them better).

[keltingmeith]

Ignored questions I don't actually know the answer to, which hopefully is an indication of what you /actually/ have to know for VCE. (but defs look deeper if you're so inclined)

Are 'Low-resolution NMR' and 'High-resolution NMR' subsets of all types of NMR, or do they only exist for proton NMR?

Low/high-resolution only refers to the frequency that the NMR runs at - so yes, it exists for all NMR ever. To memory, I think VCE passes off that low-res has no splitting and high-res has splitting? Which is kinda true, fairly stupid (I don't think anybody uses THAT low-res anymore, the lowest I've ever found only follows the n+1 rule as opposed to previous stated earlier that don't relate to VCE), but I think VCAA tend to only use "high-res" for proton NMR. Definitely exists for carbon-13, though.

How do you work out the initial concentration of a fertiliser solution that has been diluted 10 times in the form of mgL^-1,

C1V1=C2V2

I just have a few questions because we have a SAC on carbohydrates tomorrow:

Why does Starch turn blue/black when iodine is added to it unlike reduced sugars and glucose?

The starch and iodine form a metal complex, which is coloured for reasons beyond VCE (basically, at this point, you don't have to accept why something is coloured. If you really want, in general, metal complexes are coloured, organic mixtures aren't. This definitely doesn't always hold, though, just a general statement)

Why does the addition of benedicts solution in glucose and maltose cause it to turn red but no change in sucrose?

More statements about metal complexes and colours.

Both of these questions are beyond VCE, though - last I checked, you don't need to know how to test sugars.

[lzxnl]

How could I draw the H NMR structure for Hexane indicating peak splitting?
Just has to be approximate (i.e. peak heights don't matter as we aren't actually analysing a sample).
The middle CH2 is bothering me...

You're not meant to know how to draw one...and you're definitely not meant to know how the splitting works.

As an example, the CH2 on carbon 3 is split by the two non-identical CH2 groups either side of it. Each CH2 splits the peak into 3, so overall you'd have nine peaks for the carbon 3 CH2.

The starch and iodine form a metal complex, which is coloured for reasons beyond VCE (basically, at this point, you don't have to accept why something is coloured. If you really want, in general, metal complexes are coloured, organic mixtures aren't. This definitely doesn't always hold, though, just a general statement)

More statements about metal complexes and colours.

Both of these questions are beyond VCE, though - last I checked, you don't need to know how to test sugars.

I'm sure you've anticipated this response from me but I'm going to say it anyway.

Iodine forms a complex with starch but it's not a metal. Granted, however, it has metallic character because of how far down the periodic table it is. Its lone pairs are held less tightly by the iodine nucleus and thus are available for complexation. Of course, disregard this entire paragraph for the purposes of VCE.

Are 'Low-resolution NMR' and 'High-resolution NMR' subsets of all types of NMR, or do they only exist for proton NMR?
Lastly, why does the number of nucleons have to be odd, for NMR to work?

Thanks guys.

Not true. NMR can work on nuclei with even numbers of nucleons, like nitrogen-14. It's just that you need an atom that has a nonzero nuclear spin. And, well, for lots of atoms with even numbers of nucleons, the proton and neutron spins cancel. I'm pretty hopeless at nuclear physics so I can't predict which atoms will have what spin

[KingDrogba]

So we made Aspirin for our sac in class, my teacher said that the Ethanoic Anhydride we used acts as a reactant, but also a solvent? Can someone explain how and why it is a solvent???

Real cheers xx

[cosine]

Can someone help me with Atomic Absorption Spectroscopy, I just don't get what it is and why it's used for?

[Redoxify]

Can someone help me with Atomic Absorption Spectroscopy, I just don't get what it is and why it's used for?

Hi cosine,
What is it?
AAS, or atomic absorption spectroscopy is a type of analysis/spectroscopy has three main parts, the cathode ray lamp, the atomiser and the monochromator/detector
The cathode ray lamp has a small piece of the analyte(thing your analysing) present, the metals in the lamp are excited by voltage, so the electrons bounce to the next shell, when they come back down from this high energy state they release energy in the form of light (sorta like the flame test), this light energy is shot out of the cathode ray lamp,
The atomiser is a flame that forms a gas form of the metal sample being tested, these floating metal gas particles pick up some of the light energy that was produced from the cathode ray lamp
finally the detector basically detects how much was absorbed
What is this used for?
It is basically used to determine the amount of absorption of light (from cathode ray lamp) in comparison to the concentration (sample in atomiser)
Hope this helped

[cosine]

Hi cosine,
What is it?
AAS, or atomic absorption spectroscopy is a type of analysis/spectroscopy has three main parts, the cathode ray lamp, the atomiser and the monochromator/detector
The cathode ray lamp has a small piece of the analyte(thing your analysing) present, the metals in the lamp are excited by voltage, so the electrons bounce to the next shell, when they come back down from this high energy state they release energy in the form of light (sorta like the flame test), this light energy is shot out of the cathode ray lamp,
The atomiser is a flame that forms a gas form of the metal sample being tested, these floating metal gas particles pick up some of the light energy that was produced from the cathode ray lamp
finally the detector basically detects how much was absorbed
What is this used for?
It is basically used to determine the amount of absorption of light (from cathode ray lamp) in comparison to the concentration (sample in atomiser)
Hope this helped

Thanks so much!

So how does the cathode lamp work? Is the light energy emitted as the electrons move to a lower energy level, hence emitting energy? Also if the same energy is required for the sample being analysed, how do we achieve this? Cheers!