WJEC AS Chemistry CH1 May 22nd 2015

Woaaah, wait a second. I'm pretty sure that's more the case in chemistry. In physics, I can assure you that every single thing you learn, you can (and should!) understand. If you just learn formulas and use them without understanding how they work (the 'GCSE method' in A Level, you won't get too far.
Admittedly even in GCSE you could understand everything if you tried/had the maturity to try. I didn't XD! But I do in A Level, and it's so satisfying when it all clicks in.

There are a lot of things to just accept in Chemistry. But I get the feeling that when I memorise things in Chemistry, it has logic. For example the equations you learn, for me, I can imagine that the Number of moles divided by the volume gives the the concentration. I can picture it in my head.

Although things I can't picture are the mechanisms where you just have to 'blindfoldly' learn and accept them. CH1 is logic to me. CH2 is accepting everything you are told. But to be honest I still see logic in the mechanisms and I can understand why they work.

I had a bad experience in Physics in school. In the GCSE I had 88% UMS which isn't bad (A). I was 1 mark off an A*. But that was purely memorising the formulas and sticking numbers in. Nor did I, or will I ever imagine that Potential Difference is equal to Work Done divided by Time. It doesn't fit into my head in a logical way.

I very much agree with that last statement, since teachers always teach voltage (they don't even call it pd) as the 'push' of current, which is stupid because students never understand what it actually is. Understanding that it is the electrical energy converted into other forms of energy per coulomb of charge passing between two points sounds far harder than it actually is and gives students a much better understanding of circuits. And the same goes for current! I can see why you learnt physics formulaically at GCSE; it's taught really badly! But it's great when you actually understand it and it's as deep as in A Level.

Yeah, CH2 is a LOT of blindfolded memorisation...
On the other hand, in CH1 you can understand almost everything without too much difficulty!

Woah I just did that Jan 2012 paper and I think it was beautiful! Literally brilliant paper. I got 85% Raw, and 74% was an A.

That paper was a real confidence booster, I don't want it to give me false hope though!

Yeah sure, I made flash cards to memorise my reactions & conditions...

1) Reagents & conditions to convert a halogenoalkane into an alkene: Concentrated NaOH in hot Ethanol, heat under reflux

2) Reagents & conditions to convert a halogenoalkane into an alcohol: AQUEOUS NaOH and Heat [below 110 degrees C]

3) Reagents & conditions to convert an alkene into alcohol: 300 degrees C, 70 atm, and Phosphoric Acid as a catalyst (H3PO4)

4) Test to distinguish between alkene & alkane: Adding bromine water; in presence of an Alkene the solution will become colourless. In presence of Alkane, the solution will remain brown.

5) Reagents and conditions for conversion of alcohol into alkene: CONCENTRATED H2SO4, 170 Degrees C

6) Reagents and conditions to convert alcohol into an acid: Acidified Potassium dichromate; You will observe a colour change from Orange to Green

7) Conditions & reagents for CATALYTIC Cracking: 500 Degrees C, Low pressures, Zeolite catalyst

8) Conditions for THERMAL Cracking: 450-750 Degrees C, 70 atm.

9) Conditions for polymerisation of Ethene: 250 Degrees C, 2000 atm, traces of O2 to initiate.

10) 3 Catalysts that can be used in hydrogenation of an alkene into an alkane: Platinum or Palladium or Nickel

11) Test for a Carboxylic Acid, what would you observe: Adding Sodium Carbonate/Bicarbonate then testing for CO2 with lime water

Learn all that (along with your mechanisms) and that your CH2 done my friend. Well 90% done anyway.

The only things I struggle with in CH1 are emission spectra and ionisation energies. Problem is these are always 2-5 mark questions I feel.

Thank you,

emission spectra is easy and I have a feeling it'll come up on Friday. This is the main thing you need to remember...

When you apply energy to gaseous atom (Eg. Hydrogen), the electrons absorb the energy and become excited. Because they've absorbed energy, they can move to higher energy levels (shells). When they are at higher energy levels, they become unstable (because the more energy an electron gains, the more unstable it gets) and fall back down to lower energy levels (shells), and when they fall back down, the energy they absorbed earlier is released, but this time they release a quantum the form of light (quantum just means a packet or a certain amount). This is how lines show at a spectrum. Different lines on the spectrum means electrons falling FROM different energy levels. The reason that the lines are discrete and do not converge is because the energy levels are Quantised. This just means that the energy levels are fixed. And because they are fixed, electrons can only fall from an energy level to another, they cant fall from inbetween 2 energy levels. You can picture a cup on a table, the cup can only fall from the table onto the floor. The cup CANT fall from beneath the table nor above, because it ONLY exists on the table. The same thing is with electrons, they can ONLY exist INSIDE shells, not between shells.

Now when an electron from the first shell (n=1) gains enough energy to move to n=infinity. The electron becomes no longer under influence of the nuclear, and so the Hydrogen atom is ionised. So this allows us to calculate the first ionisation energy of Hydrogen.

I tried to make it as clear as possible although it can be tricky to explain, I hope that helps?

Yeah thanks that helps. What the difference between emission and absorption spectra then ?

Absorption Spectra,

It's when white light shines through gaseous atoms, so photons (a photon is basically a quantum of light) of a particular energy will be absorbed by an atom causing an electron to move from a lower energy level to a higher one. So this means that the light of a certain frequency (colour of light) corresponding to the quantum of light absorbed will be removed. So against a rainbow background there will be dark lines in certain areas. The positions of these dark lines correspond to the same colour released by the emission spectrum. (if that makes any sense). So if an emission spectrum shows the colour red. Then on an absorption spectrum, the colour red will be replaced by a dark line as it has been absorbed.

It annoys me that there are 5 mark questions for knowing how to prepare 250cm3 of copper sulfate solution. I mean I know you can argue it's important to know but it's not really chemistry in my opinion!

I get really mixed up and confused with ionisation energies.
I understand the basics, but could you explain nuclear charge in more detail? That's the one that gets me.

Can somebody explain how the ionisation energy ties in with the emission spectrum? That thing with the Lyman series... I don't get it

Basically learn this definition:

Ionisation Energy: The energy required to remove one MOLE of electrons, from one MOLE of gaseous atom.

The reason it's one mole, is because it's very very difficult to calculate the energy to remove one single electron from one single atom, they are incredibly tiny and its difficult to work with microscopic objects. Understanding this actually helps you memorise it rather than accepting it.

For nuclear charge, as you go across a period on the periodic table, you will realise that the atomic number goes up by 1 each time. This means there is one more PROTON in the nucleus that the element before it.

Take Lithium and Beryllium for example. Lithium has 3 electrons. 2 in it's inner shell and one outer electron. Now because the nucleus of Lithium has a +3 charge due to the 3 protons. It has a pull on the outer electron on the outer shell.
In Beryllium, there are 4 electrons. There are also 2 shells (the same as beryllium). In the inner most shell, there's 2 electrons, in the outermost shell of Beryllium, there are 2 electrons. Now the nucleus has a +4 charge. This +4 charge is greater than Lithium's +3, so it's more difficult to remove an electron from Beryllium because there is a greater attraction between the nucleus and the outer electrons. (due to the greater positive charge, think of it like a magnet and a piece of metal, the stronger your magnet, the harder it is to separate from the piece of metal).

As you go down a group, the atomic radius increases, so the outermost electron is FURTHER away from the nucleus. Although there is an increase in Nuclear Charge, the outer electron is still too far from the nucleus for it to attract it with much force. There are also more shells between the nucleus and the outer electron. The increase in shells is actually what causes the larger atomic radius. So to describe this we say that the increase in nuclear charge is OUTWEIGHED by shielding of the outer electron.

Just a rule of thumb, whenever you describe a trend in ionisation energy. ALWAYS refer to the outer electron.
If you say, "As we go across a period, the nuclear charge increases so therefore the/an electron is harder to remove" you will get 0 marks.

You must say: "As we go across a period, the nuclear charge increases so the outer electron is harder to remove"

When energy is applied to gaseous atoms (such as Hydrogen). Electrons absorb the energy. Now say an electron from the shell n=1 absorbed enough energy to move to n=infinity, this is a point where the nucleus has lost complete control of the electron. So the atom is now ionised as it has lost an electron. So calculating the transition of the electron from n=1 to n=infinity gives us the ionisation energy for Hydrogen.

The ionisation energy is then calculated using E=hf

When energy is applied to gaseous atoms (such as Hydrogen). Electrons absorb the energy. Now say an electron from the shell n=1 absorbed enough energy to move to n=infinity, this is a point where the nucleus has lost complete control of the electron. So the atom is now ionised as it has lost an electron. So calculating the transition of the electron from n=1 to n=infinity gives us the ionisation energy for Hydrogen.

The ionisation energy is then calculated using E=hf

Thank you very much!

Found this in a markscheme for reference.

Find frequency of convergence limit(1)for Lyman series (1) Ionisation energy is given by E=hf / Energy ∝ frequency (1)