HSC Chemistry Questions Thread - Please ask any questions here!

[thushan]

Hey everyone! I'm a long-time user of ATARNotes and am just posting here, hoping to get some nice stimulating discussion on HSC Chemistry here. I am very happy to help you guys out; feel free to ask ANY Chemistry question over here - remember, no question is ever a dumb question.

It would also be absolutely brilliant if you guys also answer each other's questions - gives you excellent practice in using your Chemistry knowledge.

I'll be popping into this thread periodically!

If you'd like to ask me a question, just make an ATAR Notes account here. Once you've done that, a little 'reply' button will come up when you're viewing threads, and you'll be able to make and reply to threads as you please!

[heids]

^ With a silver medal in the International Chemistry Olympiad, plus a Premier's Award (like the top 5 in VCE Chemistry), and a lecturer and book author in Chemistry, he does kinda know his stuff.  Might as well get in on some of his brilliance

[brenden]

I would very strongly urge any HSC Chemistry students to make the use of this resource. In 2011, Thushan Hettige won a silver medal in the International Chemistry Olympiad, scored a 99.95 with a top rank in Chemistry and achieved 99th percentile in the UMAT for admission into undergraduate medicine. Since then, he's tutored professionally, become a published Chemistry author and given lectures at the high-school and university levels.

If you'd like to ask Thushan a question, just make an ATAR Notes account here. Once you've done that, a little 'reply' button will come up when you're viewing threads, and you'll be able to make and reply to threads as you please!

[jamesey]

Hey, so I slacked a lot during prelim chemistry and as a result didn't develop the skills for equation writing in general. This led to me getting intimidated when it comes to equation writing during exams and that's not meant to happen obviously. I know this is really broad so are there any resources you can link me to that can give me a heads up on the basics or any recommendations?

[thushan]

Hey Jamesey!

Not sure so much about resources specifically on equation writing, but I'll give you a couple of pointers that hopefully may help you:

Firstly - try not to rote-learn rules such as "acid + metal carbonate --> salt + carbon dioxide + water"; rote-learning these will make it more difficult to learn and also you may misapply the rule in writing your equations. I see this in my students all the time who rote-learn formulae and when trying to solve a problem, "pick formulae off a shelf" and try apply them and see where they go, which often leaves them stuck.

Many chemical reactions are one of the following:
- acid-base (where there is a proton transfer from one species to another species)
- redox (where there is an electron transfer between species); combustion reactions in oxygen are a particular type of redox reaction
- precipitation reactions (where two aqueous species 'combine' to form a solid product)

Acid-Base
The trick here is to actually see the proton transfer.

For example - say you want to write an equation of a reaction between MgO and HCl.

The first thing you need to pick out is that MgO is an ionic solid that contains the Mg²⁺ and O^2- ion, which is a pretty damn strong base; you can 'stick' two protons (H⁺) onto this ion to form H₂O.

The second thing is that HCl is an acid, and can donate its proton to MgO (the base).

Now, what happens when HCl donates a proton? It becomes Cl-.
What happens when O^2- accepts protons? It becomes H₂O.
In the meantime, the Mg²⁺ is liberated into solution with the Cl- ions; MgCl₂ is soluble in water.

So we are left with MgO + HCl --> MgCl2 + H2O. No formulae needed here!

To balance the equation - remember that you need 2 H⁺ ions to protonate the O^2- all the way to water.

So the equation would be:
MgO + 2 HCl --> MgCl2 + H2O

States - most oxides are solid (including MgO), HCl is presumably in solution, as is MgCl2, and water is obviously a liquid.

MgO (s) + 2 HCl (aq) --> MgCl2 (aq) + H2O (l).

This seems a little long, but it's so intuitive that you'll be able to go through this thought process quickly.

Another example: HCl and CaCO₃.

HCl is again the proton donor. CaCO3 is an ionic solid with Ca²⁺ and CO₃^2- ions, the latter being a base. You can stick two protons onto it to form H₂CO₃.

Now, if you remember Coca Cola (which contains H₂CO₃), once you open the top it fizzes; that is because H₂CO₃ is pretty unstable and will just dissociate into H₂O and CO₂.

When HCl gives off a proton, it becomes Cl-.
When CO₃^2- takes up two protons, it becomes H₂CO₃ which just decomposes to H₂O and CO₂.
Meanwhile, the Ca²⁺ ion just wanders off and does its own thing, joining Cl^- in swimming around solution. You basically have a CaCl₂ salt solution.

Your equation would just be then HCl + CaCO₃ --> CaCl₂ + CO₂ + H₂O.

Redox
http://www.chemguide.co.uk/inorganic/redox/equations.html

Chemguide.co.uk is the BIBLE for Year 12 Chemistry in general. I used it religiously when I was doing Chemistry at school!

Did you want me to take you through combustion reactions of hydrocarbons specifically?

Precipitation
This is basically two ions combining to form a solid. The best thing here is to know your solubility table here.
For example if you mixed BaCl₂ and Na₂SO₄, you look at the ions and you should recognise that Ba₂₊ and SO₄^2- will not coexist in solution; they will precipitate out. So instantly you have the ionic equation -

Ba₂₊ (aq) + SO₄^2- (aq) --> BaSO₄ (s)

If you want the overall equation, then slot in the spectator ions where appropriate, and note that they 'associate' together in aqueous solution after the reaction - as NaCl (aq) on this occasion:
BaCl₂ (aq) + Na₂SO₄ (aq) --> BaSO₄ (s) + 2 NaCl (aq)

Hope that helped somewhat!

[jamesey]

Thanks a lot for that! Found that very useful. I'll have a lot more questions coming up

[jamesey]

Can I also ask about the galvanic cell? I'm confused about the role of the anodes and cathodes, which solution is where, and also determining the voltage and all that. Thankss

[thushan]

Can I also ask about the galvanic cell? I'm confused about the role of the anodes and cathodes, which solution is where, and also determining the voltage and all that. Thankss

Sure!

The galvanic cell is just the fancy name for a battery. You're using a chemical reaction to drive an electrical current.

To drive this current, you separate the redox half equations into two half cells, so that the electrons produced at one redox half equation have to physically travel through a wire to get to the other half cell to complete the redox reaction.

Each half cell is connected to an electrode, a piece of metal that allows electrons to pass from solution into the wire. One of the electrodes is called the anode, the other the cathode.

The anode is the electrode at which oxidation occurs.
The cathode is the electrode at which reduction occurs.

These two statements are true by definition.

You can predict the direction of electron flow ("which solution goes where") by looking at the position of the two reaction half equations in the electrochemical series; the further up you go in the series, the further forward the half-equation tends to go.

To determine the voltage (under standard conditions), take the difference between the Eo values of the two half-equations (in the reduction form, as shown in the electrochemical series).

[Angie.iyer]

does anybody here have any useful notes for the option module of forensic chemistry?