HSC Chemistry Question Thread

[itssona]

anyone know how Q5 from the 2014 paper is C?? Where is the chlorine atom really from? thanks

[KT Nyunt]

anyone know how Q5 from the 2014 paper is C?? Where is the chlorine atom really from? thanks

An addition reaction is when an unsaturated molecule (a molecule with a double bond) reacts with another molecule, making it saturated (all the possible bonds are used up) without the loss of any atoms. So, the reactant must have a double bond (hence A and B are out) and the product must be saturated (hence D is out and the answer is C). 

The chlorine atom comes from HCl: where there was once a double bond, one H joined to one CH (they showed this by having a CH2) and the Cl joined the other.

 

[cnimm2000]

Hey guys,
for module 3 section 2: maximising production and ensuring quality and control, do we need to know the haber process in depth (if so could i get a quick explanation on it), or just how to increase the yield and rate of reaction
thanks in advance

[Victorious]

Hi,
Could anybody please explain the production of LDPE and HDPE???

I'm so confused about the two seperate processes.

Thank you in advanced!

[RuiAce]

Hi,
Could anybody please explain the production of LDPE and HDPE???

I'm so confused about the two seperate processes.

Thank you in advanced!

The two separate processes follow the same steps. The only difference between the production of LD and HD PE is that a Ziegler-Natta catalyst replaces the peroxide initiator for the latter. So if you ever require the production of HDPE, you just write the exact same thing for LDPE, except with a Ziegler-Natta catalyst instead.

In short:
0. Activation of the peroxide initiator
1. Initiation - the peroxide initiator attacks the first ethene molecule (which produces the radical)
2. Propagation - the radical now attacks another ethene, and so on. (We don't know how many ethene atoms will be attacked, we just care that this happens)
3. Termination - eventually, it'll collide with another ethene radical and then you have your polyethylene.

(Remark: I did this off memory. Might've mixed up the words activation and initiation other way around.)

Hey guys,
for module 3 section 2: maximising production and ensuring quality and control, do we need to know the haber process in depth (if so could i get a quick explanation on it), or just how to increase the yield and rate of reaction
thanks in advance

It's more of those highly-recommended band 6 pieces of knowledge. What's good about understanding the Haber process is that if you properly understand its method, then you can potentially figure out "why we do this, why we do that" on the spot.

I haven't done this in ages so this explanation is very loose. Essentially, we pump a mixture of nitrogen and hydrogen into the reaction chamber, and we note that the N:H ratio is roughly 3:1 (why?).

We want to rely on LCP to drive the forward reaction, without breaking the delicate glassware. So we make the pressure around 250atm. (Why 250 and not something higher like 1000?)

We want to rely on LCP to drive the forward reaction, but we still leave the temperatures relatively high. (Why?)

The reaction chamber involves a catalyst. We need to maintain the purity of this catalyst. (What is the catalyst and why do we maintain its purity? Is the catalyst the only purity that we want to maintain; what if argon or even worse oxygen creeps into the reaction chamber?)

Of course, most of the important stuff happens there. The last bit is relatively easy to remember. Essentially, we also note that ammonia has a lower condensation (boiling) point in contrast to nitrogen and oxygen. Because the gas will go up by itself, we set the temperature somewhere high enough in the pipe (or whatever) so that ammonia will condense into the liquid state (and thus get collected).

[KT Nyunt]

Hi,
Could anybody please explain the production of LDPE and HDPE???

I'm so confused about the two seperate processes.

Thank you in advanced!

If you wanted any more information, I have some screenshots from my notes for production of materials: (the full notes are also up on the ATAR notes site)
  

[meditatingwalrus]

Hi!
So they give you this information because we need to find the number of moles of nitric acid (HNO3) in this reaction as well as the number of moles of Zinc (Zn) we have, in order to see which one is the limiting reagent...

From the answers:
n(HNO3) = 0.1 mol [we find this by doing n = Cv]
n(Zn) = 0.153 mol [we find this by doing mass/molar mass]

As there are fewer moles of nitric acid than Zinc, this is the limiting reagent, meaning only 0.1 moles of HNO3 reacted with 0.1 moles of Zn and some Zn was left in excess.
From the equation, we can deduce the molar ratios, which will help us find the volume of the gas produced, H2. Now that we know 0.1 moles of HNO3 reacted, and the molar ratio between HNO3 and H2 is 2:1, the number of moles of H2 produced is 0.05 mol.
Then we put this into the equation n x 24.79, giving us the answer: 1.24L as required.

Hope this helps!

legend! thanks so much

[cocopops201]

Hello,

I recently received my chemistry mid-course exam mark and honestly I'm not that too happy with it due to external factors effecting my proactivity during the exam. *internally cries* AND a true genuine question I have is how do I find my strengths and weaknesses to later fix, before any upcoming exams in Term 2 turn up? As well as, how would you specifically study for chemistry? I understand practice papers and revising syllabus notes are core elements to succeed but how does one know what to do? i.e calculation questions (step-by-step guide), production of [ethanol] using flowcharts etc.

Thanks,
Fellow struggling year 12 student in chem

[KT Nyunt]

Hello,

I recently received my chemistry mid-course exam mark and honestly I'm not that too happy with it due to external factors effecting my proactivity during the exam. *internally cries* AND a true genuine question I have is how do I find my strengths and weaknesses to later fix, before any upcoming exams in Term 2 turn up? As well as, how would you specifically study for chemistry? I understand practice papers and revising syllabus notes are core elements to succeed but how does one know what to do? i.e calculation questions (step-by-step guide), production of [ethanol] using flowcharts etc.

Thanks,
Fellow struggling year 12 student in chem

Hi!
The way that I've found to really work with studying for my science subjects - and this is a little different to normal methods of study, but hey it may work for you too! - is to teach.
Basically what I would do is:
1) create study notes (often handwritten is the way to go but this is up to you, but handwriting may improve your writing speed in exams)\
2) use these notes as 'prompters' when I teach someone (I would normally teach like a parent or a fellow classmate - setting up a study group is really good for this - this is just so they can ask you questions or give feedback)
3) You can measure how well you understand a topic by how clearly and succinctly you were able to communicate this
4) taking note, revising, then re-teaching topics that you struggled with is another way to improve understanding and this will also help to make content stick!

I still suggest working through practice papers as more of a final step, to practice writing responses and applying your knowledge. But the idea is, if you can verbally communicate your ideas, you are more than likely able to write your ideas as well.

when it comes to how to answer types of questions in exams:
- When a question says 'compare' - a quick way to do this is with a table
- when a question says 'outline a process' - a flowchart is great! (if it were to say 'describe a process' I would go in further depth than just a flowchart)
- Whenever you know a diagram for something - use it! (it shows you really know your stuff)
- calculations - when studying for this, it's always good to expose yourself to as many examples and then write yourself like a step by step guide on how to approach these types of questions (ie. a step-by-step for molar heat of combustion question, a step-by-step for concentrations in titrations etc)
- always have a look at the criteria in the answers (if it is provided) so that you can look on how to improve responses - or if they give a sample answer, add what you didn't have in your response into your own notes.

This is just my way of studying and I find it works well for me, but this may vary depending on the person. Nonetheless, I hope this was helpful!

[cocopops201]

This is just my way of studying and I find it works well for me, but this may vary depending on the person. Nonetheless, I hope this was helpful!

Thank you so much!

[Dragomistress]

What do I have to memorise for the catalyst used for the Haber process? Do I have to just remember that it is Fe3O3 and not the other small powders?

[KT Nyunt]

What do I have to memorise for the catalyst used for the Haber process? Do I have to just remember that it is Fe3O3 and not the other small powders?

Yes or more accurately, just say iron (II/III) oxide

[cnimm2000]

Hey guys,
I’m struggling with ppm calculations. If anyone has any tricks on how to do these questions, or how to begin to calculate it would be great!

[KT Nyunt]

Hey guys,
I’m struggling with ppm calculations. If anyone has any tricks on how to do these questions, or how to begin to calculate it would be great!

Hi!
This website helped me a lot with understanding ppm: http://www.ausetute.com.au/partspm.html
It goes through a lot of the types of ppm questions you may encounter and goes through them step by step.

To summarise it:
Here are the units you'd be working with when it comes to parts per million...
In weight/volume (w/v) terms,
1 ppm = 1g m-3 = 1 mg L-1 = 1 μg mL-1
In weight/weight (w/w) terms,
1 ppm = 1 mg kg-1 = 1 μg g-1

A lot of sources will use this formula:
mass of the solute/volume of the solution x 1000000
This is fine and dandy but another trick is just taking any information they give you (mass of solutes and volumes or mass of solution) and convert it into one of the units above (i.e. if they give you a solute mass in grams and a solution volume in millilitres, convert it into milligrams and litres). Then your final step is to divide the mass of the solute with the volume or mass of the solution ( e.g. mass of solute/volume or mass of solution).

Essentially, how this works is... if we look at an example of a ppm unit : mg kg-1
A milligram is a millionth of a kilogram. So by converting any information into these units first then we can really see how many "parts per million" we have.

I'm not too sure about how well I explained that... Let me know if this makes sense 

[itssona]

hey:)
how do I write an equation for the incomplete combustion of ethyne (c2h2) that produces a ratio of soot to CO2 of 5:1