Chemistry New Syllabus Advice / Sample Answers by an Experienced HSC Tutor (1 Viewer)

[jazz519]

Tip on how to set out K constant calculations:

When we set out K constant calculations there is a way you can make your working out laid out neat, easy to read for the marker and most importantly gives you a template for things you need to do each calculation (which will make getting the answer a lot easier)

The approach you should take is the following as shown in the question and answer attached:

1. Write out the K constant expression for the reaction with variables (it is really important that you write the one with variables before subbing in even if it gives you the answer, because often marking criterias have words like “correctly writes a K expression for the reaction”. Now if you just subbed in then you haven’t really satisfied that criteria, so you run the risk of losing a mark even if you get the correct answer)

2. Draw up a RICE table and fill with data from question (even if the question seems easy this is an important part of the working and in hard questions the RICE table is really useful in giving you hints on what to do)

3. Fill in the rest of the rice table (using molar ratios and etc)

4. sub in the equilibrium conc to the K constant expression from

5. round your answer to the appropriate significant figures

If you follow these steps solving these questions should feel like doing the same thing over and over (which is good because it means you will get the marks)

 

[jazz519]

Tip: How to answer boiling point questions:

In my tutoring experience a lot of students struggle with questions on boiling point, however, there is a set procedure you can follow to answer all these types of questions

All these steps are shown in the sample answer below

Step 1: Define boiling point (by this I mean give a definition that relates to intermolecular forces, NOT the physical change of going from liquid to gas --> this tells you basically nothing about the actual reason behind why boiling point changes, which is related to intermolecular forces)

Step 2: Draw all the molecules in the question. You can see below the question states "explain why in terms of intermolecular forces" - key words here is intermolecular forces --> means draw the molecules because these intermolecular forces are related to the structure (if you are more advanced and aiming for band 6 you should show the interactions between the molecules i.e. dispersion forces, dipole-dipole or hydrogen bonding)

Step 3: Analyse each compound one by one following this approach:
- Discuss polarity (if it is a symmetrical molecule it is non-polar, if it asymmetrical it is polar)
- State what intermolecular forces it will have (non-polar things only have dispersion forces. Polar things have dipole-dipole interactions and SOME have hydrogen bonding if they have an OH, NH or HF group)
- Relate this to the boiling point (i.e. if it has strong intermolecular forces it has a higher boiling point. If it has weak intermolecular forces it has a low boiling point)
- Repeat this again for the next molecule over and over till you address each molecule

 

[jazz519]

Tip: How to remember titration equipment washing:

A very common multiple choice or 1-3 marker short answer question is being asked to state what we need to wash different titration equipment with

Here is a simple trick you can use to memorise this:
- Anything ending with the word FLASK - is washed with water
- Everything else is washed with the solution to be transferred in it

Applying this to an example:
A student makes a standard solution of sodium carbonate and using 25 mL aliquots of this solution in a conical flask, titrates it against a solution of hydrochloric acid of an unknown concentration

Question: what should the student wash the following equipment with:
- Volumetric flask
- Volumetric pipette
- Conical flask
- Burette

ANSWER:
Using our simple trick from before, we can say volumetric flask and conical flask should be washed with water (because they both end in the word FLASK)

Now for the other two they don't end in FLASK so we need to wash with the solution to be transferred

Aliquots are transferred using a volumetric pipette (therefore wash the volumetric pipette with the sodium carbonate solution)

Burette is washed with the solution that titrates against aliquot (therefore wash burette with acid)

 

[jazz519]

Tip: How to answer NEW syllabus Spectroscopy Questions:

One of the new topics in the new HSC Chemistry syllabus is the use of structural determination techniques and their use in finding the structure or key features of a compound (particularly organic compounds)

There are 3 main spectroscopy techniques in the HSC these are:

- Mass spectrometry
- NMR (Nuclear Magnetic Resonance)
- IR (Infrared)

In the new syllabus, the page 2 on the data sheet contains the key characteristic absorbances for different types of functional groups. Students must be able to use this list and other information in the question to determine the structure


Here is an example of a question that could be asked for this new section:

Below is a way to structure your answer. I recommend you break the discussion of each graph into a separate paragraph and underline where you have started talking about the other graph to outline clearly to the marker that you are making reference to each graph

 

[Hithcock]

This is a very valuable thread, bookmarking for later

What topics do you think will be potential 8-9 mark questions in this year's hsc? I know this is a big ask, but it would be helpful for me and others for preparation.

 

[jazz519]

This is a very valuable thread, bookmarking for later

What topics do you think will be potential 8-9 mark questions in this year's hsc? I know this is a big ask, but it would be helpful for me and others for preparation.

Quite hard to say because often the long response question in the HSC they try to combine multiple modules sometimes, but basing my answer on the trial papers by many companies and sample questions released by NESA. I think it will likely be in one of these areas:
- Organic Reaction Pathways (so you are given a big flow chart and have to figure out what the structure of each chemical is. I have seen this in a lot of trial papers)
- Describing the factors that need to be considered in chemical process (8.3 section in syllabus - which is a multi-module section because it links to module 5 in the equilibrium / Le Chatelier's principle aspect of many industrial process, and potentially module 6 and 7 depending on the uses of the chemical)
- Analysing a concentration vs time graph (with different changes there's an example in the additional NESA sample questions)
- Spectroscopy question on all the 4 spectroscopy (IR, H-MNR, C-13 NMR and Mass spec)

I think it will likely be in one of those 4 areas

 

[erucibon]

(2011 HSC) Explain the role of the conjugate acid/base pair, H2O4-/HPO42-, in maintaining the pH of living cells

Thanks

 

[erucibon]

The Ka of 1.75x10^-5. Calculate the pH when 50mL of 0.10 M acetic acid and 50mL 0.10M sodium acetate are mixed. (I got 4.76 but not sure if this is right). Calculate pH when 0.0025 mol of NaOH is added to the previous solution, assuming the volume doesn't change. Comment on the effectiveness of the solution as a buffer.

 

[jazz519]

(2011 HSC) Explain the role of the conjugate acid/base pair, H2O4-/HPO42-, in maintaining the pH of living cells

Thanks

I think this one was four 4 marks. So I will outline to you based on that criteria


1 mark: define buffer
1 mark: provide buffer equation
1 mark: define LCP + makes relation of buffer action linked to LCP
1 mark: relates back to question --> maintaining pH of living cells

So the answer would be something like this:
A buffer is a mixture of a weak acid/base and its conjugate in equimolar concentrations. It resists changes in pH due to the addition of acids and bases. The buffer equation for H2PO4- and HPO42- is: H2PO4- (aq) + H2O(l) <---> HPO42-(aq) + H3O+(aq). When an acid is added such as HCl (HCl(aq) + H2O(l) --> Cl-(aq) + H3O+(aq)), this increases the [H3O+] which disturbs the equilibrium. According to Le Chatelier's Principle, when a system at equilibrium is disturbed it undergoes a chemical reaction to minimise the disturbance. This causes the equilibrium to shift to the left, which increases the rate of the reverse reaction. Therefore, [H3O+] decreases, which means the addition of acid has been minimised. Similarly, when base is added (OH-), this reacts with the H3O+ (OH-(aq) + H3O+(aq) <--> 2H2O(l)), which decreases [H3O+]. By LCP this shifts the equilibrium to the right increasing [H3O+] and therefore minimising pH change. This is important in living cells because certain biological processes only work in a narrow pH window

 

[jazz519]

The Ka of 1.75x10^-5. Calculate the pH when 50mL of 0.10 M acetic acid and 50mL 0.10M sodium acetate are mixed. (I got 4.76 but not sure if this is right). Calculate pH when 0.0025 mol of NaOH is added to the previous solution, assuming the volume doesn't change. Comment on the effectiveness of the solution as a buffer.

The acetic acid provides the CH3COOH in the equilibrium, and sodium acetate provides the CH3COO- in the equilibrium:

CH3COOH(aq) + H2O(l) <--> CH3COO-(aq) + H3O+(aq)

We know [CH3COOH] = 0.10 M (but there is a dilution when the two things are combined, because you are moving from 50 mL to 100 mL total volume)

So this means [CH3COOH] in end solution = 0.05 M (because we have diluted by a factor of two)

Similarly, the acetate is diluted so [CH3COO-] in end solution = 0.05 M

Ka = [CH3COO-][H3O+] / [CH3COOH]
1.75 x 10^-5 = (0.05) [H3O+] / (0.05)

This means [H3O+] = 1.75 x 10^-5 M

pH = -log[H3O+]
pH = -log(1.75 x 10^-5)
pH = 4.76


Now next part:
I'm not 100% sure if you are allowed to use this in the syllabus but I can't think of any other way to do it, but there is something called the Hendersen-Hasslebalch equation which is used for predicting pH in buffers after stuff is added

The equation is: pH = pKa + log10([A^-]/[HA])

A^- here is CH3COO-
HA here is CH3COOH

pKa is just -log(Ka) = -log(1.75 x 10^-5) = 4.756....

So 0.0025 mol of NaOH has conc of 0.0025 / 0.100 = 0.025 M

This NaOH is going to react with the acetic acid: CH3COOH + NaOH --> NaCH3COO + H2O

So this means CH3COOH will reduce in conc by 0.025 M. So it goes from 0.05 M to 0.05 - 0.025 = 0.025 M

Now at the same time you made NaCH3COO so this one has to go up. Therefore 0.05 M goes to 0.05 + 0.025 = 0.075 M

Now sub in our equation:
pH = pKa + log10([A^-]/[HA])
pH = 4.756... + log(0.075/0.025)
pH = 5.23

Therefore, the solution is effective in minimising pH change as it only changed by 0.5 units approximately

 

[jazz519]

Tip: How to set out a titration calculation:

One of the most common questions is being asked to perform a titration calculation. These are usually 4 marks. The marking criteria would be something like this:
- 1 mark: writes a balanced chemical equation with states included
- 1 mark: correctly calculates moles of one component
- 1 mark: correctly finds concentration
- 1 mark: correct significant figures

You can see below also that the calculation is written in a way that is easy to follow. You can also see that the formula is written before any numbers are subbed in (i.e. you can't just jump straight to the n = 2.00 x 20.00 x 10^-3, you must write the formula first)

 

[_Anonymous]

Tip: How to answer NEW syllabus Spectroscopy Questions:

One of the new topics in the new HSC Chemistry syllabus is the use of structural determination techniques and their use in finding the structure or key features of a compound (particularly organic compounds)

There are 3 main spectroscopy techniques in the HSC these are:

- Mass spectrometry
- NMR (Nuclear Magnetic Resonance)
- IR (Infrared)

In the new syllabus, the page 2 on the data sheet contains the key characteristic absorbances for different types of functional groups. Students must be able to use this list and other information in the question to determine the structure


Here is an example of a question that could be asked for this new section:

Below is a way to structure your answer. I recommend you break the discussion of each graph into a separate paragraph and underline where you have started talking about the other graph to outline clearly to the marker that you are making reference to each graph

View attachment 27350

View attachment 27351
View attachment 27352

You mentioned integration in your answers for HNMR. Is there a way to calculate the integration in the exam if they're not provided? My teacher said it's very unlikely they'd ask us about integration, hence why we never were taught of it. But i've seen a few past papers include the term and added some values. I'm a bit unsure on how it works and how we can figure it out in an exam, any advice?

 

[jazz519]

You mentioned integration in your answers for HNMR. Is there a way to calculate the integration in the exam if they're not provided? My teacher said it's very unlikely they'd ask us about integration, hence why we never were taught of it. But i've seen a few past papers include the term and added some values. I'm a bit unsure on how it works and how we can figure it out in an exam, any advice?

Integration just means how many hydrogens are in that environment. So you won't be asked to calculate this for HSC but its already displayed on the H-NMR spectrum. Above each peak it says a xH. x here is the number of hydrogens in that environment which is the definition of integration

The reason why we call it integration is technically because the way the H amount is found is by finding the area under each peak (which a computer does)

 

[_Anonymous]

Integration just means how many hydrogens are in that environment. So you won't be asked to calculate this for HSC but its already displayed on the H-NMR spectrum. Above each peak it says a xH. x here is the number of hydrogens in that environment which is the definition of integration

The reason why we call it integration is technically because the way the H amount is found is by finding the area under each peak (which a computer does)

Got it, thank you.

 

[frog0101]

How should we prepare notes for Questions on 8.3 in syllabus? Are we meant to have in depth knowledge of ONE chemical process, or just know that if given a chemical process, we need to consider those factors?

 

[jazz519]

How should we prepare notes for Questions on 8.3 in syllabus? Are we meant to have in depth knowledge of ONE chemical process, or just know that if given a chemical process, we need to consider those factors?

I would memorise a chemical process because I have seen both types of questions, ones where you are asked to analyse a given process and ones where it doesn’t give a specific process

 

[Arrowshaft]

Hey Jazz! If I may pick your brain a little, for polymers revision I’ve just memorised a lot of content on the production, use and properties of additional polymers: polyethylene (HDPE and LDPE), PVC, PTFE and PS and condensation polymers: Nylon and Polyester (particularly Polyethylene terephthalate), how much of this info would we be required to regurgitate in a question. Because in the sample questions I’ve seen a 7 marker asking to contrast one addition and one condensation polymer but they didn't really include a proper sample answer; rather, a "answers could include:" kinda bullet pointed list. I also don't know to what extent they expect the top bands to perform in that question with regards to depth, so any light on this would be helpful! Your input is highly appreciated!

EDIT: Also, do you recommend that we memorise 2 processes for 8.3, like contact and Haber, or stick with one? Thanks!

 

[jazz519]

Hey Jazz! If I may pick your brain a little, for polymers revision I’ve just memorised a lot of content on the production, use and properties of additional polymers: polyethylene (HDPE and LDPE), PVC, PTFE and PS and condensation polymers: Nylon and Polyester (particularly Polyethylene terephthalate), how much of this info would we be required to regurgitate in a question. Because in the sample questions I’ve seen a 7 marker asking to contrast one addition and one condensation polymer but they didn't really include a proper sample answer; rather, a "answers could include:" kinda bullet pointed list. I also don't know to what extent they expect the top bands to perform in that question with regards to depth, so any light on this would be helpful! Your input is highly appreciated!

EDIT: Also, do you recommend that we memorise 2 processes for 8.3, like contact and Haber, or stick with one? Thanks!

For polymers this was a topic in the old syllabus as well (just some minor changes to it) so the questions will be relatively the same. It is quite a content heavy section and you will have to memorise those things in depth because the questions can be 4+ marks, so unfortunately there really isn't any way around not memorising

Usually for each polymer you want to do the following: describe its structure (so what atoms are in it, does it have branching), link this structure to intermolecular forces, the properties due to the intermolecular forces, uses related to the properties. Also including a diagram of the structure is very important

(Click on my tutoring link and you can see the sample booklet for 7.6 theory, it has like sort of how to write answers on this stuff)

But those are not the only questions on the polymers section. A very common multiple choice question is being given the structure of a polymer (normally condensation polymer because these are harder) and being asked to determine what the monomers used to make it where (see 2014 HSC Q18). You can also be asked the other way around i.e. going from the monomers to the polymer (see 2015 HSC Q11)

 

[jazz519]

In terms of the chemical process I would just memorise one. It's not likely you need to talk about two and memorising one means you can do that more in depth

 

[Arrowshaft]

For polymers this was a topic in the old syllabus as well (just some minor changes to it) so the questions will be relatively the same. It is quite a content heavy section and you will have to memorise those things in depth because the questions can be 4+ marks, so unfortunately there really isn't any way around not memorising

Usually for each polymer you want to do the following: describe its structure (so what atoms are in it, does it have branching), link this structure to intermolecular forces, the properties due to the intermolecular forces, uses related to the properties. Also including a diagram of the structure is very important

(Click on my tutoring link and you can see the sample booklet for 7.6 theory, it has like sort of how to write answers on this stuff)

But those are not the only questions on the polymers section. A very common multiple choice question is being given the structure of a polymer (normally condensation polymer because these are harder) and being asked to determine what the monomers used to make it where (see 2014 HSC Q18). You can also be asked the other way around i.e. going from the monomers to the polymer (see 2015 HSC Q11)

Thank you so much for the clarification! Also, just. A quick question; for properties like tensile strength and etc. are we meant describe how the intermolecular forces provide that feature, i.e. the Ziegler Natta process in HDPE helps straighten and provide longer chains resulting in higher tensile strength, or can we just list the properties? Because, I feel otherwise it would take a loong time to explain each feature. Thanks!