Common problematic Chemistry Questions

[Lear]

Feel free to contribute tricky questions you have found in Chemistry exams/SACs below
I'll start

Unit 3
- Specific knowledge on the environmental impacts of specific fuels in study design
- Detailed comparison of biodiesel and petrodiesel in terms of various properties
- Thermochemical equations require delta H value. This may need to be multiplied from the data book depending on balanced equation
- Purpose of semi-permeable membrane in electrolytic cells
- Advantages and disadvantages of various cells
- Factors affecting the battery life of secondary cells.
- Effect of using an aqueous electrolyte or reactive electrodes an electrolytic cell
- Knowing how aspects of secondary cell change depending on whether it is acting as a secondary cell or primary.
- Being able to balance half equations in both acidic and alkaline conditions
- Energy conversions within a galvanic cell depending on whether half cells are split apart or simply in one cell
- Use of PV=nRT in larger scenarios such as 2016 Question 9
- Proper wording when describing effect of catalyst, changes to equilibrium mixture and changes to rate of reaction

Unit 4
- IUPAC naming priorities and other conventions
- Complex reaction pathways to produce esters and amides
- Difference between oxidation of primary and secondary alcohols
- Specific knowledge on how IR, Mass spec and other analytical techniques work,
- n+1 splitting rule, peak area and chemical shift
- Digestion of carbs, proteins, fats and how these are used by the body
- Composition of starch and it's effect on GI
- Importance of the tertiary structure of enzymes, coenzymes, different models and enzyme ability to differentiate enantiomers
- Effect of double bonds on the melting point of a molecule. Cis and Trans too.
- Cellulose and why we cannot digest it.
- Rancidity and antioxidants, especially vitamin C as an antioxidant.


Experimental design
- Definitions of validity, precision, reliability, reproducibility, accuracy
- Common errors in titrations, calorimetry, electrolysis, enzyme practicals
- Personal errors, systematic errors, random errors
- Writing aim, conclusions, and discussions.

[vceme]

While not a problematic question, I think the state often gets confused on factors that increase the rate of reaction:

Rate of reactions:
The rate of reaction only increases if the amount of successful collisions increases.
Temperature & Catalyst: These two factors are the only ones that increase the proportion of successful collisions, hence increasing the rate of reaction.  An increase in temperature also increase the frequency of total collision and successful collisions. **Note: a catalyst will increase the frequency of successful collisions but not the frequency of total collisions*

Increasing concentration/ Pressure: These factors increase the frequency of total collisions (yes, even the ones that aren’t strong enough to cause a reaction) which correlates to an increase in successful collisions and thus increase in the rate of reaction. 

 

[Maya24]

This is more in general
- Make sure you read the question twice to avoid simple mistakes
- Check that you have the right number of significant figures
- Check your units

[Freddie Hg]

Electrolytic cells and galvanic cells are both redox reactions yet one is non-spontaneous and the other is spontaneous.
seems very basic but i remember i wrote on a sac  the difference between them is a galvanic cells is a redox reaction and the other isnt.
they are both redox!!
idk why i got that wrong, but its a stupid mistake 

[Lear]

Note the difference between the IR of an Aldehyde and that of a ketone. An aldehyde will have a peak next to the C=O peak due to
stretching caused by C-H

[passbleh24]

Redox:

Purpose of membrane cells:
- Prevent spontaneous reaction between the products
- Could be to prevent contamination
- Look at the aim of the cell and link back.
- Maybe products are explosive/ toxic.

Why can't an alternative electrode/electrolyte be used
- Usually because it is a stronger oxidant or reductant that one of the species and prevents desired reaction from occurring.
- Write the product that results instead ( usually a mark )

Determining Half equations:
- Strongest oxidant will react with strongest reductant. Highest and then Lowest.
- Don't omit SO4 even if spectator.
- Use ox numbers to determine which has reduced or oxidised
- Might need to look at the labels on the cell.

[zenith101]

-When listing safety precautions/handling measures, remember to tailor very specifically to the question. The q's are generally one mark, so anything even slightly vague and assessors won't accept it.

-Know how to calculate Avogadro's constant through electrolysis (i have a gut feel it might be in this year's)

-Understand the basic concept of competing equilibria through the example of carbon monoxide poisoning

[Freddie Hg]

-When listing safety precautions/handling measures, remember to tailor very specifically to the question. The q's are generally one mark, so anything even slightly vague and assessors won't accept it.

-Know how to calculate Avogadro's constant through electrolysis (i have a gut feel it might be in this year's)

-Understand the basic concept of competing equilibria through the example of carbon monoxide poisoning

your last is 100% gonna be there (or has it been asked in 2018 nht? i cant remember)

[Lear]

-Understand the basic concept of competing equilibria through the example of carbon monoxide poisoning

That’s is indeed a really good question they could ask. They’ll probably give the two equations and ask something like ‘why is pure oxygen given to those suffering from Carbon Monoxide poisoning’

[Freddie Hg]

That’s is indeed a really good question they could ask. They’ll probably give the two equations and ask something like ‘why is pure oxygen given to those suffering from Carbon Monoxide poisoning’

maybe.
however i think they may ask the effect of a large equilibrium constant on the yield of another reaction in competing equilibria with reference to co poisoning.
or both in an extended stem

While we are on this topic, can you please explain question 8d from the NHT 18 exam? I was a bit unclear on how we deduce that K2 is larger. Cheers

8d isnt about equilibrium maybe you got confused about which question it is. 8d is about reliability

[Lear]

Just a note to all, it would be excellent if questions on above topics directly be placed in the chemistry question thread rather than here. If we keep our discussion here mainly centred around problematic questions we can ensure not to lose the flow of a discussion. Also, this means this thread can be much easier to follow for students in upcoming years.

[Freddie Hg]

Sorry i meant from the VCAA sample exam ahaha.

delete your previous messages as lear said keep this about the discussion on hand.

so its the exact same question i said vcaa might bring. we assume that the higher equilibrium constant belongs to equation 2 as that equation determines the yield of equation 1.

more in-depth: in competing equilibria we have two equations which share one or more reactant in common. in this case we have one reactant shared between the two equations haemoglobin. the equation with the higher equilibrium constant will force the one with the lower equilibrium constant to give up its haemoglobin, it does this by reversing the equation with a lower constant.

they tell us that "anyone who is
exposed to this will quickly lose consciousness" so we can assume that haemoglobin-oxygen
complex has decreased by equation 2 due to k2 being bigger than k1.  therefore k2 must be greater than k1 if we have a decrease in oxy-haemoglobin

sorry if it took long i got side tracked, again ill be willing to copy past this in the question thread if you move your question there.

[grestal]

-Know how to calculate Avogadro's constant through electrolysis (i have a gut feel it might be in this year's)

how to do this? i can't find it on the internet. help!!!

[Freddie Hg]

-Know how to calculate Avogadro's constant through electrolysis (i have a gut feel it might be in this year's)

how to do this? i can't find it on the internet. help!!!

dont panic its most likely not going to be there. but if you want to know https://www.thoughtco.com/experimental-determination-of-avogadros-number-602107
i dont recommend you study it now. just relax

[Lear]

dont panic its most likely not going to be there

Agreed.
As per the study design - ‘the application of stoichiometry and Faraday’s Laws to determine amounts of product, current or time for a particular electrolytic process.’