HSC Chemistry Question Thread

[MisterNeo]

Regarding to picking equations for electrolytic cells, I thought that if a solution was aqueous, you would have to consider water in a redox reaction. So in this question I thought that water would oxidise instead of the chromate because it is more easier to oxidise. Obviously my answer wasn'tin the option, but just wondering why this is the case.
Thank you

Hey
This question is referring to a galvanic cell rather than an electrolytic cell because the potentials are positive. So on the reduction table, you would look for the species closest to the top or bottom rather than in the middle.

Now why is water not oxidised in this galvanic cell?
First of all, we need to see what species are present in solution:
-Water H₂O
-Potassium ion K⁺
-Dichromate ion Cr₂O₇^2-
-Sulfur dioxide SO₂

Now we need to find out which species will be preferenced at the anode and cathode by looking for the largest E^o value.
-K⁺: Cannot be further oxidised. Reduction potential is non-spontaneous (-2.94V). Thus, we eliminate this.
-Cr₂O₇^2-: Can be reduced, 1.36V is spontaneous and is closer to the bottom of the reduction table than all the other species, thus this is going to be reduced at the cathode.
-H₂O: Water's oxidation potential is more negative (-1.23V) than SO₂ (-0.16V) hence sulfur dioxide has anode preference as it is more readily oxidised. Water cannot be reduced as dichromate has the cathode preference.
-SO₂: Sulfur dioxide's potential is less negative than water's hence why it gets oxidised easier.

So we concluded that the anode preference is sulfur dioxide with a potential of -0.16V, and the cathode preference is dichromate with a potential of 1.36V. Just add these together and you will get a total potential of 1.2V, thus the answer is A.

Thanks for reading lol

[winstondarmawan]

Shipwrecks
Do sacrificial anodes donate their electrons to the metal that is to be protected? Or do they just corrode preferentially to the protected metal? Or both?
TIA.

Would appreciate help with the following, TIA.
https://scontent-syd2-1.xx.fbcdn.net/v/t35.0-12/21534556_816544521861838_437814374_o.jpg?oh=a6d0cea1ffd670dc4ed57713635bc9b0&oe=59B4FD34

Just bumping my posts.

[VanillaRice]

EDIT: Shipwrecks
Do sacrificial anodes donate their electrons to the metal that is to be protected? Or do they just corrode preferentially to the protected metal? Or both?
TIA.

They become oxidised preferentially to the metal they are intended to protect. They will donate their electrons to whatever the oxidant is (e.g. molecular oxygen).

Would appreciate help with the following, TIA.
https://scontent-syd2-1.xx.fbcdn.net/v/t35.0-12/21534556_816544521861838_437814374_o.jpg?oh=a6d0cea1ffd670dc4ed57713635bc9b0&oe=59B4FD34

Begin by calculating the number of moles of ZnO and Al₂O₃. Knowing this, can we work out how many moles of Zn and Al there are (i.e. consider n(Zn) = n(ZnO))? We are assuming all zinc and aluminium came from the alloy being studied. Once you have found the number of moles of each metal, find the masses of zinc and aluminium this equates to. Now, you will have the masses of zinc and aluminium extracted from the alloy. How can we calculate the percentage composition of each?

Hope this helps

[anotherworld2b]

Thank you for your help Misterneo
I was wondering if someone could explain what a zwitterion is and it's importantance to me. I don't quite understand at the moment   

[VanillaRice]

Thank you for your help Misterneo
I was wondering if someone could explain what a zwitterion is and it's importantance to me. I don't quite understand at the moment   

A zwitterion is a neutral molecule that has internal charges. That is, it's positive and negative charges cancel out.
I assume you're learning about zwitterions in the context of amino acids? If so, think about how basic and acidic groups can cancel each other out.
One use of having a knowledge about the zwitterionic forms of an amino acid is that it allows us to predict the pHs at which the amino acid is overall neutral. This is useful for things like electrophoresis, where, if we know the charges of the amino acids at a particular pH, we can separate a sample of amino acids based on their charge, etc.

Hope this helps

[phoebegresham]

Hey!! If in an exam we are given a list of combustion equations and the question asks which one is incomplete combustion, what is the best way of knowing which one it is? Thanks

[Natasha.97]

Hey!! If in an exam we are given a list of combustion equations and the question asks which one is incomplete combustion, what is the best way of knowing which one it is? Thanks

Hi!

Incomplete combustion will result in the production of Carbon Monoxide (CO)/Carbon (C)/both and water. Complete combustion will result in the production of Carbon Dioxide (CO₂) and water.

Hope this helps

[Bubbly_bluey]

For eutrophication, is there a quantitative method to monitor the levels of phosphates and nitrate ions? I learnt about colorimetry but that seems to be a qualitative analysis. Would AAS work even though its not a trace element?
Thank you 

[Natasha.97]

For eutrophication, is there a quantitative method to monitor the levels of phosphates and nitrate ions? I learnt about colorimetry but that seems to be a qualitative analysis. Would AAS work even though its not a trace element?
Thank you 

Hi!

These are the tests that could be used to monitor the levels of phosphate/nitrate ions:

Nitrates
- Brown ring test: Add Iron (II) Sulphate to conc. H₂SO₄. When combined with a nitrogen-containing solution, a brown ring will form at the intersection of the solutions (Qualitative)
- Kjedahl digestion: Heat sample with conc. H₂SO₄ (forms ammonium sulphate). Add NaOH to form ammonia. Titrate with a standardised HCl solution, then calculate the levels of ammonia. (Quantitative)
- Colorimetry: Add Nessler’s reagent, which reacts with nitrogenous compounds to form a yellow compound (Qualitative)

Phosphates
- Colorimetry: Add ammonium molybdate (forms pale yellow substance). Add solid ascorbic acid (Forms blue compound: ‘molybdenum blue’). The intensity of the blue colour is measured by a colorimeter and compared to a series of standards to determine the concentration (Quantitative)
- Adding Lead (II) nitrate : Forms a white precipitate of lead phosphate (Qualitative)

Hope this helps

[anotherworld2b]

I was wondering if anyone had information about how oil produces plastics.
i have a research assignment that requires me to do a flow chart with diagrams using relevant equations for the Synthesis Process but I can't find any information or equations

[Kle123]

I was wondering if anyone had information about how oil produces plastics.
i have a research assignment that requires me to do a flow chart with diagrams using relevant equations for the Synthesis Process but I can't find any information or equations

The flow chart would go something like this:
Crude Oil --(fractional distillation)--> ethylene (insufficient amounts to meet market demand) + high molecular weight fractions --(Thermal and Catalytic Cracking of large carbon fractions)--> ethylene {{--(substitution reactions for other plastics other than polyethylene)--> monomers such as vinyl chloride and styrene }} --(radical addition polymerisation and/or ziegler natta catalyst polymerisation)--> LDPE and/or HDPE

I don't think the syllabus asks for the production of PVC and Polystyrene plastics, which is why it is in brackets.
You could include equations of cracking stage of high molecular weight fractions such as C₂₀H₄₂(l)->C₂H₄ + C₁₈H₃₈
and also the stages of the polymerisation such as initiation, propagation and termination


_____________________________

1. Random(but not really) question:
Why does the carboxylic group only lie at the end of a carbon chain
2. In separating esters from reaction mixture in esterification why can we add water to the products then use a separating funnel to extract the ester on the top layer? Aren't esters polar.. aren't they soluble in water? why can they be separated like this?

[Natasha.97]

Random(but not really) question:
Why does the carboxylic group only lie at the end of a carbon chain

Hi!

The carboxyl group is -COOH. The carbon that is involved must be bonded to two O molecules. The first oxygen shares a double covalent bond with carbon (carbonyl: C=O), whilst the second oxygen shares a single covalent bond with carbon and a hydroxyl bond with hydrogen (See diagram below). Therefore, the carboxyl group is at the end of the carbon chain as carbon only has 4 valence electrons (thus can form a maximum of 4 bonds.)



Hope this helps

[Kle123]

Oh I was thinking more about why the carboxylic group lies on carbon 1 in naming, but now i realise the its just standard that we don't count it as a side branch. THANKSS

Also,
In separating esters from reaction mixture in esterification why can we add water to the products then use a separating funnel to extract the ester on the top layer? Aren't esters polar.. aren't they soluble in water? why can they be separated like this?

[VanillaRice]

In separating esters from reaction mixture in esterification why can we add water to the products then use a separating funnel to extract the ester on the top layer? Aren't esters polar.. aren't they soluble in water? why can they be separated like this?

Somewhat, but not to the same extent as the carboxyl and alcohol groups from which the ester was made. While those groups would indeed make the molecule much more soluble in water, they are lost in the condensation reaction (in the form of water molecules). Further to this, the ester linkage is located in the middle(ish) of the molecule, somewhat shielded by the hydrophobic alkyl groups in the molecule.

Hope this helps

[MisterNeo]

2. In separating esters from reaction mixture in esterification why can we add water to the products then use a separating funnel to extract the ester on the top layer? Aren't esters polar.. aren't they soluble in water? why can they be separated like this?

Esters are polar but not very because esters can only participate in hydrogen bonding via the negative double bonded oxygen lone pair.
The small esters are quite soluble and solubility decreases as ester size increases because the hydrocarbon chains start to get in the way of the hydrogen bonding.
Hence, big esters with around 5+ carbons start to become insoluble in water as they have weaker interactions with water.