HSC Chemistry Question Thread

[justwannawish]

A few random chemistry questions
1. Are saturated substances necessarily pure?
2. And with groups on polymer chains, why do they have to be on every second one?

[MisterNeo]

A few random chemistry questions
1. Are saturated substances necessarily pure?
2. And with groups on polymer chains, why do they have to be on every second one?

It depends on what "pure" is. Saturated substances can include alkanes such as methane in natural gas, which is a mixture. Mixtures contain multiple pure substances mixed. The definition of impure can be physical or chemical. Like, iron ore is impure, and dirty water is also impure.
For the polymer question, do you have a specific polymer? Something like PVC has the chlorine on every second carbon because the monomer is 2 carbons long.

[hansolo9]

Hi
can someone explain the different types of detergents and their strengths/weaknesses?

[MisterNeo]

Hi
can someone explain the different types of detergents and their strengths/weaknesses?

Hey
So there are 3 types of detergents: Anionic, cationic, and non-ionic.
Normal soap is bad at cleaning in hard or acidic water because of the forming of scum.
Anionic
-Doesn't form precipitates in hard of acidic water like soap.
-It's effectiveness is low because of the positive ions disturbing the sulfonate head's interaction with water.
-Phosphate builders are usually added to get rid of the ions, but this causes eutrophication.
Cationic
-Very effective in hard or acidic water because the positive charge repels positive ions.
-Bad in alkaline water because it attracts OH^-.
-Non-biodegradable, as cationic detergents are antimicrobial and kill the bacteria responsible for decomposition.
-Sucks at cleaning glass because of glass having a slight negative charge, which causes cationic molecules to cling.
Non-ionic
-Good at cleaning both acidic, basic, and hard water because it has no charge. However, it is still weak in acidic/hard water because all the oxygens in the structure cause it to have a slight negative charge that can attract positive ions.
-Doesn't form scum.

High branching reduces biodegradability of all detergents.
Hope this helps

[justwannawish]

It depends on what "pure" is. Saturated substances can include alkanes such as methane in natural gas, which is a mixture. Mixtures contain multiple pure substances mixed. The definition of impure can be physical or chemical. Like, iron ore is impure, and dirty water is also impure.
For the polymer question, do you have a specific polymer? Something like PVC has the chlorine on every second carbon because the monomer is 2 carbons long.

Thank you for your answer! I was wondering about polystyrene but also in general. So is it  always based on the number of carbons in the monomer?
2. What are the conditions for two compounds go form a condensation polymer?
3. How do we deconstruct polymers into their respective monomers?

[bsdfjnlkasn]

Thank you for your answer! I was wondering about polystyrene but also in general. So is it  always based on the number of carbons in the monomer?
2. What are the conditions for two compounds go form a condensation polymer?
3. How do we deconstruct polymers into their respective monomers?

Hey there!

So since polystyrene has the addition of a benzene ring (a.k.a group) to the carbon chain, it is not pure.

A condensation polymer is one that upon polymerisation, releases a small molecule (usually water). So it's crucial that the monomers that go into forming a condensation polymer chain contain functional groups that can eject these small molecules (NH3 is another example) upon joining. Some functional groups involved in condensation polymerisation include(where R is some molecular structure)

•   Alkanol: R – OH
- When it joins with another alkanol, a water molecule is released, causing the remaining oxygen to become apart of the polymer
•   Carboxylic acid: R – COOH
•   Amine: R – NH2
•   Ester: R – COO – R

3. In terms of 'deconstructing polymers' to find what the original monomer was, we need to consider whether it initially underwent condensation or addition polymerisation. If it was condensation, we have to consider what molecule was released and what functional group that hints at being present in the monomer. Then by becoming familiar with the list above, we can pick which it was most likely to be (especially if we know the compound).

On the other hand, if it was addition polymerisation, we would simply need to look at a diagram for repetition, from which we can section off the different monomers and eventually identify theme. For example, PVC will have a Cl on every alternating carbon and so because that repeats, we can deduce that the monomer is just chloroethene. We know it's ethene (i.e. presence of a double bond) because during addition polymerisation, we break the double bond of the unsaturated molecule and pair monomers across the newly released bonds (free electrons)

[winstondarmawan]

Thank you for your answer! I was wondering about polystyrene but also in general. So is it  always based on the number of carbons in the monomer?
2. What are the conditions for two compounds go form a condensation polymer?

I think for the HSC, the polymers we focus on are simple and usually an alkene of some variation, which is why the ones we encounter you see the side chain on 'every second one'. Don't need to worry about this too much.
However, you should note that the benzene ring (for polystyrene) actually contains more carbons. So you shouldn't say 'carbons in the monomer'.
2. Monomers must have a functional group each.
Hope this helps!

Anyways, I have a couple of questions:
a) Compare the process of polymerisation of ethylene and glucose. Include relevant chemical equations. (3)
b) Explain the relationship between the structures and properties of three different polymers from ethylene and glucose, and their uses. (5)
For this question, I chose to talk about LDPE, HDPE and cellulose. I could talk about relationship between LDPE and HDPE and their uses but I'm stumped for cellulose - in terms of the relationship between LDPE and HDPE.

And also this question:
https://scontent-syd2-1.xx.fbcdn.net/v/t34.0-12/22054551_1336533976472122_2117348658_n.jpg?oh=35cfe5c35729edbffc00a89dce11c179&oe=59CDE1E2

Thanks in advance!

[bsdfjnlkasn]

Anyways, I have a couple of questions:
a) Compare the process of polymerisation of ethylene and glucose. Include relevant chemical equations. (3)
b) Explain the relationship between the structures and properties of three different polymers from ethylene and glucose, and their uses. (5)
For this question, I chose to talk about LDPE, HDPE and cellulose. I could talk about relationship between LDPE and HDPE and their uses but I'm stumped for cellulose - in terms of the relationship between LDPE and HDPE.

And also this question:
https://scontent-syd2-1.xx.fbcdn.net/v/t34.0-12/22054551_1336533976472122_2117348658_n.jpg?oh=35cfe5c35729edbffc00a89dce11c179&oe=59CDE1E2

Thanks in advance!

Hey! Let's have a look at these questions

I would structure the first one in a table format to ensure that you're directly showing differences and similarities (i.e. comparing). So check out the screenshot to help you clarify

[MisterNeo]

Anyways, I have a couple of questions:
a) Compare the process of polymerisation of ethylene and glucose. Include relevant chemical equations. (3)
b) Explain the relationship between the structures and properties of three different polymers from ethylene and glucose, and their uses. (5)
For this question, I chose to talk about LDPE, HDPE and cellulose. I could talk about relationship between LDPE and HDPE and their uses but I'm stumped for cellulose - in terms of the relationship between LDPE and HDPE.

And also this question:
https://scontent-syd2-1.xx.fbcdn.net/v/t34.0-12/22054551_1336533976472122_2117348658_n.jpg?oh=35cfe5c35729edbffc00a89dce11c179&oe=59CDE1E2

Thanks in advance!

So for the polymerisation of the ethylene and glucose, you would talk about the fact that polyethylene is produced via addition, whereas cellulose is produced via condensation. You would draw the monomer units in the equation, showing the water being a byproduct.
For the second question, cellulose has a lot of hydroxyl groups in its monomer units that create very strong bonds between polymer chains. This doesn't allow water to interact with the cellulose, hence cellulose is insoluble in water and really good as plant material.
For the last question:
Zn E^o= -0.76V
Fe E^o= +0.44V
Ca E^o= +2.87V
Cu E^o= -0.34V
Mn E^o= +1.18V
A potential would need to be positive since it needs to be spontaneous. Just add them up for each one, and the Ca/Mn are positive values with Mn being the lowest.

[clarence.harre]

This question is from the 2007 paper. I still don't agree with the answer. If someone could please explain why I'm wrong, that'd be much appreciated.

Question:
Which term best describes the role of the chlorine free radical in this process?
(A)    Anion
(B)    Catalyst
(C)    Initiator
(D)    Oxidant

Response:
I lean towards B and C. Since we're taught catalysts just lower the activation energy and not actually participate in the reaction, and given that in polymerisation the initiator series joins in, why is the answer B and not C?
 So, I figure it must be C - in polymerisation the initiator series catalyses the reaction and joins in. But the answer is B. Why?

[justwannawish]

I think for the HSC, the polymers we focus on are simple and usually an alkene of some variation, which is why the ones we encounter you see the side chain on 'every second one'. Don't need to worry about this too much.
However, you should note that the benzene ring (for polystyrene) actually contains more carbons. So you shouldn't say 'carbons in the monomer'.
2. Monomers must have a functional group each.
Hope this helps!

Anyways, I have a couple of questions:
a) Compare the process of polymerisation of ethylene and glucose. Include relevant chemical equations. (3)
b) Explain the relationship between the structures and properties of three different polymers from ethylene and glucose, and their uses. (5)
For this question, I chose to talk about LDPE, HDPE and cellulose. I could talk about relationship between LDPE and HDPE and their uses but I'm stumped for cellulose - in terms of the relationship between LDPE and HDPE.

And also this question:
https://scontent-syd2-1.xx.fbcdn.net/v/t34.0-12/22054551_1336533976472122_2117348658_n.jpg?oh=35cfe5c35729edbffc00a89dce11c179&oe=59CDE1E2

Thanks in advance!

Hey there!

So since polystyrene has the addition of a benzene ring (a.k.a group) to the carbon chain, it is not pure.

A condensation polymer is one that upon polymerisation, releases a small molecule (usually water). So it's crucial that the monomers that go into forming a condensation polymer chain contain functional groups that can eject these small molecules (NH3 is another example) upon joining. Some functional groups involved in condensation polymerisation include(where R is some molecular structure)

•   Alkanol: R – OH
- When it joins with another alkanol, a water molecule is released, causing the remaining oxygen to become apart of the polymer
•   Carboxylic acid: R – COOH
•   Amine: R – NH2
•   Ester: R – COO – R

3. In terms of 'deconstructing polymers' to find what the original monomer was, we need to consider whether it initially underwent condensation or addition polymerisation. If it was condensation, we have to consider what molecule was released and what functional group that hints at being present in the monomer. Then by becoming familiar with the list above, we can pick which it was most likely to be (especially if we know the compound).

On the other hand, if it was addition polymerisation, we would simply need to look at a diagram for repetition, from which we can section off the different monomers and eventually identify theme. For example, PVC will have a Cl on every alternating carbon and so because that repeats, we can deduce that the monomer is just chloroethene. We know it's ethene (i.e. presence of a double bond) because during addition polymerisation, we break the double bond of the unsaturated molecule and pair monomers across the newly released bonds (free electrons)

Thank you so much! It really means a lot to me!

[angelahchan]

This question is from the 2007 paper. I still don't agree with the answer. If someone could please explain why I'm wrong, that'd be much appreciated.

Question:
Which term best describes the role of the chlorine free radical in this process?
(A)    Anion
(B)    Catalyst
(C)    Initiator
(D)    Oxidant

Response:
I lean towards B and C. Since we're taught catalysts just lower the activation energy and not actually participate in the reaction, and given that in polymerisation the initiator series joins in, why is the answer B and not C?
 So, I figure it must be C - in polymerisation the initiator series catalyses the reaction and joins in. But the answer is B. Why?

This question is from the 2007 paper. I still don't agree with the answer. If someone could please explain why I'm wrong, that'd be much appreciated.

Question:
Which term best describes the role of the chlorine free radical in this process?
(A)    Anion
(B)    Catalyst
(C)    Initiator
(D)    Oxidant

Response:
I lean towards B and C. Since we're taught catalysts just lower the activation energy and not actually participate in the reaction, and given that in polymerisation the initiator series joins in, why is the answer B and not C?
 So, I figure it must be C - in polymerisation the initiator series catalyses the reaction and joins in. But the answer is B. Why?

The initiator, e.g. in making polyethylene isn't a catalyst because it gets incorporated into the reaction, whereas chlorine is a catalyst because as you said, it doesn't get used up

(so it's attached along the chain as it propogates)

[MisterNeo]

Question:
Which term best describes the role of the chlorine free radical in this process?
(A)    Anion
(B)    Catalyst
(C)    Initiator
(D)    Oxidant

From my understanding, initiators are the species that creates the free radicals to be used in the initiation stage and is used up, which would be the CFC itself (? kinda like the peroxide in LDPE). The chlorine radical does in a way act as a catalyst because it provides an alternate reaction pathway.


Overall equation is:

A catalyst does react with the species involved but it reforms itself, as that is how it lowers activation energy since it is easier using chlorine path rather than ozone + oxygen radical.

[anotherworld2b]

Hi I was hoping if someone could help me. I received my test back and got quite a few multiple choice questions wrong.
I was wondering how can you compare the pH of solutions of these compounds? (ethanol, ethanoic acid, enthanamine)?

Q2. Which one of the following compounds is least likely to be one of the amino acids that undergo a condensation reaction to form a protein?
a) H2NCH(CH2OH)COOH
b) H2NCH(CH2OOH)COOH
c) H2NCH2(CH2SH)COOH
d) H2NCH(CH2CH2CONH2)COOH

Q3. Which of the following substances will not act as a soap?
a) CH3(CH2)16COOK
b) CH3(CH2)16COOH
c) CH3(CH2)14COONa
d) CH3(CH2)12C6H4SO3Na

[MisterNeo]

I was wondering how can you compare the pH of solutions of these compounds? (ethanol, ethanoic acid, enthanamine)?

In increasing order: ethanoic acid, ethanol, ethylamine.
Acid is pretty straight forward. Ethanol is fairly neutral with a very slight basic pH because of its -OH group. Most amines are based because they are derived from ammonia.

Which of the following substances will not act as a soap?
a) CH3(CH2)16COOK
b) CH3(CH2)16COOH
c) CH3(CH2)14COONa
d) CH3(CH2)12C6H4SO3Na

Answer B will not act as a soap because it is insoluble scum. One of the limitations of carboxylate soaps is that it sucks in acidic water because the soap ion gains a H⁺ ion to form scum. The other answers are soaps, and D is an anionic detergent.
Not sure about the second question though since without a structural diagram, it's a bit difficult.