Predictions for Chemistry 2014 HSC? (7 Viewers)

[faisalabdul16]

- Bold 1: Poor leading sentence. Ozone depletion is caused by various substances not only CFC's which is what you implied
- Red: Useless
- Bold 2: Poor cause and effect (I know what you're trying to say, but it's not simply that free radicals directly cause ozone depletion)
- Overall: Poor answer because you didn't really tackle what is causing ozone depletion. All you said is CFC's destroy ozone. Sure, but overall the effect could be nil since the ozone layer is always naturally regenerating. (I.e. say that the increase in usage of ozone depleting substances has resulted in the rate of ozone destruction > rate of ozone regeneration and hence the phenomena "ozone depletion")


+1 (not on mark though)

Thanks for the advice! but for such a simple question worth 6 marks, as if you're not expected to mention what cfc's are? How is a marker supposed to know what they are or why they were used without you explaining?

Could you explain the "not simply that free radicals directly cause ozone depletion."

 

[iStudent]

Energy released when a compound undergoes combustion (burns)

Properrrrr pls

That's actually insufficient. I'm assuming heat of combustion i.e. molar heat of combustion. You would need to then explain how it's the heat released when 1 mol of the substance undergoes complete combustion with oxygen at standard atmospheric pressure and maybe say with the final products being carbon dioxide and water.

In chemistry it's all about going into detail

 

[QZP]

Critique so you can improve @emilios:
"While there is some market for the calcium chloride byproduct, the supply exceeds the demand" Providing an example of the "demand" is good if this Q worth >=4 marks
"significantly increase the level of TDS" NOPE...... (increased pH & hardening of water)
"resulting in decreased oxygen solubility" Low solubility is not a problem but the effect of it is (decreased DO)
"water used in the process must be sufficiently cooled before being discharged" How?

Random points: - Your last sentence was off. "majority of raw materials are recycled" Really? Aren't we using them?
- You could also mention other environmental issues such as ammonia leakage

 

[faisalabdul16]

That's actually insufficient. I'm assuming heat of combustion i.e. molar heat of combustion. You would need to then explain how it's the heat released when 1 mol of the substance undergoing complete combustion with oxygen at standard atmospheric pressure and maybe say with the final products being carbon dioxide and water.

It's not molar heat of combustion.

You can have heat of combustion on it's own. Plus they're not going to give 3+ marks for a question that requires you to define something

Hmm actually nah the last sentence with the oxygen should be in there, my bad thanks

 

[emilios]

Why does TDS effect marine life/water quality? Elaborate.

You did answer the question on point however. So that's a good call.

Haha I actually have no idea. Care to share?

 

[Queenroot]

That's actually insufficient. I'm assuming heat of combustion i.e. molar heat of combustion. You would need to then explain how it's the heat released when 1 mol of the substance undergoes complete combustion with oxygen at standard atmospheric pressure and maybe say with the final products being carbon dioxide and water.

In chemistry it's all about going into detail

That would be a proper definition lol

 

[QZP]

Thanks for the advice! but for such a simple question worth 6 marks, as if you're not expected to mention what cfc's are? How is a marker supposed to know what they are or why they were used without you explaining?

Could you explain the "not simply that free radicals directly cause ozone depletion."

You just needed to elaborate on what the free radicals do Your logic in that solution was: formation of free radicals --> boom ozone depletion. Instead it should be more like: formation of free radicals --> reaction with existing ozone --> destroy and cause ozone depletion. Just walk through it slowly otherwise the marker can pick at loopholes like that

 

[Queenroot]

Haha I actually have no idea. Care to share?

If you have no idea then re-read your notes. Make sure you have a model answer for the long response questions and adjust to the type of question it is. It would really help someone like you.

 

[iStudent]

It's not molar heat of combustion.

You can have heat of combustion on it's own. Plus they're not going to give 3+ marks for a question that requires you to define something

Hmm actually nah the last sentence with the oxygen should be in there, my bad thanks

https://www.google.com.au/search?ie...on+define&gfe_rd=cr&ei=FK8mVLmcN8aA4AKh4ID4BQ

googled heat of combustion definition.

also i have seen 2 markers asking "define molar heat of combustion". what i gave you is a sample answer i ripped off them.

 

[faisalabdul16]

You just needed to elaborate on what the free radicals do Your logic in that solution was: formation of free radicals --> boom ozone depletion. Instead it should be more like: formation of free radicals --> reaction with existing ozone --> destroy and cause ozone depletion. Just walk through it slowly otherwise the marker can pick at loopholes like that

Oh yeah of course, I just got a little lazy and didn't explain in words just wrote the equations which show all that!

In an exam, of course I would go into detail !

 

[faisalabdul16]

https://www.google.com.au/search?ie...on+define&gfe_rd=cr&ei=FK8mVLmcN8aA4AKh4ID4BQ

googled heat of combustion definition.

also i have seen 2 markers asking "define molar heat of combustion". what i gave you is a sample answer i ripped off them.

Ooooo okay thanks!

 

[iStudent]

"Define molar heat of combustion of a compound" is actually a syllabus dot point. I don't see why they wouldn't ask it (hsc papers like to steal questions directly from the syllabus sometimes)

 

[emilios]

Critique so you can improve @emilios:
"While there is some market for the calcium chloride byproduct, the supply exceeds the demand" Providing an example of the "demand" is good if this Q worth >=4 marks
"significantly increase the level of TDS" NOPE...... try again.
"resulting in decreased oxygen solubility" Low solubility is not a problem but the effect of it is
"water used in the process must be sufficiently cooled before being discharged" How?

Random points: - Your last sentence was off. "majority of raw materials are recycled" Really? Aren't we using them?
- You could also mention other environmental issues such as ammonia leakage

1) True - I should have some of the industrial applications of CaCl2 under my belt
2) I'm fairly certain that discharging calcium chloride into the environment does in fact increase the TDS to undesirable levels. Otherwise would you just say it causes an undesirable 'increase in these ions?'
3) Low solubility of oxygen --> fishies dying does it not? I said threatening aquatic life
4) Fair point about cooling the water, but that's kinda obscure isn't it. I can't find any information regarding it
5) Yeah I should've been more specific, it's not really the 'raw materials' that are recycled per se: the CO2 sourced from the decomposition of limestone is regenerated later on from the decomposition of sodium hydrogen carbonate. The ammonia is regenerated from the reaction of ammonium chloride and calcium hydroxide. I believe the brine can be recycled? The limestone cannot.
6) I definitely remember reading that this process was very mechanically efficient and virtually no ammonia was lost.

 

[emilios]

If you have no idea then re-read your notes. Make sure you have a model answer for the long response questions and adjust to the type of question it is. It would really help someone like you.

I'm googling 'effect of TDS on ecosystem' but all i'm getting is that "Spawning fishes and juveniles appear to be more sensitive to high TDS levels."

 

[QZP]

1) True - I should have some of the industrial applications of CaCl2 under my belt
2) I'm fairly certain that discharging calcium chloride into the environment does in fact increase the TDS to undesirable levels. Otherwise would you just say it causes an undesirable 'increase in these ions?'
3) Low solubility of oxygen --> fishies dying does it not? I said threatening aquatic life
4) Fair point about cooling the water, but that's kinda obscure isn't it. I can't find any information regarding it
5) Yeah I should've been more specific, it's not really the 'raw materials' that are recycled per se: the CO2 sourced from the decomposition of limestone is regenerated later on from the decomposition of sodium hydrogen carbonate. The ammonia is regenerated from the reaction of ammonium chloride and calcium hydroxide. I believe the brine can be recycled? The limestone cannot.
6) I definitely remember reading that this process was very mechanically efficient and virtually no ammonia was lost.

1) Just memorise that it's used as an additive in concrete to de-ice roads
2) Oh crap... I read it wrong. Yes youre right it does increase TDS SORRY. I guess you could talk about increased salinity for it (I'll need to google the effects of this and add it to my notes). Thanks Learnt something
3) Decreased DO not decreased solubility is what is harmful. Just being pedantic
4) They cool CaCl2 using heat exchangers and cooling towers that utilise ocean water
5) ://// I'd avoid all that because it's not really significant. Stick with only ammonia being recycled in the list of raw materials
6) Yes you are right It was an evaluate question. Mention something like the monitoring of equipment to check for ammonia leakages is very effective such that there is virtually no ammonia losses

 

[madharris]

just a heads up, it's going to be a question from the visual arts paper.

Those of you who don't do VA are in some deep shit.

You're welcome

 

[Queenroot]

I'm googling 'effect of TDS on ecosystem' but all i'm getting is that "Spawning fishes and juveniles appear to be more sensitive to high TDS levels."

lol

 

[faisalabdul16]

just a heads up, it's going to be a question from the visual arts paper.

Those of you who don't do VA are in some deep shit.

You're welcome

Nao

 

[faisalabdul16]

Does anyone know if one gets marked down for having extra info (info not required)

 

[photastic]

Does anyone know if one gets marked down for having extra info (info not required)

If criteria mentions, cohesive or succinct you would lose a mark but science marking criteria is super dodgy so KIS.