HSC 2012-2015 Chemistry Marathon (archive) (3 Viewers)

[Drsoccerball]

re: HSC Chemistry Marathon Archive

In the attempt to reduce the amount of ozone depletion in the atmosphere the montreal protocol was put into place. Describe the effectiveness of another method. 2-3 marks

 

[BlueGas]

re: HSC Chemistry Marathon Archive

In the attempt to reduce the amount of ozone depletion in the atmosphere the montreal protocol was put into place. Describe the effectiveness of another method. 2-3 marks

HFCs are compounds that contain hydrogen, fluorine and carbon. They have been used as alternatives for CFCs as they have a zero ozone depleting potential, they are good alternatives of CFCs in terms of atmospheric health. However they aren't effective refrigerants compared to CFCs as they are slightly expensive.

 

[Drsoccerball]

re: HSC Chemistry Marathon Archive

HFCs are compounds that contain hydrogen, fluorine and carbon. They have been used as alternatives for CFCs as they have a zero ozone depleting potential, they are good alternatives of CFCs in terms of atmospheric health. However they aren't effective refrigerants compared to CFCs as they are slightly expensive.

I would also mention HCFC's

 

[BlueGas]

re: HSC Chemistry Marathon Archive

I would also mention HCFC's

You said describe another "method" not "methods". Here's another question: Describe how you would measure the concentration of dissolved oxygen in a water sample. (2 marks)

 

[BlueGas]

re: HSC Chemistry Marathon Archive

What is algal bloom and what is eutrophication? (2 marks).

 

[Drsoccerball]

re: HSC Chemistry Marathon Archive

What is algal bloom and what is eutrophication? (2 marks).

I've never heard of an "algal" bloom... Is that part of the new syllabus ?

 

[Mr_Kap]

re: HSC Chemistry Marathon Archive

You said describe another "method" not "methods". Here's another question: Describe how you would measure the concentration of dissolved oxygen in a water sample. (2 marks)

There are two main methods in measuring the concentration of dissolved oxygen in a water sample. The Winkler Method, and Electrolysis Method.

The Winkler Method is a titration method, where the dissolved oxygen oxidises manganese from Mn2+ to Mn(IV) in alkaline solution. Mn(IV) then oxidises iodide (I-) ions to iodine (I2) in acidic conditions. Iodine is then titrated against sodium thiosulfate (Na2S2O¬3) using starch as in indicator, to determine the concentration of dissolved oxygen.

The electrolysis method (oxygen sensor) is where electrolytic oxygen sensors use a platinum cathode and a silver anode, using the rate of electrolysis of oxygen at the platinum cathode to determine its concentration. This electrolysis causes current to flow, the current is measured and converted to an oxygen concentration.

 

[Mr_Kap]

re: HSC Chemistry Marathon Archive

I've never heard of an "algal" bloom... Is that part of the new syllabus ?

Are you serious. You've never heard of "algal bloom" before.

 

[BlueGas]

re: HSC Chemistry Marathon Archive

Are you serious. You've never heard of "algal bloom" before.

I know the answer to this lol, but I thought it might trick some of you, but seems not haha.

 

[Ekman]

re: HSC Chemistry Marathon Archive

This is how 1st in chem explained it to me. Combinations. Since the structure is 3D don't count isomers twice.

What do you mean by 'combinations'? I hope you don't mean the combinations, as in the mathematical topic for 3U known as 'Permutation and Combinations'.

 

[Drsoccerball]

re: HSC Chemistry Marathon Archive

Are you serious. You've never heard of "algal bloom" before.

Nop :L never heard of it

 

[Ekman]

re: HSC Chemistry Marathon Archive

What is algal bloom and what is eutrophication? (2 marks).

Algal booms are excessive algae growth, whereas eutrophication is the result of excessive nutrients in water.

 

[Drsoccerball]

re: HSC Chemistry Marathon Archive

What do you mean by 'combinations'? I hope you don't mean the combinations, as in the mathematical topic for 3U known as 'Permutation and Combinations'.

I "C" what you did there

 

[Mr_Kap]

re: HSC Chemistry Marathon Archive

Why is hardness quantitatively measured as mg/L of CaCO3, when Mg2+ ions also contribute to Hardness?

 

[BlueGas]

re: HSC Chemistry Marathon Archive

Because calcium carbonate is one of the more common causes of hardness, when the hardness is expressed as CaCO3, it is also calculated as if the magnesium were there as calcium.

 

[Drsoccerball]

re: HSC Chemistry Marathon Archive

Why is hardness quantitatively measured as mg/L of CaCO3, when Mg2+ ions also contribute to Hardness?

I think its because theres barely any magnesium ions in water when compared to calcium ?

 

[Mr_Kap]

re: HSC Chemistry Marathon Archive

Because calcium carbonate is one of the more common causes of hardness, when the hardness is expressed as CaCO3, it is also calculated as if the magnesium were there as calcium.

Magnesium can turn in calcium. WTF!

 

[porcupinetree]

re: HSC Chemistry Marathon Archive

There are two main methods in measuring the concentration of dissolved oxygen in a water sample. The Winkler Method, and Electrolysis Method.

The Winkler Method is a titration method, where the dissolved oxygen oxidises manganese from Mn2+ to Mn(IV) in alkaline solution. Mn(IV) then oxidises iodide (I-) ions to iodine (I2) in acidic conditions. Iodine is then titrated against sodium thiosulfate (Na2S2O¬3) using starch as in indicator, to determine the concentration of dissolved oxygen.

The electrolysis method (oxygen sensor) is where electrolytic oxygen sensors use a platinum cathode and a silver anode, using the rate of electrolysis of oxygen at the platinum cathode to determine its concentration. This electrolysis causes current to flow, the current is measured and converted to an oxygen concentration.

Just want to raise an interesting point about this - an information sheet I have about the Winkler method states that Mn2+ is oxidised to Mn3+. There's even a statement on Wikipedia concerning the confusion regarding the specific oxidation of manganese (https://en.wikipedia.org/wiki/Winkler_test_for_dissolved_oxygen#Sample_method).
Thought you might be interested to know that. Does anyone else have any info regarding the oxidation of manganese? I'd be curious to read it. Perhaps in an exam it would simply be better to be more general and say "manganese is oxidised by dissolved oxygen".

 

[leehuan]

re: HSC Chemistry Marathon Archive

I don't think we actually need to know about the Winkler method and what chemistry is involved in it, just that it's used to measure DO

 

[porcupinetree]

re: HSC Chemistry Marathon Archive

I don't think we actually need to know about the Winkler method and what chemistry is involved in it, just that it's used to measure DO

True, but briefly going over the method in a 5/6/7 marker will impress the hsc markers