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

[BlueGas]

re: HSC Chemistry Marathon Archive

 

[Drsoccerball]

re: HSC Chemistry Marathon Archive

1) For something to be a primary standard it needs to be pure, anhydrous, high molecular weight etc... These properties are all in sodium bicarb. However is NaOH most of these properties are missing. Also titrating NaOH with another stronger base would be dodge af. Therefore NaOH isnt a suitable primary standard.

 

[Drsoccerball]

re: HSC Chemistry Marathon Archive

2)
i) Silver is the cathode
ii) KCl would be an inappropriate salt bridge as in reacts with the other solutions. Cl- forms a precipitate with Ag+ and therefore the cell would stop working.

 

[Drsoccerball]

re: HSC Chemistry Marathon Archive

3) Because cobalt 59 is found in nature and is easier to just add a neutron then starting with a smaller element.

 

[Mr_Kap]

re: HSC Chemistry Marathon Archive

Why is Cellulose insoluble?

 

[psyc1011]

re: HSC Chemistry Marathon Archive

Why is Cellulose insoluble?

Not enough information.

 

[Drsoccerball]

re: HSC Chemistry Marathon Archive

Why is Cellulose insoluble?

Its molecular weight is heeps high which means its not able to be soluble ( something like this)

 

[Mr_Kap]

re: HSC Chemistry Marathon Archive

Its molecular weight is heeps high which means its not able to be soluble ( something like this)

What about something to do with intramolecular and intermolecular forces. I remember learning something about Hydrogen bonds or something like that.

 

[Ekman]

re: HSC Chemistry Marathon Archive

1) For something to be a primary standard it needs to be pure, anhydrous, high molecular weight etc... These properties are all in sodium bicarb. However is NaOH most of these properties are missing. Also titrating NaOH with another stronger base would be dodge af. Therefore NaOH isnt a suitable primary standard.

Lol not sure why you would be titrating a base with another base, so that isn't a valid point. The reasons are because:
-its hygroscopic
-low molar mass
-its hard to obtain in pure solid form in order to the measure mass

 

[Mr_Kap]

re: HSC Chemistry Marathon Archive

Describe and Justify the conditions under which fermentation of sugars is promoted? 3 MARKS.

Discuss the properties of cellulose in relation to its structure. 2 MARKS.

 

[Drsoccerball]

re: HSC Chemistry Marathon Archive

Describe and Justify the conditions under which fermentation of sugars is promoted? 3 MARKS.

Discuss the properties of cellulose in relation to its structure. 2 MARKS.

Fermentation of sugars such as glucose require to have:
0 oxygen present - To not take part in the reaction?
Presence of yeast- Break down the sugar
37 degrees Celsius - perfect temperature for yeast to maximise production
and a low concentration of the alcohol - As the alcohol kills the yeast

Cellulose is a condensation polymer which is constructed of a chain of beta glucoses which are insoluble due to the large chain. It is also strong due to its extensive chaining.

 

[Mr_Kap]

re: HSC Chemistry Marathon Archive

Fermentation of sugars such as glucose require to have:
0 oxygen present - To not take part in the reaction?
Presence of yeast- Break down the sugar
37 degrees Celsius - perfect temperature for yeast to maximise production
and a low concentration of the alcohol - As the alcohol kills the yeast

Cellulose is a condensation polymer which is constructed of a chain of beta glucoses which are insoluble due to the large chain. It is also strong due to its extensive chaining.

Yeh, I'm not sure about the reason for having anerobic conditions either. I'm assumin the yeast reacts with oxygen and then fermentation doesn't occur as efficiently but i'm not sure.

 

[Drsoccerball]

re: HSC Chemistry Marathon Archive

lmaooo i remember during the fermentation experiment one of my mates pushed the cotton balls in the flask while no one was watching...

 

[BlueGas]

re: HSC Chemistry Marathon Archive

Yeh, I'm not sure about the reason for having anerobic conditions either. I'm assumin the yeast reacts with oxygen and then fermentation doesn't occur as efficiently but i'm not sure.

Low oxygen levels are need so the yeast doesn't prefer respiration over fermentation of glucose.

 

[BlueGas]

re: HSC Chemistry Marathon Archive

Quick question, how to test for the presence of lead? Add iodide ions or HCl? Or either will do?

 

[Drsoccerball]

re: HSC Chemistry Marathon Archive

Quick question, how to test for the presence of lead? Add iodide ions or HCl? Or either will do?

The thing is HCl also precipitates with silver and mercury hence that would not suffice. As a confirmation test we use KI and if a yellow precipitate forms then it shows that lead is present.

 

[Ekman]

re: HSC Chemistry Marathon Archive

The thing is HCl also precipitates with silver and mercury hence that would not suffice. As a confirmation test we use KI and if a yellow precipitate forms then it shows that lead is present.

Why not just go straight to the confirmation test and add iodide ions? If this was a question, I would just say add KI, and if a yellow precipitate forms, then lead is present since that is only ion that precipitates that colour.

 

[Drsoccerball]

re: HSC Chemistry Marathon Archive

Why not just go straight to the confirmation test and add iodide ions? If this was a question, I would just say add KI, and if a yellow precipitate forms, then lead is present since that is only ion that precipitates that colour.

I was telling him what would happen if he used HCl.

 

[BlueGas]

re: HSC Chemistry Marathon Archive

Because there was a HSC question and it said add "excess HCl" and the reaction was Pb2+ + 2Cl- -------> PbCl2(s)

 

[Drsoccerball]

re: HSC Chemistry Marathon Archive

Because there was a HSC question and it said add "excess HCl" and the reaction was Pb2+ + 2Cl- -------> PbCl2(s)

It probably gave the possible things insider and mercury and silver were not there