HSC 2012-2015 Chemistry Marathon (archive) (1 Viewer)

[BlueGas]

re: HSC Chemistry Marathon Archive

Need help for all questions on this picture guys.

 

[BlueGas]

re: HSC Chemistry Marathon Archive

Question: Use these equations to account for magnesium oxide's classification as an amphoteric oxide.

 

[Mr_Kap]

re: HSC Chemistry Marathon Archive

Need help for all questions on this picture guys.

what year / where is this question from so i can check my answer before I post how i did it.

 

[BlueGas]

re: HSC Chemistry Marathon Archive

what year / where is this question from so i can check my answer before I post how i did it.

2008 lol.

 

[leehuan]

re: HSC Chemistry Marathon Archive

So weird seeing a galvanic cell after so many electrolytic cells recently....

a) The anode is the Ni||Ni2+ electrode, the cathode is the Pt||Cl-||Cl2 electrode. Electrons flow from the andoe to the cathode as always, so the arrow goes to the right
b) Ni(s) <--> Ni2+ + 2e-; Cl2(g) + 2e- <--> 2Cl-
So the overall equation must be Ni(s) + Cl2(g) --> Ni2+ + 2 Cl-
c) Standard cell potential = 1.36V + 0.24V = 1.50V
d) Chlorine gas concentrations decrease, thus the pipe connected to the source of Cl2 will appear less yellow in colour.
Because nickel (II) nitrate is pale green, as mentioned in the diagram, and we are increasing concentration of Ni2+, the anolyte will appear darker green.


(My brain is switched off so apologies if my answers happen to be in reverse or anything)

 

[leehuan]

re: HSC Chemistry Marathon Archive

Question: Use these equations to account for magnesium oxide's classification as an amphoteric oxide.

Because magnesium oxide can neutralise the hydrochloric acid (producing a salt and water), in effect it serves as a base in this regard.
Magnesium oxide dissolves to form a base in water, which means it is considered a basic oxide in this regard.

So your question confuses me. In fact... Read the third line you highlighted. You just said that MgO acts as a base, which evidently it does. If anything I'm pretty sure the amphoteric oxide is Al2O3

 

[Mr_Kap]

re: HSC Chemistry Marathon Archive

So weird seeing a galvanic cell after so many electrolytic cells recently....

a) The anode is the Ni||Ni2+ electrode, the cathode is the Pt||Cl-||Cl2 electrode. Electrons flow from the andoe to the cathode as always, so the arrow goes to the right
b) Ni(s) <--> Ni2+ + 2e-; Cl2(g) + 2e- <--> 2Cl-
So the overall equation must be Ni(s) + Cl2(g) --> Ni2+ + 2 Cl-
c) Standard cell potential = 1.36V + 0.24V = 1.50V
d) Chlorine gas concentrations decrease, thus the pipe connected to the source of Cl2 will appear less yellow in colour.
Because nickel (II) nitrate is pale green, as mentioned in the diagram, and we are increasing concentration of Ni2+, the anolyte will appear darker green.


(My brain is switched off so apologies if my answers happen to be in reverse or anything)

Some solid MATH there. lol. Should be 1.60 V.

Also for a) you know the Pt is not going to do anything as it is inert (and not on the data sheet), and the KCl is the electrolyte solution so that won't do anything, so therefore it must be Cl2 GAS, that occurs in the reaction.

 

[Ekman]

re: HSC Chemistry Marathon Archive

So weird seeing a galvanic cell after so many electrolytic cells recently....

a) The anode is the Ni||Ni2+ electrode, the cathode is the Pt||Cl-||Cl2 electrode. Electrons flow from the andoe to the cathode as always, so the arrow goes to the right
b) Ni(s) <--> Ni2+ + 2e-; Cl2(g) + 2e- <--> 2Cl-
So the overall equation must be Ni(s) + Cl2(g) --> Ni2+ + 2 Cl-
c) Standard cell potential = 1.36V + 0.24V = 1.50V
d) Chlorine gas concentrations decrease, thus the pipe connected to the source of Cl2 will appear less yellow in colour.
Because nickel (II) nitrate is pale green, as mentioned in the diagram, and we are increasing concentration of Ni2+, the anolyte will appear darker green.


(My brain is switched off so apologies if my answers happen to be in reverse or anything)

Not sure why the pipe will be changing colour. Also we aren't told that Chlorine gas is yellow, so im not sure if we could assume that.

 

[Mr_Kap]

re: HSC Chemistry Marathon Archive

Not sure why the pipe will be changing colour. Also we aren't told that Chlorine gas is yellow, so im not sure if we could assume that.

Yeh, I would have only talked about the Nickel turning into Ni2+ making the nickel half cell more green colour.

 

[leehuan]

re: HSC Chemistry Marathon Archive

I thought 1.5 seemed wrong, whatever.

Pt is inert, which is why it is the metal used to help connect the circuit. Personally in the exam I would've clumsily said that the chlorine gas is the required electrode, or I would've used the galvanic cell notation. It just seems counter-intuitive to me to put the gas there.

(The pipe was a dumb assumption that Cl2 wasn't being pumped, which of course it is. However 2 marks for the pale-green change seemed generous to me.)

 

[BlueGas]

re: HSC Chemistry Marathon Archive

Because magnesium oxide can neutralise the hydrochloric acid (producing a salt and water), in effect it serves as a base in this regard.
Magnesium oxide dissolves to form a base in water, which means it is considered a basic oxide in this regard.

So your question confuses me. In fact... Read the third line you highlighted. You just said that MgO acts as a base, which evidently it does. If anything I'm pretty sure the amphoteric oxide is Al2O3

What do you mean by this? Where did I mention Al2O3 on the page or in my post?

 

[leehuan]

re: HSC Chemistry Marathon Archive

You didn't. That was me saying I'm pretty sure MgO is basic 'because' Al2O3 is amphoteric.

(Reasoning - Across a period, oxides get more acidic. But in Period III, I remember there is only one amphoteric species. And I know P4O10 is thoroughly acidic and Na2O is very basic. Which leaves me with the options - MgO is quite basic, SiO2 is a bit acidic, and Al2O3 is amphoteric.)

 

[BlueGas]

re: HSC Chemistry Marathon Archive

What's the relationship between concentration and volume? For example a decrease in volume cause an increase in pressure, how about conc. and volume?

 

[Ekman]

re: HSC Chemistry Marathon Archive

What's the relationship between concentration and volume? For example a decrease in volume cause an increase in pressure, how about conc. and volume?

Depends on what definition of volume you are referring to. When you talk about the volume-pressure relationship, volume refers to the amount of gas in the container. However when you talk about the volume-concentration relationship, volume refers to the amount of liquid present in the sample. So assuming that mass of a particular substance, that is in solution, is constant then concentration is inversely proportional to volume.

 

[BlueGas]

re: HSC Chemistry Marathon Archive

Depends on what definition of volume you are referring to. When you talk about the volume-pressure relationship, volume refers to the amount of gas in the container. However when you talk about the volume-concentration relationship, volume refers to the amount of liquid present in the sample. So assuming that mass of a particular substance, that is in solution, is constant then concentration is inversely proportional to volume.

I read somewhere that a decrease in volume say on a reaction vessel, causes an increase in concentration of the reactants/products (it's something like that I'm not sure if it's the other way around), do you know what I'm trying to say?

 

[Ekman]

re: HSC Chemistry Marathon Archive

I read somewhere that a decrease in volume say on a reaction vessel, causes an increase in concentration of the reactants/products (it's something like that I'm not sure if it's the other way around), do you know what I'm trying to say?

That's what I said. concentration is proportional to 1/volume. Meaning that an increase in volume results in a decrease in concentration and vice versa.

 

[BlueGas]

re: HSC Chemistry Marathon Archive

Q: Previous theories of acids and bases could not explain why certain substances could be classified as them.

(i) Despite this, Arrhenius' theory was arguably a big step in understanding how acids and bases work. What did Arrhenius refer to as acids and bases, and what were some of the consequences of his theory? Include relevant equations where appropriate. (4)
(ii) Currently, Bronsted-Lowry theory is commonly accepted to explain acids and bases. Explain the principles of this theory and why it can be considered more valid than Arrhenius' theory.

Anyone wanna take a shot at this question too?

 

[leehuan]

re: HSC Chemistry Marathon Archive

Don't worry too much about that one since I already posted some tips which could be considered solutions.

I'm going to leave this question here because I struggled badly to find an answer ('08 HSC):

Using TWO examples, analyse how the features of catchment areas will determine the water treatment necessary to make the water safe to drink. (5)

 

[BlueGas]

re: HSC Chemistry Marathon Archive

Explain why alkanes and their corresponding alkenese have similar physical properties, but very different chemical properties. (3 marks)

 

[Drsoccerball]

re: HSC Chemistry Marathon Archive

Explain why alkanes and their corresponding alkenese have similar physical properties, but very different chemical properties. (3 marks)

Alkanes and their corresponding alkenes have similar physical properties because they both have similar dispersion forces and intermolecular forces. Therefore this results in both having very similar physical properties as physical properties are dictated by the forces in the molecule. On the other hand, they have different chemical properties dues to the difference in bonding. Alkenes have a double bond which makes them significantly more reactive than alkanes with only single bonds.