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

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bleakarcher

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re: HSC Chemistry Marathon Archive

Just re-read the question and realised my answer was unnecessarily long lol.
 

nightweaver066

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re: HSC Chemistry Marathon Archive

A galvanic cell is a battery consisting of two seperate half-cells both which consist of a metal (electrode) in an electrolyte solution, one at which a oxidation reaction occurs (anode-negative electrode) and one at which a reduction reaction occurs (cathode-positive electrode), a salt bridge (usually immersed in some electrolyte solution) which allows for the migration of ions between the electrolyte solutions, and a wire connecting the anode to the cathode. A galvanic cell therefore runs on a redox reaction. This redox reaction will form a total EMF (electromotive force - a measure of the induced voltage at a negligibly small current) which allows for the movement of electrons through the wire. The direction of flow of electrons is such that they move from the anode (more reactive metal) to the cathode (less reactive metal). An example of a galvanic cell is the copper-zinc cell in which copper metal is immersed in copper sulfate solution and zinc metal is immersed in zinc sulfate solution with a neutral salt bridge. At the anode: Zn->Zn(2+)+2e- and at the cathode: Cu(2+)+2e->Cu. However, this means an increase in the Zn(2+) concentration in the aqueous ZnSO4 solution and an decrease in the Cu(2+) concentration in the aqueous CuSO4 solution. In order to maintain electrical neutrality in the half cells, a salt bridge is connected from from one electrolyte to the other so that Zn(2+) ions can flow to the CuSO4 solution (opposite to the direction of electron flow) and SO4(2-) can flow to the ZnSO4 solution (in the direction of the electrons). This creates a continuous current throughout the copper-zinc cell.

Please be as critical as you wish.
It all sounds pretty good, just that you should include that the way the cell is set up forces electrons to move from the anode to the cathode through an external wire thus creating an electric current (flow of charges). The salt bridge simply acts to maintain electrical neutrality (due to build up of charges) thus completing the circuit allowing for, as you said, a continuous current.
 

bleakarcher

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re: HSC Chemistry Marathon Archive

It all sounds pretty good, just that you should include that the way the cell is set up forces electrons to move from the anode to the cathode through an external wire thus creating an electric current (flow of charges). The salt bridge simply acts to maintain electrical neutrality (due to build up of charges) thus completing the circuit allowing for, as you said, a continuous current.
Okay thanks for that. I don't quite understand what you by the bolded though. If you could please clarify, that would be great :).

Edit: Did the bolding lol.
 
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deswa1

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re: HSC Chemistry Marathon Archive

WHY does a galvanic cell work? As in, you have two seperate half cells (say of zinc and copper). How does the zinc know to oxidise?
 

nightweaver066

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Okay thanks for that. I don't quite understand what you by the bolded though. If you could please clarify, that would be great :).
I just bolded it to show that i changed something. Most sources use the word "migration" so its best to stick to what is taught as keywords are pretty important.
 

bleakarcher

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WHY does a galvanic cell work? As in, you have two seperate half cells (say of zinc and copper). How does the zinc know to oxidise?
Good question, I don't know the exact answer to why it all happens but here's what I think. Zinc has a tendency to be oxidised and so, when it is, a voltage is induced which allows for the movement of electrons from the zinc electrode to the copper one. Hence Cu(2+) is oxidised and moves from the solution to the metal forming Cu. Probably not right, I've asked my teacher this question before and he didn't really answer it properly.
 

bleakarcher

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I just bolded it to show that i changed something. Most sources use the word "migration" so its best to stick to what is taught as keywords are pretty important.
lol shit sorry. When I said 'I don't quite understand what you mean by the bolded' I meant what I was going to bold and forgot to lol.

/fail
 

nightweaver066

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lol shit sorry. When I said 'I don't quite understand what you mean by the bolded' I meant what I was going to bold and forgot to lol.

/fail
The cell is set up with two separate half cells. As zinc oxidises, electrons are released and travel to the cathode. By separating the cells by an external wire, the electrons are forced to travel through this (the path with minimal resistance) to reach the cathode. An electric current is produced as it is a flow of charges.
 

bleakarcher

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The cell is set up with two separate half cells. As zinc oxidises, electrons are released and travel to the cathode. By separating the cells by an external wire, the electrons are forced to travel through this (the path with minimal resistance) to reach the cathode. An electric current is produced as it is a flow of charges.
Oh right, okay thanks for that. Also, special thanks to you and deswa for offering everyone constructive criticism, should help us a lot :).
 

nightweaver066

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re: HSC Chemistry Marathon Archive

The structure below shows one example of a polymer classified as a type of nylon.



Draw structural formulae of the monomers which produced the nylon polymer shown above.
 

Eg155

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re: HSC Chemistry Marathon Archive

WHY does a galvanic cell work? As in, you have two seperate half cells (say of zinc and copper). How does the zinc know to oxidise?
I don't know but here's a stab:
Because chemical reactivity, which the table of standard potential relays in order of increasing reactivity, correlates to the physical property- first ionization energy.
Zinc has a higher ionization energy than copper and thus will oxidize more readily than a metal with a lower ionization energy, in this case- Copper. Cu is more reactive than Zn and so the ability of the the Cu metal to be reduced is increased. Something like 'a more reactive metal will displace a less reactive metal in a solution of one of its salts'. Cu's higher electronegativity, ability to receive e- to fill the outer shell(?) also means that the galvanic cell works to satiate the need to fulfill the octet rule-> stability.

It's probably not right, don't hate :(.
 

jarrodoliver1

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I don't know but here's a stab:
Because chemical reactivity, which the table of standard potential relays in order of increasing reactivity, correlates to the physical property- first ionization energy.
Zinc has a higher ionization energy than copper and thus will oxidize more readily than a metal with a lower ionization energy, in this case- Copper. Cu is more reactive than Zn and so the ability of the the Cu metal to be reduced is increased. Something like 'a more reactive metal will displace a less reactive metal in a solution of one of its salts'. Cu's higher electronegativity, ability to receive e- to fill the outer shell(?) also means that the galvanic cell works to satiate the need to fulfill the octet rule-> stability.

It's probably not right, don't hate :(.
Sounds pretty spot on to me :3
 

bleakarcher

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re: HSC Chemistry Marathon Archive

New question.

A 0.2845 g sample of impure sodium carbonate required 24.65 mL of an HCl solution for
titration. A 0.2204 g sample of pure sodium carbonate required 20.06 mL of the HCl solution
of the same concentration.
What is the percentage of sodium carbonate in the sample?
 

tabasomarya

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Discuss how knowledge of the structure of the atom led to the development of such things as the nuclear reactor, radioisotopes and Geiger counter.
 

Eg155

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re: HSC Chemistry Marathon Archive

I don't know but here's a stab:
Because chemical reactivity, which the table of standard potential relays in order of increasing reactivity, correlates to the physical property- first ionization energy.
Zinc has a higher ionization energy than copper and thus will oxidize more readily than a metal with a lower ionization energy, in this case- Copper. Cu is more reactive than Zn and so the ability of the the Cu metal to be reduced is increased. Something like 'a more reactive metal will displace a less reactive metal in a solution of one of its salts'. Cu's higher electronegativity, ability to receive e- to fill the outer shell(?) also means that the galvanic cell works to satiate the need to fulfill the octet rule-> stability.

It's probably not right, don't hate :(.
It's wrong people, the flaws have been seen. So.many.flaws. Very sorry, I should have thought out a proper answer before slashing the keyboard.
New answer:


Ionization energy also increases as you follow the periodic table from left to right across the periods and decreases going down the groups; so, from this it is correct to say that Zn has a higher ionization energy than Cu (even though they are right next to each other lol).

However, the reactivity of a metal increases as their ionization energy decreases. So Zn has a lower ionization energy than Cu which makes Zn more reactive than Cu (which would mean that Cu would oxidise more easily and Zn be reduce because it's much easier to plonk off those electrons being more reactive ;D )
But:
A negative reduction potential (EMF (Zn)= -0.76 V): tells us that the zinc ion is harder to reduce (is a worse oxidizing agent) than is the hydrogen ion; this also tells us that zinc metal is a better reducing agent (meaning it will, in turn, become oxidised itself) than is hydrogen gas (every standard potential is measured against Hydrogen ( 0.00 V)

The standard reduction potential for the copper electrode is : +0.337 V:
A positive reduction potential tells us that the copper ion can be reduced more easily (is a better oxidizing agent) than is the hydrogen ion; this also tells us that copper metal is a worse reducing agent than is hydrogen gas.

From this, it can deduced that yes, Zn> Cu in terms of reactivity (according to the periodic table and the the elements' ionization energies) but when viewed on the Standard Potentials Table, the negative reduction potential of Zn means that an electrolytic cell is required for the reaction to occur as the reaction will not occur spontaneously as a positive reducing potential (such as Cu in this case) will.

Zn being more reactive will more easily 'give-up' its electrons would make much more sense, but since the standard potentials dictates that the -/+ voltage and the reactivity series that followed from it, the opposite is true. Zn needs more energy due to its neg voltage and so is more easily oxidised than reduced.

When I said "Cu is more reactive than Zn"I made a typo there, very sorry if I confused you; I wrote it very quickly and must of lost my train of thought- where's a periodic table and a txt book when you need one hey :). So, sub in "Zn is more reactive than Cu" :) and the rest that follows is incorrect too- it's correct if you switch the elements around (like Cu where Zn is and vice versa) haha. But what I have said in terms of negative and positive reduction potentials is a better justification for the question.

A member said: "Don't metals with lower ionization energies oxidise more readily compared to metals with higher ionzation energies?"- this is completely correct and follows as a trend on the SPT, but I suppose with Zn and Cu being transition metals and only one atomic number apart there is an exception to the 'rule' due to the +/- voltage charge as explained above .
 

someth1ng

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re: HSC Chemistry Marathon Archive

For those questions, you talk about the tendency of certain certain elements to oxidise/reduce more than others.

In this case, zinc has a higher tendency to oxidise than copper and copper ions have a higher tendency to reduce than zinc ions. Due to this tendency, in a galvanic cell, the atoms in the zinc electrode to oxidise and copper ions to reduce in the adjacent half cell to produce an emf.
 

bleakarcher

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For those questions, you talk about the tendency of certain certain elements to oxidise/reduce more than others.

In this case, zinc has a higher tendency to oxidise than copper and copper ions have a higher tendency to reduce than zinc ions. Due to this tendency, in a galvanic cell, the atoms in the zinc electrode to oxidise and copper ions to reduce in the adjacent half cell to produce an emf.
lol, was off by the word 'higher'. Don't know how to forgot to include that.
 

Magical Kebab

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WHY does a galvanic cell work? As in, you have two seperate half cells (say of zinc and copper). How does the zinc know to oxidise?
Des can u confirm an answer to this?
 

Magical Kebab

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Compare the process of polymerisation of ethylene and glucose. Include 3 relevant chemical equations in your answer. (3 marks)
 

someth1ng

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re: HSC Chemistry Marathon Archive

Compare the process of polymerisation of ethylene and glucose. Include 3 relevant chemical equations in your answer. (3 marks)
A bit strange to allocate 3 marks to that question, a comparison and 3 equations is a bit too much - I think.
 
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