HSC 2012-2015 Chemistry Marathon (archive) (5 Viewers)

[Kaido]

re: HSC Chemistry Marathon Archive

Easiest two cells are Drycell (primary) and button cell (secondary, although rarely asked)
Know the cathode, anode and overall reaction (include the expected voltage). Be able to assess the impact on society, environment and future implications

 

[leehuan]

re: HSC Chemistry Marathon Archive

Easiest two cells are Drycell (primary) and button cell (secondary, although rarely asked)
Know the cathode, anode and overall reaction (include the expected voltage). Be able to assess the impact on society, environment and future implications

It's just unfortunate I did an assignment on the secondary cell where we drew out of a hat our cell. I got the Gratzell cell.

 

[Drsoccerball]

re: HSC Chemistry Marathon Archive

It's just unfortunate I did an assignment on the secondary cell where we drew out of a hat our cell. I got the Gratzell cell.

Talk about bad luck lol

 

[BlueGas]

re: HSC Chemistry Marathon Archive

Easiest two cells are Drycell (primary) and button cell (secondary, although rarely asked)
Know the cathode, anode and overall reaction (include the expected voltage). Be able to assess the impact on society, environment and future implications

Dry cell? Where can I find notes on that?

 

[BlueGas]

re: HSC Chemistry Marathon Archive

Yeh i looked at it and was like wtf, looks so hard.

I'm usually really efficient at rote learning things but i've dropped off on my efficent recently, and after a few days i'm starting to forget things more than i used to. What works for you when you rote learn things. Need to rote learn the end of my option topic for chem, the second half of option in biology, and the second half of the option in physics.

You know what Kap, stick with the dry cell, I think that's easier than the lead acid cell because I didn't really know the chemistry of it until I looked it up, and it is easier lol.

 

[BlueGas]

re: HSC Chemistry Marathon Archive

What is the answer and why?

 

[Ekman]

re: HSC Chemistry Marathon Archive

What is the answer and why?

Answer should be A.

By process of elimination, B is wrong because that is the electrolyte for the anode half-cell as the magnesium electrode is the anode. C is wrong as platinum doesn't take part in redox reactions because of its extremely low reactivity. D is wrong because if potassium was to be an electrode, then magnesium would of been the cathode as potassium is higher on the standard potentials, hence it contradicts the information given to us so it is wrong. A is correct because after looking at the info given to us, a platinum electrode is used, indicating that the oxidant must be some gas. After inspecting the standard potential, this gas must be chlorine gas, as it provides the desired cell potential. Therefore it would be appropriate to use HCl as an electrolyte as the equation of the reduction of chlorine gas is: Cl2 - > 2Cl- + 2e-. So the chlorine ions are able to enter HCl solution without reacting to anything.

 

[BlueGas]

re: HSC Chemistry Marathon Archive

Quick question about the dry cell, apparently it has small amounts of zinc so that isn't considered as an environmental issue, the main issue with dry cells is the increase in landfill as they are not rechargeable. But is zinc really not a problem since there's only small amounts?

 

[BlueGas]

re: HSC Chemistry Marathon Archive

Guys, the answer is D, but why?

 

[rand_althor]

re: HSC Chemistry Marathon Archive

One Dobson Unit is: 1 m * 1 m * 0.01 mm = 1 m * 1 m * (0.01)x10^-3 m = 1x10^-5 m³. So we have:
2.7x10²⁰ molecules / DU
4.48x10^-4 moles / 1x10^-5 m³ (Since there are 6.022x10²³ molecules in a mole)
44.84 moles / m³ (Dividing through by 1x10^-5)
Multiplying through by 220 we have 9863.83 moles / m³ which is roughly 10000 moles / m³

 

[Kaido]

re: HSC Chemistry Marathon Archive

Quick question about the dry cell, apparently it has small amounts of zinc so that isn't considered as an environmental issue, the main issue with dry cells is the increase in landfill as they are not rechargeable. But is zinc really not a problem since there's only small amounts?

By itself, dry cells don't pose much problems. However, it is precisely the vast accumulation of zinc that WILL POSE problems (because as you mentioned, batteries are disposed fairly quickly). You can now mention leeching into soils and all the other bs related to heavy metal ions (from your study in monitoring)

 

[BlueGas]

re: HSC Chemistry Marathon Archive

Need help for these two questions:

 

[BlueGas]

re: HSC Chemistry Marathon Archive

Easiest two cells are Drycell (primary) and button cell (secondary, although rarely asked)
Know the cathode, anode and overall reaction (include the expected voltage). Be able to assess the impact on society, environment and future implications

How is the button cell secondary when it's not rechargeable?

 

[Mr_Kap]

re: HSC Chemistry Marathon Archive

Need help for these two questions:

first one is B? and second one is D? just checking its correct so i dont explain a wrong method.

 

[BlueGas]

re: HSC Chemistry Marathon Archive

first one is B? and second one is D? just checking its correct so i dont explain a wrong method.

Yep both are correct.

 

[rand_althor]

re: HSC Chemistry Marathon Archive

For the first one: Using H=-mCΔT, we have H=-12540J=-12.54kJ. -12.54/-1367=0.00917 moles. Multiplying by the molar mass of ethanol you get 0.423g.

For the second one: The reaction is: (COOH)₂.2H₂O + 2NaOH -> Na₂(COO)₂.2H₂O + H₂O. Using n=m/M, we have 0.100 moles of oxalic acid. Using c=n/V the concentration of oxalic acid is 0.200M. Using c=n/V the number of moles in 25mL of oxalic acid is 0.005 moles. The amount of moles of sodium hydroxide is double the amount of moles of oxalic acid, so n(NaOH)=0.010. Using c=n/V, the concentration of sodium hydroxide is 0.549M.

 

[Kaido]

re: HSC Chemistry Marathon Archive

How is the button cell secondary when it's not rechargeable?

No, 'secondary' in this context means that it is NOT one of the primary cells you should learn about. The primary ones are Dry cell or Lead acid.
Every other cell is 'secondary'.

 

[porcupinetree]

re: HSC Chemistry Marathon Archive

I need some help with this question y'all. The marking guidelines state that we should list several problems with current use of petrochemicals, and several environmental effects. For the former, I thought of:
1. Petroleum is a non-renewable source: our supplies will most likely run out within the next century (if not the next few decades)
2. Using petrochemicals introduces more greenhouse gases (e.g. CO2) into the atmosphere than are being absorbed: it is an unbalanced process leading to excess greenhouse gases

For the latter, I thought of:
1. Generally, using biomass leads to a more carbon-neutral system, however, CO2 is often released by the processes involved in biomass production (e.g. agriculture). If efficient production systems are developed, greenhouse gases in atmosphere could be reduced, however, at present, economically viable systems have not been developed.
2. Using bacteria to produce biodegradable polymers (instead of producing non-biodegradable polymers using petrochemicals) will reduce the plastic in landfill, positively impacting the environment.

Are there any other factors I could address that you guys can think of?

 

[Ekman]

re: HSC Chemistry Marathon Archive

I need some help with this question y'all. The marking guidelines state that we should list several problems with current use of petrochemicals, and several environmental effects. For the former, I thought of:
1. Petroleum is a non-renewable source: our supplies will most likely run out within the next century (if not the next few decades)
2. Using petrochemicals introduces more greenhouse gases (e.g. CO2) into the atmosphere than are being absorbed: it is an unbalanced process leading to excess greenhouse gases

For the latter, I thought of:
1. Generally, using biomass leads to a more carbon-neutral system, however, CO2 is often released by the processes involved in biomass production (e.g. agriculture). If efficient production systems are developed, greenhouse gases in atmosphere could be reduced, however, at present, economically viable systems have not been developed.
2. Using bacteria to produce biodegradable polymers (instead of producing non-biodegradable polymers using petrochemicals) will reduce the plastic in landfill, positively impacting the environment.

Are there any other factors I could address that you guys can think of?

For the former;
-Due to the scarcity of petrochemicals, there is greater price volatility, resulting in the risk of economic and diplomatic crisis's.
-Petrochemicals are non-biodegradable, hence they fill up land-fills and cause extra issues of pollution to the environment

For the latter;
-Use of biomass results in issues such as food depletion, as crops that would normally be used for food, would instead be used for petrochemicals. Also there is the issues of excessive crop growth leads to nutrient deficiency in soils and increased demand for water, which is scarce especially during times of drought.
-Use of bacteria is also unethical
-When talking about the efficiency of converting biomass into petrochemicals, talk about how it is expensive and time-consuming.

 

[Mr_Kap]

re: HSC Chemistry Marathon Archive

For industrial chem people:

Can someone explain to me why Water is reduced in the Concentrated NaCl.

In Ahmad Shah notes it says "A lower Eo value means the reaction is more likely to occur", however:

the Na one is (-2.71V), while the water one is (-0.83V). Here the lower Eo value is Na, but i know that water is reduced, not Na.
Is Ahmad Shah statement wrong? Is it meant to say, "A less negative Eo value means the reaction is more likely to occur"?