VCE Chemistry Question Thread

[mary1911997]

please need help specially part b as it does not make sense
In an electrolysis experiment, a student observed that an unknown metal anode with an oxidation number of +2 lost 0.208g while a total of 3.5mg of hydrogen gas was produced at the cathode. The temperature of the solution was 25C and the pressure was 740 mmHg
   a. How many Faradays of electricity was passed through the cell?
   b. What is the metal in the anode?

thanks

[HasibA]

questions:
1) primary cells are rechargeable, secondary cells arent. Would this be enough (better worded) for a distinguish question between primary and secondary cells?
2)do i have to know each example for primary cells, secondary cells and fuel cells? eg, fuel cell- phosphoric acid fuel cell, solid oxide fuel cell etc.
3) to what extent do i need to know electroplating? enough to do a definition and diagram or. ... more?

[jyce]

questions:
1) primary cells are rechargeable, secondary cells arent. Would this be enough (better worded) for a distinguish question between primary and secondary cells?
2)do i have to know each example for primary cells, secondary cells and fuel cells? eg, fuel cell- phosphoric acid fuel cell, solid oxide fuel cell etc.
3) to what extent do i need to know electroplating? enough to do a definition and diagram or. ... more?

1) You have this the wrong way around - primary cells AREN'T rechargeable, secondary cells ARE rechargeable. But yeah, that's the difference. You should also be able to explain why some cells are rechargeable, and why others aren't.

2) No. But you should be able to apply general information about galvanic cells and redox chemistry to given examples.

3) You should also be able to apply Faraday's laws of electrolysis to electroplating cells, and be able to explain which metals in the presence of an aqueous electrolyte can't be electroplated.

But the best way to figure out to what extent you need to know these things is to get an idea for yourself by doing VCAA exams!

please need help specially part b as it does not make sense
In an electrolysis experiment, a student observed that an unknown metal anode with an oxidation number of +2 lost 0.208g while a total of 3.5mg of hydrogen gas was produced at the cathode. The temperature of the solution was 25C and the pressure was 740 mmHg
   a. How many Faradays of electricity was passed through the cell?
   b. What is the metal in the anode?

thanks

Hi, mary1911997. I outlined how I would answer this question a few days ago. Was that not enough?

To be honest, this question isn't a very good one. Is the anode an ion? If so, there a few different possibilities with different mole ratios that you could apply. For example, Fe²⁺ and Sn²⁺ both have an oxidation number of +2 but they are oxidised in different mole ratios with electrons. And we need to know what the mole ratio with electrons is in order to determine the moles of the metal/metal ion and determine the metal's identity.

But I think the answer might be tin/tin(II) ions.

[HasibA]

-snip-

i always mix it up *sigh* thanks

[Adequace]

http://m.imgur.com/g30uwPg

For this question, I'm getting 1.75 * 10^4 but the book has 1.75 * 10^-6. My teacher also says the book is correct but I'm not so sure. I've used my calculator and it's pretty much a simple substitution question..

Can anyone confirm which answer is correct, thanks.

[sweetiepi]

http://m.imgur.com/g30uwPg

For this question, I'm getting 1.75 * 10^4 but the book has 1.75 * 10^-6. My teacher also says the book is correct but I'm not so sure. I've used my calculator and it's pretty much a simple substitution question..

Can anyone confirm which answer is correct, thanks.

I believe you are correct

[Swagadaktal]

(Image removed from quote.)

I believe you are correct

Beautiful working out +1

[jyce]

the question does not specify anything about the anode
also i asked other people and they told me it might be tin but how did you conclude that it is tin thats what is confusing me

Well, did you see my post from a few days ago?

I'm at uni right now so I'm not going to actually give you the answers, but I'll briefly describe the process of how to answer them.

a. You have the mass of hydrogen gas, and the molar mass of hydrogen gas is just 2 g mol^-1. n = m/M. Make sure that you convert the mass to grams, though!

b. Find the relevant half-equation in the electrochemical series for the production of hydrogen gas and apply stoichiometry to calculate the moles of electrons that must have been consumed to produce the 3.5 mg of H₂. The Faradays is the same as the moles of electrons.

c. As the oxidation number for the anode material is 2+, the mole ratio of anode material reacted : electrons released is 1 : 2. Use this knowledge to calculate the moles of anode material/metal. You also have the mass of anode material that reacted, so you can calculate the molar mass of the metal (again, use n = m/M). Once you have the molar mass, it's a simple matter of finding the correct metal from the periodic table.

I can't see why you need the temperature or pressure; maybe I've missed something. Can have a closer look when I get home if my above method doesn't get you the answer.

Is it tin??

[mary1911997]

Well, did you see my post from a few days ago?

yeah but the last part still does not make sense to me
i found the half equation which is 2H+ (aq) + 2e- ------> H2(g)
and i got the ratio is 1:2
but i do not know how to do  the rest of it

[Elizawei]

Answer's B right?

[Swagadaktal]

(Image removed from quote.)

Answer's B right?

Indeed.
As it is an ideal gas, PV=nRT,
V = nRT/P - with new conditions you get
V = 2 * T*n*R/2*P --- the 2 2's cancel out (hehe 2 2's)
so what you're left with is v = nRT/P  - same as initial as the two changes cancel each other out

[Elizawei]

Indeed.
As it is an ideal gas, PV=nRT,
V = nRT/P - with new conditions you get
V = 2 * T*n*R/2*P --- the 2 2's cancel out (hehe 2 2's)
so what you're left with is v = nRT/P  - same as initial as the two changes cancel each other out

Thanks Swag!   
Very helpful explanation 　　２　２　s haha

[mary1911997]

Well, did you see my post from a few days ago?

the question does not specify anything about the anode
also i asked other people and they told me it might be tin but how did you conclude that it is tin thats what is confusing me
Modify message

[jazzaa36]

I NEED URGENT HELP!
Given a temp vs time graph of well insulated calorimeter with 100ml of water, and using the sample conditions (V,I,t)

1. how would the graph alter if we use 200ml of good insulated calorimeter ?

2. how would the graph alter if we us 50ml of a poorly insulated calorimeter?

Like what effects will these have of the delta T need help asap! 

Thanks in advance though

[jyce]

I NEED URGENT HELP!
Given a temp vs time graph of well insulated calorimeter with 100ml of water, and using the sample conditions (V,I,t)

1. how would the graph alter if we use 200ml of good insulated calorimeter ?

2. how would the graph alter if we us 50ml of a poorly insulated calorimeter?

Like what effects will these have of the delta T need help asap! 

Thanks in advance though

1. The rate of change in temperature (i.e., the gradient of the graph) would be half.

2. The rate of change in temperature would be double.

This is assuming you haven't changed voltage or current, or the solvent being used.