Galvanic Cells Prac (1 Viewer)

[kloudsurfer]

Hey,
Could someone please help me with these questions?

Why does the volume of electrolyte in the half cell affect the current but does not significantly alter the voltage produced by the cell?

When I did the experiment it was the other way around!! Anyway, i dont fully undestand what current and voltage are, so that doesnt help. Would anyone be able to explain it to me? Also,

Why does the current output of the cell decrease steadily with time? Does it have to do with the ions from one half cell moving across the salt bridge and 'contaminating' the solution in the other half cell?

 

[Forbidden.]

Hey,
Could someone please help me with these questions?

Why does the volume of electrolyte in the half cell affect the current but does not significantly alter the voltage produced by the cell?

When I did the experiment it was the other way around!! Anyway, i dont fully undestand what current and voltage are, so that doesnt help. Would anyone be able to explain it to me? Also,

Why does the current output of the cell decrease steadily with time? Does it have to do with the ions from one half cell moving across the salt bridge and 'contaminating' the solution in the other half cell?

1)

Remember Le Chatelier ? A system would shift to maintain equlibrium ? Shoving in more reactants will make the reaction produce more products to even out.

Let's use Magnesium and Silver as an example.
Working it out, the redox reaction is Mg_(s) + 2Ag⁺ --> Mg²⁺ + 2Ag_(s) ...

Oh as from my experiment with the magnesium ans silver half-cells, the magnesium electrode will dissolve into magnesium ions and the silver electrode will expand and the ion solution will be depleted.
"Contamination" does not occur, if you refill more magnesium ions in the magnesium half-cell there will be a "fresh" supply of current.

Current (and similarly voltage) can be affected if the two solutions:

Are more or less than ONE MOLE.
Operate at more or less than SLC (Standard Laboratory Conditions, 25 degrees Celsius and 1 atmosphere).
Have dirty electrodes .


2)

"i dont fully undestand what current and voltage" - it's statements like these that make me wanna say I WISH YOU DID PHYSICS. But I'll be happy to try to explain to you.


Current is the actual flow of electrons in a circuit, the unit is Amps
Resistance is how much electrons can be prevented from flowing is the the unit is Ohms

High current kills you, not the voltage (unless the resistance is low enough)
It's the volts that jolts but its the mils that kills.



What a nice problem to solve ?

Use the formula V=IR

Note:
100mA current is enough to kill you even if your skin is dry.
20mA is enough to shock you silly, just less than 10mA is noticeable.

Theres two uninsulated cables, A has resistance of 1000 Ohm and B has resistance of 55000 Ohms. Both are 5000 Volts, sounds dangerous ? Which one ?

If you touch cable A:

I = V/R = (5000/1000) = 5A

That's 50 times more than enough to kill you.


If you touch cable B:

I = V/R = (5000/55000) = 0.09A=9mA

You could feel some tingling, won't kill you.

As you can see, both voltages of 5000V sounds dangerous, but it's the low resistance of cable A that has a high current to kill you.

 

[rama_v]

Hey,
Could someone please help me with these questions?

Why does the volume of electrolyte in the half cell affect the current but does not significantly alter the voltage produced by the cell?

When I did the experiment it was the other way around!! Anyway, i dont fully undestand what current and voltage are, so that doesnt help. Would anyone be able to explain it to me? Also,

Why does the current output of the cell decrease steadily with time? Does it have to do with the ions from one half cell moving across the salt bridge and 'contaminating' the solution in the other half cell?

Who says the voltage does not change? The voltage of a cell changes with varying concentrations of electrolytes. The amount by which it changes is given by the Nernst equation, E(cell) = E(cell, standard conditions) - RT/nF * lnQ

where R is the gas constant, T is temperature in kelvin, n is the number of moles of electrons, F is Faraday's constant, and Q is the reaction quotient. You don't need to know this information for the HSC sylabus, btw, but you can read about it here.

http://en.wikipedia.org/wiki/Nernst_equation

 

[Forbidden.]

Who says the voltage does not change? The voltage of a cell changes with varying concentrations of electrolytes. The amount by which it changes is given by the Nernst equation, E(cell) = E(cell, standard conditions) - RT/nF * lnQ

where R is the gas constant, T is temperature in kelvin, n is the number of moles of electrons, F is Faraday's constant, and Q is the reaction quotient. You don't need to know this information for the HSC sylabus, btw, but you can read about it here.

http://en.wikipedia.org/wiki/Nernst_equation

The voltages on the standard potentials table (Given in the HSC, or by your teacher to use in class) are for 1 Molar solutions.

Simply put, different concentrations result in different voltages.

 

[kloudsurfer]

Yeah...I have learnt none of this. My teacher is terrible. Oh well.

A correction to the thing about the salt bridge - i know its not the ions from the other half cell that travel across it, its the ions from the salt bridge itself that travel into each half cell to balance out the charge.

The voltages on the standard potentials table (Given in the HSC, or by your teacher to use in class) are for 1 Molar solutions.

Simply put, different concentrations result in different voltages

Ooops, i should have been clearer. I meant the actual volume of electrolyte, not the concentration of electrolyte. I understand that different concentrations produce different voltages. In the experiment we added more elctrolyte to the beaker, we didnt increase the concentration.

Why does the volume of electrolyte in the half cell affect the current but does not significantly alter the voltage produced by the cell?

 

[kloudsurfer]

1)

Remember Le Chatelier? A system would shift to maintain equlibrium ? Shoving in more reactants will make the reaction produce more products to even out.

Let's use Magnesium and Silver as an example.
Working it out, the redox reaction is Mg_(s) + 2Ag⁺ --> Mg²⁺ + 2Ag_(s) ...

Oh as from my experiment with the magnesium ans silver half-cells, the magnesium electrode will dissolve into magnesium ions and the silver electrode will expand and the ion solution will be depleted.
"Contamination" does not occur, if you refill more magnesium ions in the magnesium half-cell there will be a "fresh" supply of current.

Current (and similarly voltage) can be affected if the two solutions:v

Are more or less than ONE MOLE.
Operate at more or less than SLC (Standard Laboratory Conditions, 25 degrees Celsius and 1 atmosphere).
Have dirty electrodes .


2)

"i dont fully undestand what current and voltage" - it's statements like these that make me wanna say I WISH YOU DID PHYSICS. But I'll be happy to try to explain to you.


Current is the actual flow of electrons in a circuit, the unit is Amps
Resistance is how much electrons can be prevented from flowing is the the unit is Ohms

High current kills you, not the voltage (unless the resistance is low enough)
It's the volts that jolts but its the mils that kills.



What a nice problem to solve ?

Use the formula V=IR

Note:
100mA current is enough to kill you even if your skin is dry.
20mA is enough to shock you silly, just less than 10mA is noticeable.

Theres two uninsulated cables, A has resistance of 1000 Ohm and B has resistance of 55000 Ohms. Both are 5000 Volts, sounds dangerous ? Which one ?

If you touch cable A:

I = V/R = (5000/1000) = 5A

That's 50 times more than enough to kill you.


If you touch cable B:

I = V/R = (5000/55000) = 0.09A=9mA

You could feel some tingling, won't kill you.

As you can see, both voltages of 5000V sounds dangerous, but it's the low resistance of cable A that has a high current to kill you.

Ahhhh....that actually makes sense!!!! I understand why the voltage does not change (with volume, not concentration!), but i still dont understand why the current changes.


EDIT: Wait, I think I understand something. Why does voltage decrease steadily over time? Is it because the concentration of the solutions changes? The solution containing the reductant would have a higher concentration of ions over time as the reaction progresses, and the solution containing the oxidant would have a lower concentration of ions. Duh. I think thats what you were trying to say. Thanks. Still stuck on the other question though.

 

[Forbidden.]

Yeah...I have learnt none of this. My teacher is terrible. Oh well.

A correction to the thing about the salt bridge - i know its not the ions from the other half cell that travel across it, its the ions from the salt bridge itself that travel into each half cell to balance out the charge.


Ooops, i should have been clearer. I meant the actual volume of electrolyte, not the concentration of electrolyte. I understand that different concentrations produce different voltages. In the experiment we added more elctrolyte to the beaker, we didnt increase the concentration.

Why does the volume of electrolyte in the half cell affect the current but does not significantly alter the voltage produced by the cell?

Let's say you perform an experiment with magnesium and zinc half cells, working it out, the Magnesium electrode shall dissolve while the Zinc electrode would be bigger as in the equation Mg +Zn²⁺ --> Zn +Mg²⁺
The solutions relatively have the same electrical resistance, so the voltage isn't affected as much.

Now obviously if your Magnesium electrodes disappear or you run out of Zinc ion solution there will be no current.
But if you add a "fresh" supply of ion solution or use new electrodes there shall be more current because the electrical conductivity is the same hence the voltage is not affected as much but the current increases.

Now why do they use thick cables in overhead power lines and small wires in small electronic toys ?
The resistance is inversely proportional to the cross-sectional area.
If you use even thicker cables the resistance is even less and more current can run through freely.
That's why if you add more solution more current can be produced and resistance is less because there is more space to travel through.

 

[kloudsurfer]

Let's say you perform an experiment with magnesium and zinc half cells, working it out, the Magnesium electrode shall dissolve while the Zinc electrode would be bigger as in the equation Mg +Zn²⁺ --> Zn +Mg²⁺
The solutions relatively have the same electrical resistance, so the voltage isn't affected as much.

Now obviously if your Magnesium electrodes disappear or you run out of Zinc ion solution there will be no current.
But if you add a "fresh" supply of ion solution or use new electrodes there shall be more current because the electrical conductivity is the same hence the voltage is not affected as much but the current increases.

Now why do they use thick cables in overhead power lines and small wires in small electronic toys ?
The resistance is inversely proportional to the cross-sectional area.
If you use even thicker cables the resistance is even less and more current can run through freely.
That's why if you add more solution more current can be produced and resistance is less because there is more space to travel through.

Ahhhh. I understand.

Ive been trying to get my teacher to explain it but he isnt very good at explaining. He changed his mind about which of the elctrodes was the cathode and which was the anode about 4 times when he was explaining the whole experiment lol.

Thanks

 

[Forbidden.]

Because I hear you are dedicated to English I do hope you remember what a consonant and vowel is. You know cathode, anode, reduction and oxidation.
C and R are both consonants and A and O are vowels, sooo ....


Remember this pattern.:
The cathode undergoes redction.
The anode undergoes oxidation.