Trying to understand relevance of a salt bridge

[xeluc]

Ok. So I have two cups with NaCl in solution. I have a wide peice of string between them acting as a salt bridge. As expected. I see the blue color of Cu2+ in the solution in the cup that contains the anode. Here is where my question is. Just two seperate cups with electrolyte don't conduct electricity. that means that ions MUST physically flow through the solution to donate, take electrons.....Right? So why is only the Anode blue? Why don't the Cu2+ ions run over to the Cathode? I understand that a salt bridge is suposed to seperate compound created in the Anode and Cathode, but I don't understand how it's seperated. SOMETHING is obviously moving through the bridge. If not Cu2+, then what?

[P-man]

You're talking electrochemistry here man! I have the same questions as you!

[Skye]

Well electrolytes in water conduct electricity, and a salt bridge is electrolytes in water.

[xeluc]

Could you answer the rest of my questions?

[woelen]

Suppose, you have a salt bridge of NaCl-solution. You also have NaCl at the anode beaker and NaCl at the cathode beaker.

 

Now, if you apply a potential over the circuit, then the following happens:

 

Cathode gives off electrons. These are absorbed by water molecules and H2 is formed, together with OH(-):

 

2H2O + 2e ---> 2OH(-) + H2

 

This would cause buildup of negative charge at the cathode beaker. The salt bridge, however, supplies positively charged sodium ions, which neutralize the charge at the cathode beaker.

 

Now, you would have charge buildup in the salt bridge. This is compensated for, because chloride ions are moving from the salt bridge towards the anode beaker.

 

Now, you would have negative charge buildup at the anode beaker, due to migration of chloride ions from the salt bridge. This in turn is compensated for, because the copper anode 'eats' electrons:

 

Cu ---> Cu(2+) + 2e

 

The Cu(2+) goes in solution and compensates for the extra Cl(-).

 

In case of a graphite anode, you get 2Cl(-) --> Cl2 + 2e. In that case, the chloride ions are discharged and the electrons are 'eaten' by the anode.

 

This closes the circuit.

 

So, the net effect of the salt bridge is that positively charged ions from the bridge move to the cathode beaker and negatively charged ions move to the anode beaker. The salt bridge slowly is depleted and must be replaced by a fresh salt bridge every now and then. If electrolysis continues without replenishing, then the electrolysis goes slower and slower, but due to diffusion and electromotive forces also ions from the anode beaker (in your case Cu(2+)) can move towards the cathode beaker, although that would take a lot of time.

 

In industrial electrolysis processes, special membranes are used, which are selective and only certain (small) ions can pass through them. So, in such processes there is no need to replenish. But for home use, a salt bridge is a very good alternative.

[xeluc]

alright thanks

[P-man]

So the NaCl in woelen's explenation is the salt bridge?

[xeluc]

there is NaCl in both cups and the salt bridge, yes

[P-man]

Yeah, ok.