electrochemical cell (1 Viewer)

[Mr Chi]

I am having trouble understanding concepts of the anode and cathode in an electrochemical cell where current is passed through dilute solutions. A galvanic cell's anode is negative and cathode is positive, but in an electrochemical cell where current is supplied the anode is positive and cathode negative? Is this because the flow of electrons is always from anode to cathode outside the cell and from cathode to anode inside the cell and with a electrochemical cell there is only cathode to anode?

 

[tennille]

There is some information on anodes/cathodes here:

http://en.wikipedia.org/wiki/Anode#Battery_or_galvanic_cell_anode

It explains the difference between galavanic cells and electrolytic cells.

 

[Riviet]

There was a discussion about the anode and cathode used in physics/chemistry in the physics forum. After some debate, the score was finally settled with this:

Definitions of anode (from Dictionary.com)
- A positively charged electrode, as of an electrolytic cell, storage battery, or electron tube.
- The negatively charged terminal of a primary cell or of a storage battery that is supplying current.

Definitions of cathode (from Dictionary.com)
- A negatively charged electrode, as of an electrolytic cell, a storage battery, or an electron tube.
- The positively charged terminal of a primary cell or a storage battery that is supplying current.

In electrochemistry, when looking at a galvanic cell, the anode is assigned to be negative because it is the NEGATIVE TERMINAL. Why?....because this is where electrons come from so it is initially negative. However, as electrons LEAVE the anode, the ELECTRODE has an OVERALL POSITIVE CHARGE because there is a deficiency of electrons.
Similarly with the cathode, it is assigned to be positive in electrochemistry because it is the POSITIVE TERMINAL. Why?....because this is where the electrons come in, so it is initially positive. However, as electrons ENTER the cathode, the ELECTRODE has an OVERALL NEGATIVE CHARGE because there is an excess of electrons.
Remember that all this occurs WITHIN A BATTERY and as a result:
- cathode (electrode) has become negatively charged
- anode (electrode) has become positively charged

Using that result, in an external circuit, a battery contains a negatively charged cathode and a positively charged anode. Now we can manipulate this and extract electrons to move FROM the cathode of the battery to flow to the anode on the other side of the battery. Why?....because the cathode has been negatively charged so there are electrons hovering around the cathode (as a result of the galvanic cell above). Hence, electrons (or cathode rays) are said to flow from cathode to anode. Hence, in a circuit diagram the cathode is assigned a shorter line than the anode. Therefore the cathode is the negative terminal and the anode is the positive terminal for an EXTERNAL CIRCUIT.

NOTE: The first definition of cathode and anode only applies for galvanic cells and batteries. The second definition of anode and cathode only applies for external circuits.

NOTE: The definition states that a cathode is POSITIVELY CHARGED terminal in a power supply. This is different to a POSITIVE TERMINAL.
A POSITIVE TERMINAL is where the electrons enter as a result of attraction.
A POSITIVELY CHARGED terminal is where the terminal has become positively charged as a result of electrons leaving the terminal. This is the case of the cathode, where electrons flow from cathode to anode in an EXTERNAL CIRCUIT. Since the electrons leave the cathode and move into the external circuit, the cathode has an overall positive charge, hence it is a positively charged terminal.
The above also applies for the anode definition.

In summary:
- Electrons flow from anode to cathode WITHIN a battery
- Electrons flow out of the battery from cathode to anode in an EXTERNAL circuit

Hopefully you'll be able to follow that, otherwise it gets really confusing...

This well explained definition of the anode/cathode by A l should hopefully clear up any problems.

 

[tennille]

Overall, in a galvanic cel,l the anode (-ve) is where oxidation occurs and thus is supplying electrons to the external circuit. In an electrolytic cell, the anode is +ve as it is connected to the +ve terminal of the power supply.