Naked Science Forum

Non Life Sciences => Chemistry => Topic started by: pharmaceutical on 09/07/2008 22:52:21

Title: What is the difference between the cathode and anode?
Post by: pharmaceutical on 09/07/2008 22:52:21

Hello everyone  [:)]  [:)]

I am a new member here and this is my 1st post  [:)]

I am having a problem understanding the difference between the anode and cathode in electrochemistry.
What I know so far is that the anode is the electrode at which oxidation takes place (and hence donates electrons) and the cathode is the electrode at which reduction takes place (and hence accepts these electrons). Is this true? Please correct me if not.

Then, I am very confused regarding the charges on the each of the electrodes.  Which is positive and which is negative? In my chem book (Chemistry: The Central Science by Brown, LeMay, and Bursten) it is says that "although the anode and cathode are labeled with - and + signs respectively, you should not interpret the labels as charges on the electrodes.  The labels simply tell us the electrode at which the electrons are released to the external circuit (the anode) and received from the external circuit (the cathode).  The actual charges on the electrodes are essentially zero."

Then I read on Wikipedia that the anode and cathode are charged.  It states that "anode polarity is not always positive but depends on the device type, and sometimes even in which mode it operates, as determined by the above electric current direction-based universal definition." and that "cathode polarity is not always negative but depends on the device type, and sometimes even in which mode it operates, as determined by the above current direction based universal definition." They say that the charges of the anode and cathode differ depending on whether they are a part of a Galvanic cell or an electrolysis cell.

I just don't get it! I am totally confused. Can anyone help me out please?

Another question: in a conventional (dry-cell) battery, does the positive sign refer to the anode or cathode?

Thank you so much and sorry for my looooong question.

Title: Re: What is the difference between the cathode and anode?
Post by: that mad man on 10/07/2008 20:33:23

Have a look here as some still get the terms wrong.

http://www.mi.mun.ca/users/cchaulk/eltk1100/ivse/ivse.htm

This may explain some of the differences.

Title: Re: What is the difference between the cathode and anode?
Post by: cconley on 11/07/2008 19:53:51

Here’s a reply from another new member.

You are completely correct in your statement that oxidation occurs at the anode and reduction at the cathode.  It can be confusing assigning positive and negative signs to the electrodes, because some electrochemical processes are spontaneous (voltaic) and others are not spontaneous (electrolytic).

In a voltaic process (spontaneous) like your common dry cell battery, the anode is giving off electrons (oxidation), so has a negative charge.  This will be the zinc on the bottom of the battery.  The cathode is taking in and consuming electrons (reduction).  This means there is a deficiency of electrons here, so it is positive.  In the battery, this occurs in a manganese-based paste.  To get the electrons to the manganese, the battery uses a graphite rod.  The tip of this rod is the knob on top of the battery, or the positive terminal.

In an electrolytic cell (not spontaneous), you need to initiate the reaction with an outside power source, like a battery.  If you wanted to silver-plate a fork, for example, the fork would be the cathode, so that silver ions in solution will reduce on the surface of the fork.  You would have to connect the negative terminal of your battery to the cathode, so that electrons can be added to the fork and reduction can occur.  In other words, the anode of you voltaic cell connects to the cathode of your electrolytic cell.

-Chris: CT, USA

Title: What is the difference between the cathode and anode?
Post by: pharmaceutical on 12/07/2008 22:13:15

Thank you Chris [:)]

I first thought that the anode would be positively charged since it's loosing electrons (and hence attracts anions in the solution) and the cathode would be negatively charged since it accepts those electrons (and hence attracts cations). Isn't this true?

Title: What is the difference between the cathode and anode?
Post by: cconley on 16/07/2008 21:17:44

In my desire to be concise, I think I over simplified a statement: "the anode is giving off electrons (oxidation), so has a negative charge"

The process of oxidation is releasing electrons.  Those electrons build up on the metal strip waiting to be taken away.  Those excess electrons make that electrode (the anode) negative.  The opposite is true at the cathode.

Again, I am referring to a simple battery here.

Sorry for the confusion.

Chris: CT, USA

Title: What is the difference between the cathode and anode?
Post by: Pumblechook on 16/07/2008 21:38:28

Confusing..  In various vacuum tubes (CRTs, valves, magnetrons, kystrons) the anode is made positive.

Title: What is the difference between the cathode and anode?
Post by: pharmaceutical on 19/07/2008 13:30:34

Hmmmmmmmm, confusing indeed. But what Chris is saying is quite convincing. I think I get it now  [:)]

Thank you Chris and everyone  [:)]

Title: What is the difference between the cathode and anode?
Post by: lyner on 19/07/2008 18:51:32

Quote from: Pumblechook on 16/07/2008 21:38:28

Confusing..  In various vacuum tubes (CRTs, valves, magnetrons, kystrons) the anode is made positive.

There need not be any confusion;  the anode is kept positive if you want current to flow for devices (thermionic or solid state) which aren't batteries or Zener Diodes (and Zener Diodes are reverse biased but will also conduct as a normal diode when forward biased and, in any case,  'zener' diodes  above about 5.6V are really 'avalance' diodes- some more confusion).
I'm not really sure that you should use the terms anode and cathode for batteries.