ATAR Notes: Forum
VCE Stuff => VCE Science => VCE Mathematics/Science/Technology => VCE Subjects + Help => VCE Chemistry => Topic started by: b^3 on July 25, 2011, 05:25:53 pm
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Ok so from what I have read in Electrolytic Cells the Anode is +ve and the Cathode is -ve. In Galvanic Cells the Anode is -ve and and the Cathode is +ve. Now the Cathode is defined as the electrode at which Reduction occurs right? So that means the electrode is gaining electrons yes? So my question is why are the Cathode and Anode those charges? In Galvanic Cells at the Cathode, the electrode is gaining electrons so its should be negatively charged until the ions form the salt bridge balance it out, not postive? Overall the charges balance out so do we take the charge on the original electrode? I am missing something here because logically it looks like they both end up with the same charge at the same electrodes but this is not the case.
Another quick question. In both cells one electrode "corrodes" while the other forms mass on it, does this "corrosion" occur at the same electrode in each Electrolytic Cells and Galvanic Cells or at the opposing electrode?
A detailed explanation would be a lot of help. Thankyou.
EDIT: Ok any explanation will help.
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Does anyone have any ideas? thoughts?
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The Anode in the galvanic cell is the source of electrons. i.e electrons are produced here and so it is negative. The cathode consumes electrons and so it is positive.
You can also see it as though because the electrons move along the wire towards the cathode, the cathode must then be positive.
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So then why do we say the anode is +ve in the Electrolytic Cells if it is producing the electrons?
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From what i remember, it had something to do with the fact that you have the external power source in an electrolytic cell, but i'm not quite sure :-\
Edit: From the Nelson Textbook:
"The anode and cathode
The anode is still defined as the electrode at which oxidation occurs and the cathode
is still defined as the electrode at which reduction occurs.
However, in the case of an electrolytic cell, the polarity of the electrodes is
determined by the external power source. Since the electrons that leave the negative
terminal of power source move towards the anode on the pathway back to the positive
terminal of the cell, the cathode is negatively charged relative to the anode. Hence
in the electrolytic cell the cathode has a negative polarity and the anode a positive
polarity, as shown in Figure 15.4."
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The +ve/-ve come from the half cell's position on the table of electrochemical potentials. I.e. if the half cell is placed higher (has a higher E potential), then it is +ve. This has got to do with the thermodynamics of the system (how energy is consumed/released).
The anode/cathode designation comes from the direction of reaction. In a spontaneous reaction, electrons will go from the negative potential to the positive potential (and release energy). Thus the -ve electrode is oxidation (anode), the +ve electrode is reduction (cathode). When you are recharging, you are driving the system with some external voltage. The reaction is reversed, and thus the -ve electrode is reduction (cathode), the +ve electrode is oxidation (anode).
As you can see, the +/- do not change going from electrochemical cell to electrolytic cell. The direction of reaction changes, and thus the anode/cathode designation flip sides.
The key points to remember are:
- +/- designation do not depend on the direction of reaction. It is determined by the electrochemical series.
- reduction is always at the cathode. oxidation is always at the anode. (AN OIL RIG CAT)
- designation of cathode/anode depend on the direction of reaction
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THANKS ALOT GUYS, this really helps :). I just want to check something now, so the higher E value is +ve. In a galvanic cell the half cell with the higher E value will always pull the electrons one way, but when we apply an external circuit (with voltage greater than the E value) we push them back the other way. So that means that in a galvanic cell the higher E value will gaining electrons i.e. reduction so it will be the cathode (since AN OIL RIG CAT) and have a positive charge. In a Electrolytic cell the half cell with the higher E value will be forced to lose electrons by the external power supply and so be undergoing oxidation, thus becoming the anode and still be positively charged.
- Galvanic Cell - Higher E Value Gaining Electrons - Reduction - Cathode - +ve
- Lower E Value Losing Electrons - Oxidation - Anode - -ve - Electrolytic Cell - Higher E Value Losing Electrons - Oxidation - Anode - +ve
- Lower E Value - Gaining Electrons - Reduction - Cathode - -ve
Is that reasoning and information correct? and thanks again guys.
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Yes. Spot on.
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Thankyou Mao.