Hydrogen oxygen fuel cell question (A level chemistry)

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Report Thread starter 1 year ago
Bit confused about the hydrogen oxygen fuel cell. My understanding is that in electrolysis the cathode is negative (where reduction happens) and anode is positive (where oxidation happens). It says here:
'electrodes are named contrary to electrodes in electrolysis reactions' but in the diagram it doesn't seem so because reduction is still at cathode and oxidation at anode (so the cathode remains negative?). Also, assuming the cathode is still negative (thus attracting the protons) why are electrons moving towards it?
Appreciated if someone could explain this contradiction and what that sentence could mean.
Badges: 9
Report 1 year ago
Hi there,

In electrolysis the anode is positive and the cathode is negative (PANCake - Positive Anode Negative Cathode). Positive ions are attracted to the cathode (hence we call them cations) and gain electrons in a reduction reaction. Negative ions (anions) are attracted to the anode and lose electrons in an oxidation reaction. The positive and negative charges are generated by an externally applied current, and the electrons produced in the reaction will flow from the anode to the cathode. Which electrode is the anode and which is the cathode is determined by which electrode is attached directly to the positive and negative terminal of the power supply. Swap the terminals and your anode will become your cathode (and vice versa).

In electrochemical cells there is no application of an external electric current. Instead the current is generated by the potential difference between the two half-cells. Every individual half cell can be considered a reversible redox process and as such are governed by the chemistry of reversible reactions (Le Chatelier's Principle e.t.c.). The position of equilibrium of each half-cell is different. Compare Cu2+ + 2e- === Cu and Mg2+ + 2e- ===Mg. You know from your knowledge of chemistry that Magnesium is a more reactive metal than Copper and is more likely to exist as the oxidised ionic form than the elemental metallic form. This means that when you compare the position of equilibrium of these two reactions, keeping all things the same such as concentrations of solutions and temperatures, Mg is further to the left than Cu. This means that Mg will generate more electrons than Cu. When the half-cell is in isolation (a piece of metal in an ionic solution of that metal), there is nowhere for the electrons to flow to, so they build up on the piece of metal. This gives the metal a negative charge. The further the position of equilibrium is to the left for a half-cell, the more negative it will be (more oxidation at equilibrium). Therefore, half-cells with greater negative potentials favour the oxidised side of their equilibrium equation. Once you connect two half cells together in an electrochemical cell, the electrons can flow from the more negative half-cell to the more positive half-cell. Therefore, oxidation takes place at the negative electrode (anode) and reduction takes place at the positive electrode (cathode).

Whether it is electrolysis or electrochemical cells, electrons always flow from the anode to the cathode.

Hope this helps, reply if still stuck.

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