Chemistry HL: Voltaic versus Electrolytic Cells

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HL Chemistry: In Redox chemistry, what is happening at the cathode and anode (reduction or oxidation?), and which is positive and which negative in...
A) a voltaic cell
B) an electrolytic cell?

A) In Voltaic cells, electricity is produced by a redox reaction. Oxidation occurs at the anode (which is negative) and reduction occurs at the cathode (which is positive).

B) In Electrolytic cells, oxidation occurs at the anode (which is positive) and reduction occurs at the cathode (which is negative).

Voltaic Cells - these are made of two half-cells connected in a circuit by a salt bridge. The salt bridge is usually made using filter paper dipped in saturated KNO₃. Each half-cell is composed of a strip of metal suspended in a solution of its own ions, which creates an equilibrium between the metal and its ions.

An example of this equilibrium would be a zinc half-cell: Zn <--> Zn²⁺ + 2e-

When two half-cells are connected together, the metal which is higher in the reactivity series is more inclined to lose its electrons. It loses its electrons (i.e. is oxidised) and becomes the negative electrode. The other half-cell accepts the electrons and becomes the positive electrode.

Electrolytic Cells - these are made from a power source, a negative electrode, a positive electrode and a beaker of electrolyte. The electrolyte is the substance which can be split into its component ions and therefore allow the flow of electricity through the cell.

The ions within the electrolyte are free to move around in solution and so can conduct current. The electrolyte decomposes in the process, for example NaCl --> Na⁺ + Cl^-. The positive ion will be attracted to the negative electrode (so Na⁺ to the cathode) and the negative ion (Cl^-) will go to the anode.

These substances are given off at the different electrodes. The products of electrolysis will be the cation at the negative electrode, the anion at the positive electrode, and for aqueous electrolytes there will also be H⁺ and OH^- ions from the H₂O.

In this case, at the anode the Cl^- will be oxidised 2Cl^- --> Cl₂ + 2e- and as it is a gas, Cl₂ will be given off.

Related areas of the syllabus to check out at this point: how to calculate an EMF, the definition of the Standard Hydrogen Electrode and how to predict and explain the products of electrolysis.

Revision tip

A good way to remember these the right way round but not have to sit and figure it all out every time is with the acronyms CRANOV and CRAPOE.

CRANOV = Cathode Reduction Anode Negative Oxidation Voltaic (...and if the anode is negative, the cathode must be positive!)

CRAPOE = Cathode Reduction Anode Positive Oxidation Electrolytic (...and again, if the anode is positive, the cathode is negative)

We hope this helps! Any questions either post here or in the Chemistry forum.

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You can also use:

RED CAT

(Reduction Cathode)

[harveyk]

AN OX

Anode- oxidation

[ilovepotato]

I use "An OIL RIG Cat"

Anode - Oxidation is loss (of electrons)

Reduction is gain (of electrons - Cathode

Edited March 11, 2014 by ilovepotato