Notes

Electrolysis Guidebook

Terms used in electrolysis and their meanings

Electrolysis sure knows how to add an EXTRA e into chem. Terminology like electrolyte, electrode, discharge, anode, cathode etc etc makes it sound so cheem… But do not fear, as we promised, we are here to kick the e out of cheem!!

Definition of Cheem Terms

TermMeaning
Electrolysisa process of using electricity to decompose a compound
Dischargea process whereby ions gain or lose electrons to form neutral atoms or molecules
Selective Dischargea discharge of the more reactive ion in an aqueous electrolyte, while the less reactive ion remains unreacted
Electrolytea molten (l) or aqueous (aq) ionic compound with free-moving ions that conducts electricity and decomposes
Electrodean electrical conductor connecting the electrolyte to the circuit, where redox occurs
Inert Electrodean unreactive electrode made of carbon or platinum
Cathodeelectrode that reduces cations (red cat); negative electrode in electrolysis but
positive in simple cell
Anodeelectrode that oxidises anions (orange apple); positive electrode in electrolysis but negative in simple cell
Cationpositively charged ions (CATion ⇒ PAWsitive)
Anionnegatively charged ions
Electroplatinga process of depositing a metal layer on a substance at the cathode using electrolysis

Is This an Electrolytic Cell?

There is a battery in an electrolytic cell, but not in a simple cell.

An electrolytic cell looks awfully similar to a simple cell. The difference? Electrolysis requires an external supply of electricity to decompose a compound. A set-up with a battery is an electrolytic cell.

Positive and Negative Electrodes?

When you analyse an electrolysis diagram, you should always begin by labelling the battery:

  • The shorter line is the negative terminal, where electrons leave the battery.
  • The longer line is the positive terminal, where electrons enter the batter.

Next, identify and label the electrodes.

  • The electrode attached to the negative terminal of a battery is the negative electrode. The negative electrode attracts positive cations, and is hence called cathode.
  • The electrode attached to the positive terminal of a battery is the positive electrode. The positive electrode attracts negative anions, and is hence called anode.

Finally, identify the process and the product at each electrode.

  • At the negative electrode, the electrons reduce cations.
  • At the positive electrode, anions (or the reactive metal electrode, if any) oxidise.

Are the Electrodes Inert?

Inert electrodes are made of carbon or platinum. They do not react, and can be ignored in your question analysis.

However, if the positive electrode is a reactive metal like copper, then the metal will be oxidised. Visually, it will look as though the positive electrode is dissolving.

Which Ions Will Selectively Discharge?

For molten electrolyte, there are usually just one cation and one anion. They will both be discharged, since there is no other alternative.

However, for aqueous electrolytes, multiple cations are present. One cation will be selectively discharged over the other cations, and likewise for anions. To predict which ion will be selectively discharged, we can use the electrochemical series below. As you go down the series, it becomes easier to discharge the ions.

Anions like sulphate and nitrate will NEVER be discharged in electrolysis.

You may have already noticed, the cation portion in the series above the reverse of the metal reactivity series. Simply put, the less reactive the metal atom, the more likely its ion will discharge. Likewise for the anion! The less reactive the halogen, the more likely its ion will discharge. For example, iodide discharges more easily than bromide, as iodine is less reactive than bromine.

If the electrodes used are inert and the solution is dilute, here are some shortcuts you can use:

  • At the anode, hydroxide ions (OH) are usually discharged to form oxygen gas, UNLESS the electrolyte has a high concentration of halide anions like chloride, bromide, or iodide.
  • At the cathode, hydrogen ions (H+) are usually discharged to form hydrogen gas, UNLESS copper(II) ions or silver ions are present

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