The electrochemical series arranges metals by electropositivity, from potassium (most electropositive) to gold. It tells you which ion is discharged in electrolysis and which metal is the negative terminal, and the larger the gap the higher the cell voltage.
The electrochemical series arranges elements by their tendency to lose electrons and form positive ions, their electropositivity. It looks like the reactivity series but does more work: it predicts which ion is discharged during electrolysis, which metal becomes the negative terminal in a simple voltaic cell, and roughly how large the voltage will be. This reference presents the standard SPM series and the anion discharge order, arranged the way the DSKP expects you to use them.
Scope note
The tables below hold standard KSSM reference data for SPM Chemistry. Hydrogen is included because it competes with metal ions for discharge in the electrolysis of aqueous solutions. Use the metal series to place any element by electropositivity, and use the anion order for the negative-ion discharge questions that appear in Form 5 electrolysis.
The metal (cation) series
| Position | Element | Ion formed | Electropositivity | Ease of ion discharge at cathode |
|---|---|---|---|---|
| 1 (top) | Potassium (K) | K+ | Highest | Hardest to discharge |
| 2 | Sodium (Na) | Na+ | Very high | Very hard |
| 3 | Calcium (Ca) | Ca2+ | High | Hard |
| 4 | Magnesium (Mg) | Mg2+ | High | Hard |
| 5 | Aluminium (Al) | Al3+ | Moderately high | Fairly hard |
| 6 | Zinc (Zn) | Zn2+ | Moderate | Moderate |
| 7 | Iron (Fe) | Fe2+ | Moderate | Moderate |
| 8 | Tin (Sn) | Sn2+ | Low | Easier |
| 9 | Lead (Pb) | Pb2+ | Low | Easier |
| 10 | Hydrogen (H) | H+ | Reference | Reference point |
| 11 | Copper (Cu) | Cu2+ | Very low | Easy |
| 12 | Silver (Ag) | Ag+ | Very low | Very easy |
| 13 (bottom) | Gold (Au) | Au3+ | Lowest | Easiest |
Anion discharge order (for electrolysis)
| Order | Anion | Ease of discharge at anode |
|---|---|---|
| 1 | Fluoride, F- | Hardest to discharge |
| 2 | Sulfate, SO4 2- | Hard |
| 3 | Nitrate, NO3- | Hard |
| 4 | Chloride, Cl- | Moderate |
| 5 | Bromide, Br- | Easier |
| 6 | Iodide, I- | Easier |
| 7 | Hydroxide, OH- | Easiest to discharge |
How to use the series
In the electrolysis of an aqueous solution, both a metal ion and hydrogen ions reach the cathode, and the ion lower in the metal series is discharged. So in a dilute copper(II) solution copper is deposited, but in a sodium solution hydrogen is released instead, because sodium sits far too high. At the anode, the anion lower in the discharge list wins, unless a halide is at high concentration. In a simple voltaic cell made from two metals, the more electropositive metal (higher in the series) is the negative terminal, and the further apart the two metals are, the larger the voltage. Reading the series answers all three question types.
Worked reasoning
Take the electrolysis of dilute sodium chloride solution with carbon electrodes. At the cathode, sodium ions and hydrogen ions both arrive; because sodium sits high in the series and is far harder to discharge, the hydrogen ions are discharged instead and hydrogen gas bubbles off. At the anode, chloride ions and hydroxide ions both arrive; since the solution is dilute, the hydroxide ion is lower in the discharge order and is discharged, releasing oxygen. Concentrate the chloride and the outcome at the anode changes, because a high concentration of chloride is discharged in preference to hydroxide. Building each prediction from the series, rather than recalling a fixed answer, is exactly what the exam rewards, and it lets you handle a solution you have never seen before.
Common mistakes to avoid
- Confusing the two directions. Metals higher up are harder to discharge as ions but more reactive as atoms.
- Forgetting concentration. A concentrated halide can be discharged ahead of hydroxide.
- Getting the negative terminal wrong. It is the more electropositive metal, which loses electrons.
- Mixing up cathode and anode products.
How we help
Our online one-to-one teachers work through electrolysis and simple cells with the series open beside you, so every discharge prediction becomes a short, confident deduction. Lessons are in English, one to one, from RM50 per hour, with a paid one-hour trial. Because the same series governs electrolysis, voltaic cells and the reasoning behind extraction, mastering it removes much of the guesswork from Form 5 redox and electrochemistry questions.
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