Electrolysis: Half Equations at the Electrodes
A current through a molten ionic compound pulls it apart. Positive ions take electrons at the cathode and negative ions give them up at the anode, and each electrode gets its own half equation.
What a learner can do afterwards
- Explains why an ionic solid does not conduct but the melt does
- Predicts the product at each electrode for a named molten metal halide using inert electrodes
- Writes both half equations with the electrons balanced
- Combines the two half equations back into the overall reaction
1 · Read
Ion tests identify ions. Electrolysis changes them into products. An ionic solid does not conduct because its ions cannot move through the lattice. In a melt, ions can move and carry charge. A direct-current supply drives a reaction that would not occur on its own under these conditions. Electrons carry charge through the external wires.
Positive ions (cations) move to the negative cathode and gain electrons to form metal atoms. This is reduction. Negative ions (anions) move to the positive anode and lose electrons. This is oxidation. For the molten metal halides here, assume inert electrodes. Lead bromide contains Pb2+ and Br- ions. Sodium chloride contains Na+ and Cl- ions. Halide ions form halogen molecules such as Br2 and Cl2.
Balance atoms first, then total charge using electrons. For molten magnesium chloride, the cathode half is Mg2+ + 2e- -> Mg. Its left charge is +2 - 2 = 0, matching the right. The anode half is 2Cl- -> Cl2 + 2e-. Both sides have two chlorine atoms and charge -2. Reduction uses electrons on the left; oxidation releases them on the right.
Before adding halves, make electrons gained equal electrons lost. If needed, multiply every term in one or both halves. Each magnesium chloride half transfers two electrons, so they cancel. Adding gives Mg2+ + 2Cl- -> Mg + Cl2. The starting ions have the ratio in MgCl2, so write MgCl2 -> Mg + Cl2. MgCl2 gives the formula-unit ratio of this ionic compound, not a molecule in the melt.
Melt to free the ions, discharge cations at the cathode and anions at the anode, and balance each half.
This lesson, question bank and flashcards were written for this dataset.
These readings and flashcards were written for this dataset. OpenStax Chemistry sections 17.1 and 17.7 were consulted for scientific facts and teaching structure. Their prose was not adapted.
More to read
Charge moving through a cell
Having charged particles is not enough for conduction. They must be able to move through the material. Ions occupy fixed positions in a solid ionic lattice. A melt allows them to move towards the electrodes.
The wires and melt carry charge in different ways. Electrons move through the external wires; ions move through the melt. Reactions at the electrodes connect these two kinds of charge movement.
For a molten metal halide with inert electrodes, positive metal ions gain electrons at the negative cathode. Negative halide ions lose electrons at the positive anode. The supply provides energy to drive the reaction.
Two checks for a half equation
Check the atoms and total charge separately. Passing one check does not guarantee the other.
Consider Mg2+ + 2e- -> Mg. Each side contains one magnesium atom. On the left, the ion contributes +2 and the electrons contribute -2. The total is 0, matching the neutral atom on the right.
Now check 2Cl- -> Cl2 + 2e-. Each side contains two chlorine atoms. The left charge is -2. The chlorine molecule is neutral, so the two electrons give the right side a charge of -2 too.
Electron placement also tells you the reaction type. Electrons used on the left show reduction. Electrons released on the right show oxidation.
Matching the electron transfers
Molten sodium chloride gives the halves Na+ + e- -> Na and 2Cl- -> Cl2 + 2e-. One uses a single electron, while the other releases two. They cannot yet be added with all electrons cancelling.
Double every term in the sodium half: 2Na+ + 2e- -> 2Na. Both halves now transfer two electrons. Add them and cancel electrons to obtain 2Na+ + 2Cl- -> 2Na + Cl2.
The reactant ions have the ratio in sodium chloride. In formula-unit notation, the overall equation is 2NaCl -> 2Na + Cl2. There are two sodium atoms and two chlorine atoms on each side.
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