Electrolytes and the process of electrolysis

Electrolysis, from compound to products

Step through the stages in order. Each one sets up the next.

1. 1Free the ionsStart with an ionic compound and melt it, or dissolve it. Now its ions are free to move, and the liquid is an electrolyte.+
2. 2Build the cellPut the electrolyte in an electrolysis cell with two electrodes and a DC power supply. The anode joins the positive terminal, so it is positive. The cathode joins the negative terminal, so it is negative.+
3. 3Switch onThe ions are attracted to the oppositely charged electrodes. Negative ions head for the anode. Positive ions head for the cathode.+
4. 4Electrons change handsAt the anode, the negative ions lose electrons. At the cathode, the positive ions gain electrons.+
5. 5Atoms appearChemical reactions at the electrodes have changed the ions into atoms. The products are deposited at the electrodes, often seen as bubbles of gas or a solid forming on an electrode.+

Chemistry · Electrolysis

Electrolyte, or not?

Is each of these an electrolyte? Pick one, then pick a category.

5 of 5 still to sort.

Decide first. Then read why.

What you need to know

• Electrolysis uses electricity to break down (decompose) an ionic compound.
• The compound being broken down is the electrolyte: a liquid or solution containing freely moving ions.
• An ionic compound acts as an electrolyte when it is molten or dissolved, because its ions are then free to move.
• A solid ionic compound can't be electrolysed. Its ions sit in fixed positions in a giant ionic lattice.
• Have a goYour friend says: "Solid salt is made of ions, so of course it conducts!" What have they missed?

  In the solid the ions are fixed in place, so they can't move to carry charge.

  Having ions isn't enough. They have to be free to move, which is why the compound must be molten or dissolved.

• In a metal, electrons carry the charge. In an electrolyte, the ions carry it and complete the circuit.
• Electrodes are rods of metal or graphite that conduct, so current can flow in and out of the electrolyte.
• A DC supply joins the anode to the positive terminal and the cathode to the negative terminal.
• Have a goElectrode A is joined to the positive terminal of the supply. Is A the anode or the cathode, and what charge does it carry?

  It is the anode, and it is positively charged.

  The anode is the electrode joined to the positive terminal. Mixing it up with the cathode is the easy slip.

• DC is used so the electrodes keep the same polarity: anode always positive, cathode always negative.
• Negative ions move to the anode and lose electrons. Positive ions move to the cathode and gain electrons.
• Have a goA negative ion is drifting about in the electrolyte. Which electrode is it pulled towards, and does it gain or lose electrons there?

  It goes to the anode and loses electrons.

  Opposites attract, so a negative ion heads for the positive anode. It is the positive ions that gain electrons, at the cathode.

• It is a chemical change: ions become atoms, seen as bubbles of gas or a solid forming on an electrode.

The big picture

Electrolysis uses electricity to break down an ionic compound. It only works when the compound is molten or dissolved, because the ions have to be free to move. They carry the charge through the electrolyte, then change into atoms at the electrodes.

⚠ Watch out

Treating electrolysis as a physical separation, like filtering. It is a chemical change: reactions at the electrodes turn ions into atoms. Another slip is thinking a solid ionic compound can be electrolysed if you push enough current through it.

Memory hook

Ions in, atoms out. Free the ions, switch on, and the electrodes turn them into atoms.

Check yourself

From memory: name the charge carriers in a metal wire and in an electrolyte, then say what happens to a positive ion when it reaches the cathode.