Electrolysis of aqueous and molten electrolytes with inert electrodes

Chemistry · Electrolysis

Who wins at each electrode?

Inert electrodes (like graphite) only conduct the charge, they never take part. Pick a branch at each step and see what forms at the cathode and the anode.

Molten or aqueous? → Cathode: metal or hydrogen? → Anode: halogen or oxygen?

On Electrolysis with inert electrodes. 2 branches to choose from.

Follow the decision: molten or aqueous first, then one question at each electrode.

Chemistry · Apply the two rules

Six solutions, two electrodes

Each solution is electrolysed with inert electrodes. Sort it by what forms at the cathode first, then switch the rule to sort the same six by the anode.

Is the metal less reactive than hydrogen?

6 of 6 still to sort.

Collecting a gas from an electrode

Step through the method in order.

1. 1Fill the tubeFill a test tube with the electrolyte.
2. 2Invert it over the electrodeTurn the tube upside down and place it over the electrode where the gas forms.
3. 3Turn off the powerSwitch the power off once the gas has been collected.
4. 4Thumb on until testingKeep the tube closed with your thumb until the gas is tested.

Predict, then check

Copper sulfate solution is electrolysed with inert electrodes. Only one of the two electrodes gives a gas. Use the rules to work out which gas, then pick the test result.

You collect the gas from the one electrode that gives a gas in copper sulfate solution. Which test result do you get?

What you need to know

• Inert electrodes, like graphite, only conduct the charge. They do not take part in the reaction.
• In a molten salt the only ions are the metal ion and the non-metal ion, so metal forms at the cathode and non-metal at the anode.
• Have a goA classmate says, with total confidence, that molten sodium chloride gives hydrogen at the cathode. What actually forms at each electrode?

  Sodium at the cathode, chlorine at the anode.

  A molten salt has only the metal ion and the non-metal ion, so the metal goes to the cathode and the non-metal to the anode.

• Dissolve an ionic compound in water and its ions separate and spread out, free to move and carry charge.
• Some water molecules ionise into H+ and OH-, so an aqueous salt solution always contains some of both as well as the salt's ions.
• Have a goSomeone claims lithium bromide solution contains only Li+ and Br- ions. What have they forgotten?

  The H+ and OH- ions from water.

  Some water molecules ionise, so an aqueous salt solution always contains some H+ and OH- as well as the salt's ions.

• Pure distilled water does not conduct electricity, because it only has H2O molecules.
• In aqueous electrolysis, more than one type of positive ion competes at the cathode, and more than one type of negative ion at the anode.
• At the cathode, you get the metal if it is less reactive than hydrogen, and hydrogen if it is more reactive.
• At the anode, you get a halogen if there is a halide ion, and oxygen if there is not.
• Have a goPotassium bromide solution is electrolysed with inert electrodes. Use the two rules: what forms at the cathode and at the anode?

  Hydrogen at the cathode, bromine at the anode.

  Potassium is more reactive than hydrogen, so hydrogen forms. Bromide is a halide, so a halogen forms.

• To collect a gas, put a test tube of electrolyte upside down over the electrode, turn off the power, and keep the tube closed with a thumb until testing.
• Hydrogen gives a squeaky pop with a burning splint, oxygen relights a glowing splint, and chlorine turns damp blue litmus red then bleaches it white.

The big picture

In molten electrolysis the only ions are the metal ion and the non-metal ion, so you get the metal at the cathode and the non-metal at the anode. In an aqueous solution, water adds H+ and OH- ions, so ions compete: the cathode gives the metal only if it is less reactive than hydrogen, and the anode gives a halogen only if a halide ion is present, otherwise oxygen.

⚠ Watch out

Assuming aqueous electrolysis gives the same products as molten electrolysis. Molten sodium chloride gives sodium, but sodium chloride solution gives hydrogen, because sodium is more reactive than hydrogen. And with no halide ion in the solution, oxygen forms at the anode, not the non-metal from the salt.

Memory hook

Two questions for an aqueous solution. Cathode: 'Is the metal less reactive than hydrogen?' Yes gives metal, no gives hydrogen. Anode: 'Is there a halide?' Yes gives halogen, no gives oxygen.

Check yourself

Potassium nitrate solution is electrolysed with inert electrodes. Name the product at each electrode and give the reason for each.