Notes

Electrolysis of Molten Compounds

Describing electrolysis of molten ionic compounds

Now that you know how eletrolysis works, let’s look at how the different ions take in or give out electrons during the electrolysis of molten compounds.

Pay attention here, cause this is commonly seen in exams!!

Let us start by describing the electrolysis of molten compounds, specifically ZnCl2.

  • Step 1: Label the electrodes negative/ positive

    Electrons flow from the negative terminal of the battery (source of electrons) towards the left electrode, hence that electrode will be negatively charged aka negative electrode. The other electrode will be the positive electrode.

    Note that the electrodes here are platinum electrodes. Platinum is very unreactive, aka inert. Thus, it will not interfere with the electrolysis of ZnCl2. To learn more about inert electrodes, go on to Electrolysis – Guidebook.

  • Step 2: Indicate ions present in electrolyte

    The electrolyte is molten ZnCl2 (l), so the ions present are Zn2+ and Cl . Since opposite charges attract, mobile Zn2+ ions will migrate towards the negative electrode. Likewise, the positive electrode will attract Cl ions.

  • Step 3: Taking in electrons at negative electrode (reduction reaction at cathode)

    At the negative electrode, the zinc ions present will take in electrons, leading to the reaction:

    Zn2+ (l) + 2e ⟶ Zn (l)

    NOTE: Balance the total charges on both sides of the equation, thus Zn2+ will react with 2 electrons.

    We can see that zinc metal will be a product produced at the negative electrode. Observations like silvery metal will be formed at the negative electrode. Zinc produced is liquid because of the high temperature.

    We can also tell this is a reduction reaction (gain of electron, charge of zinc decreases from 2+ to 0). The electrode where reduction occurs is also known as the cathode. (Red Cat)

  • Step 4: Giving electrons at positive electrode (oxidation reaction at anode)

    We also learnt that at the positive electrode, chlorine ions present will give electrons to complete the circuit, leading to this reaction:

    2Cl (l) ⟶ Cl2 (g) + 2e

    NOTE: Balance both the number of atoms and the charges present on both sides of the equation.

    Therefore, we can also see that chlorine gas will be a product at the negative electrode. Observations like yellow-green pungent gas will be produced at the negative electrode.

    We can also tell that this is a oxidation reaction (loss of electron, charge of chlorine increases from -1 to 0). The electrode where oxidation occurs is also known as the anode. (Orange Apple)

Try it out yourself now! Follow the steps above to describe the electrolysis of
(i) molten NaCl
(ii) molten PbBr2

Remember you need to:
1. State the distribution of ions present
2. State the ionic equations of the reactions occurring at each electrode
3. State the products formed and any observations made

Psst… Answers for extra practices:
(i) molten NaCl: Na(l) produced at cathode. Cl2(g) produced at anode.
(ii) molten PbBr2: Pb(l) produced at cathode. Br2(g) produced at anode.