Electrolysis of molten ionic compounds

Chemical changes • Electrolysis

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State the movement of anions during electrolysis.

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Anions move toward the anode because it has positive charge.

Key concepts

What you'll likely be quizzed about

Ionic conduction in the molten state

Melting an ionic compound breaks the crystal lattice and frees ions to move and carry electric current. No solvent molecules intervene so the only mobile species are the compound's cations and anions. Electrical current forces cations toward the negative electrode and anions toward the positive electrode. Electrodes supply or remove electrons so ionic charges convert into neutral atoms or molecules. The absence of water avoids side reactions such as hydrogen or hydroxide formation.

Cathode reactions and reduction

Negatively charged cathode attracts positive ions (cations). Cations gain electrons (reduction) to form neutral atoms. Metal cations produce the metal at the cathode when the cation reduces to its elemental state. The reduction half-equation follows the form: M^n+ + n e^- -> M.

Anode reactions and oxidation

Positively charged anode attracts negative ions (anions). Anions lose electrons (oxidation) to form neutral atoms or molecules. Non-metal anions typically form diatomic non-metal molecules (for example, halide ions form X2 gas). The oxidation half-equation follows the form: 2 X^- -> X2 + 2 e^- for halide ions.

Simple prediction rule for binary ionic compounds

Binary ionic compounds contain one type of cation and one type of anion. Predict the products by reducing the cation at the cathode to its element and oxidising the anion at the anode to its element or elemental molecule. Examples apply directly: molten NaCl gives sodium metal at the cathode and chlorine gas at the anode; molten PbBr2 gives lead metal at the cathode and bromine at the anode. Ionic charges determine electron counts in half-equations and the stoichiometry of any gaseous products.

Limiting factors and special cases

Electrode material affects secondary reactions only when electrodes react with products; inert electrodes (graphite, platinum) avoid additional chemistry. Impurities in the molten sample can introduce other ions and change products. High temperature affects volatility of product elements and may cause rapid escape of gaseous products. Binary compounds that contain polyatomic anions or multiple oxidation states require a different approach; the simple reduction-to-metal and oxidation-to-nonmetal rule applies only to binary metal + non-metal ionic compounds in the molten state.

Key notes

Important points to keep in mind

Molten state removes water, so only the compound's own ions take part.

Cations reduce at the cathode to form metals; anions oxidise at the anode to form non-metals or molecules.

Use ionic charges to balance electrons in half-equations.

Inert electrodes avoid extra reactions that change products.

Impurities or multiple ion types change the predicted products.

Halide anions produce diatomic halogen gases at the anode.

Predict metals at the cathode and non-metals at the anode for binary ionic compounds.