Electrolysis Fundamentals, Extraction of Metals, and Aqueous Solutions

A set of vocabulary flashcards covering the core concepts of electrolysis, including mnemonics, electrode rules, metal extraction using cryolite, and half equations for molten and aqueous electrolysis.

Electrolysis

The process of using electricity to break down an ionic compound into its elements.

Electrolyte

A molten ionic compound or aqueous solution containing free-moving ions that carry electric charge to the electrodes.

Cathode

The negatively charged electrode where positively charged ions move and are reduced by gaining electrons.

Anode

The positively charged electrode where negatively charged ions move and are oxidised by losing electrons.

Inert Electrodes

Electrodes, generally made of carbon, that conduct electricity and allow electron transfer without reacting chemically during electrolysis.

Discharge

The process in which an ion gains or loses electrons at an electrode to become its original neutral atom or molecule.

PANIC

A memory tip mnemonic meaning: Positive Anode, Negative Is Cathode.

An Ox, Red Cat

A mnemonic denoting that oxidation occurs at the anode (An Ox) and reduction occurs at the cathode (Red Cat).

OIL RIG

A mnemonic describing electron movement: Oxidation Is Loss of electrons, Reduction Is Gain of electrons.

Cryolite

A compound mixed with aluminium oxide ($Al_2O_3$) to lower its melting point, reducing the energy required and making the extraction process cheaper.

Reactivity Series and Metal Extraction Methods

Metals more reactive than carbon (potassium, sodium, calcium, magnesium, aluminium) must be extracted by electrolysis; metals less reactive than carbon (zinc, iron, tin, lead) are extracted by reduction; metals below hydrogen (copper, silver, gold, platinum) are unreactive and found in their native state.

Electrolysis of Molten Lead Bromide

Molten $PbBr_2$ contains $Pb^{2+}$ and $Br^-$ ions. At the negative cathode, lead ions are reduced ($Pb^{2+} + 2e^- \rightarrow Pb$). At the positive anode, bromide ions are oxidised ($2Br^- \rightarrow Br_2 + 2e^-$). The overall equation is $PbBr_2(l) \rightarrow Pb(l) + Br_2(g)$.

Electrolysis of Molten Aluminium Oxide

Molten $Al_2O_3$ contains $Al^{3+}$ and $O^{2-}$ ions. At the cathode, aluminium ions are reduced ($Al^{3+} + 3e^- \rightarrow Al$). At the anode, oxide ions are oxidised ($2O^{2-} \rightarrow O_2 + 4e^-$). The overall equation is $2Al_2O_3(l) \rightarrow 4Al(l) + 3O_2(g)$.

Rule for Cathode Discharge in Aqueous Solutions

At the negative cathode, competing positive ions ($\text{metal ion}$ vs. $H^+$) migrate, and the ion of the least reactive element is discharged.

Rule for Anode Discharge in Aqueous Solutions

At the positive anode, if a halide ion (Group 7) is present, it is discharged; if no halide ion is present, hydroxide ions ($OH^-$) are discharged.

Electrolysis of Aqueous Sodium Chloride

Aqueous $NaCl$ contains $Na^+$, $Cl^-$, $H^+$, and $OH^-$ ions. Hydrogen gas is formed at the cathode ($2H^+ + 2e^- \rightarrow H_2$) because $H^+$ is less reactive than $Na^+$. Chlorine gas is formed at the anode ($2Cl^- \rightarrow Cl_2 + 2e^-$) because chloride is a halide. Sodium hydroxide ($NaOH$) remains in solution: $2NaCl(aq) + 2H_2O(l) \rightarrow H_2(g) + Cl_2(g) + 2NaOH(aq)$.

Electrolysis of Aqueous Copper Sulfate

Aqueous $CuSO_4$ contains $Cu^{2+}$, $SO_4^{2-}$, $H^+$, and $OH^-$ ions. Copper metal forms at the cathode ($Cu^{2+} + 2e^- \rightarrow Cu$) because $Cu^{2+}$ is less reactive than $H^+$. Oxygen and water form at the anode ($4OH^- \rightarrow O_2 + 2H_2O + 4e^-$) because no halide is present. The overall equation is $2CuSO_4(aq) + 2H_2O(l) \rightarrow 2Cu(s) + O_2(g) + 2H_2SO_4(aq)$.