Electrolysis

Electrolysis

Electrolytes

In order for electrolysis to occur there must be an electrolyte to carry the current. An electrolyte contains free-floating charged particles. Ionic compounds are electrolytes when dissolved in water, or melted into their liquid state. Molecular compounds cannot conduct electricity - graphite is the one exception!

Multiple Select

Which of the following would be electrolytes? (Check all that apply)

• A solution of salt water

• Molten sugar

• Solid KCl

• Molten Lead Bromide

• Pure Water

Electrodes

A conducting material is placed into the electrolyte and a power supply is connected, resulting in a flow of electrons. Electrons move away from one electrode, resulting in a positive charge and towards the other electrode, resulting in a negative charge.

Electrodes must be made of a conducting material such as metal or graphite.

Cathode

The negatively charged electrode is called a cathode.

The cathode attracts cations from the electrolyte.

Reduction occurs at the cathode.

Anode

The positively charged electrode is called a anode.

The anode attracts anions from the electrolyte.

Oxidation occurs at the anode.

Half-Equations

Electrolysis occurs as a result of a redox reaction. The reduction and oxidation happen separately at each electrode. For example:

Cathode:

2 Na⁺ + 2 e^- --> 2 Na

Anode:

2 Cl^- --> Cl₂ + 2 e^-

The balanced equation for the reaction would be:

2 NaCl --> 2 Na + Cl₂

Multiple Choice

Which of the following would be the correct half-equations for the electrolysis of molten lead (II) bromide?

• Pb + 2e^- --> Pb²⁺

  
  

  Br₂ --> 2 Br^- + 2 e^-

• Pb --> Pb²⁺ + 2e^-

  
  

  Br₂ + 2 e^- --> 2 Br^-

• Pb²⁺ --> Pb + 2e^-

  
  

  Br₂ + 2 e^- --> 2 Br^-

• Pb²⁺ + 2e^- --> Pb

  
  

  2 Br^- --> Br₂ + 2 e^- 

Multiple Choice

Which reaction would happen at the cathode?

• Pb²⁺ + 2e^- --> Pb

• 2 Br^- --> Br₂ + 2 e^- 

Multiple Choice

Which reaction is an oxidation?

• Pb²⁺ + 2e^- --> Pb

• 2 Br^- --> Br₂ + 2 e^-

Aqueous Solutions

When the electrolyte is an aqueous solution, things are more complicated.

In addition to the ions of the electrolyte, water always contains a small number of H⁺ and OH^- ions.

These ions will also be attracted to the electrodes and undergo redox.

Reactivity Series

What happens at the electrodes in an aqueous solution depends on the relative reactivity of the ions.

Substances that are higher up on the reactivity series tend to remain as ions, while substances lower down tend to become neutral.

Reactivity Series

Notice that sodium is more reactive than hydrogen.

In a solution that contains both hydrogen ions and sodium ions hydrogen will be reduced and the sodium ions will remain unchanged.

Electrolysis of salt water produces hydrogen gas (H₂) at the cathode not sodium metal!

Rules for Electrolysis in Solution

• At the cathode: either metal or hydrogen gas will be produced - whichever is lower on the reactivity series.

• At the anode: if the solution contains a halide ion (e.g. Cl^-, Br^- etc) AND the solution is concentrated then the halogen will be produced (e.g. Cl₂ gas)

• If the electrolyte solution is dilute or does not contain any halide ions then oxygen will always be produced at the anode

Multiple Select

Which solutions will produce oxygen gas at the anode?

• Concentrated NaCl

• Concentrated KNO₃

• Dilute KBr

• Concentrated Magnesium Phosphate

• Dilute sugar

Multiple Select

Which solutions would produced hydrogen gas at the cathode?

• Concentrated NaCl

• Dilute AgNO₃

• Concentrated Copper (II) Chloride

• Dilute K₂SO₄

• Concentrated FeCl₃

Multiple Choice

What would be produced at the cathode if an aqueous solution of concentrated KBr is electrolysed?

• Potassium metal

• Hydrogen gas

• Bromine

• Oxygen gas

Multiple Choice

What would be produced at the anode if an aqueous solution of concentrated KBr is electrolysed?

• Potassium metal

• Hydrogen gas

• Bromine

• Oxygen gas

Multiple Choice

What would be produced at the cathode if molten sodium chloride is electrolyzed?

• Sodium metal

• Hydrogen gas

• Chlorine gas

• Oxygen gas

Multiple Choice

What would be produced at the anode if molten sodium chloride is electrolyzed?

• Sodium metal

• Hydrogen gas

• Chlorine gas

• Oxygen gas

Multiple Choice

What would be produced at the cathode if a solution of concentrated AgNO₃ is electrolyzed?

• Silver metal

• Hydrogen gas

• Oxygen gas

• Nitrogen dioxide

Multiple Choice

What would be produced at the anode if a solution of concentrated AgNO₃ is electrolyzed?

• Silver metal

• Hydrogen gas

• Oxygen gas

• Nitrogen dioxide