Metallic Bonding + Electrolysis

Question 1

What does a metallic lattice consist of?

Answer 1

• Positive metal ions surrounded by delocalised electrons
• These electrons were the valence electrons of the positive metal ions, but now do not belong to any atom (so are delocalised)

Question 2

What is a metallic bond?

Answer 2

• The electrostatic force of attracting between positive metal ions and the surround delocalised electrons
• This bond is what holds all pure metals together

The higher the charge on the ion, the stronger the bond will be due to a higher charge difference

Question 3

What are the main properties of metals?

Answer 3

• High melting/boiling point
• Good conductors of electricity and heat
• Malleable and ductile (can be bent and stretched)

Question 4

Why do metals have a high melting/boiling point?

Answer 4

The electrostatic forces of attraction between positive metal ions and delocalised electrons are strong and require alot of energy to overcome

Question 5

Why does metal conduct electricity?

Answer 5

There are many free delocalised electrons in the sea of electrons within the metallic lattice which can move and carry charge

Question 6

Why are metals malleable and ductile?

Answer 6

• Layers of positive ions can slide over each other without disrupting the metallic bonds
• The electrons simply move with the positive ions, maintaining the electrostatic attraction
• This is why metals have a high melting point but are often soft and flexible

Question 7

What is electrolysis?

Answer 7

The seperation of a compound into its constituent elements using electricity

Question 8

What charge does the cathode have during electrolysis and which ions does it attract?

Answer 8

• The cathode has a negative charge
• It will therefore attract cations

The cathode has a negative charge because electrons from the power supply will flow from the annode to the cathode

Question 9

What charge does the annode have during electrolysis and which ions does it attract?

Answer 9

• The annode has a positive charge
• It will therefore attract anions

The annode has a positive charge because electrons from the power supply will flow from the annode to the cathode

Question 10

Which compounds can electrolysis be used on?

Answer 10

• Molten ionic
• Aqueous ionic

It only works for ionic compounds as covalent compounds do not contained charged ions, and it only works for molten or aqueous ionic compounds because only these compounds can conduct electricity

Question 11

What happens in the electrolysis of a molten ionic compound (Lead Bromide(II) as an example)?

Answer 11

• The positive lead ions will be attracted to the cathode
• The negative bromine ions will be attracted to the annode
• Each Pb²⁺ ion will gain two electrons at the cathode, forming Pb(s)
• Each Br⁻ ion will lose an electron, and then pair up to form Br₂ (g)

• You need to heat the lead bromide first to make it molten
• The bromine gas will bubble to the top of the molten solution, and the lead will be deposited at the bottom
• Similar things will happen with any molten ionic compound, however be aware of whether the anions will form polyatomic molecules or not
• It is important to note that bromine gas only forms when doing electrolysis of a molten compound due to the presence of heat as its boiling point is only 60 degrees celcius, therefore in a solution bromine liquid will form which is red-brown in colour

Question 12

What will be the two ionic half equations for the electrolysis of molten lead bromide (II) as well as the overall chemical equation?

Answer 12

• Pb²⁺ + 2e⁻ –> Pb
• 2Br⁻ –> Br₂ + 2e⁻
• PbBr₂ –> Pb + Br₂

Question 13

What happens in the electrolysis of an aqueous ionic compound?

Answer 13

• H⁺ ions and OH⁻ ions will always be present in the solution alongside the ions of the ionic compound
• Whether it is the non-metal anion or the OH⁻ anion which is attracted to the annode depends on which one is easier to break and loses electrons more readily - usually if it is a complicated anion such as SO₄ it will be the OH⁻ anion
• Whether it is the metal cation or the H⁺ cation will depend on the metal’s reactivity - if it is more reactive than hydrogen then the H⁺ ion will be attracted, if it is not then it will be the metal

Question 14

What is the ionic half equation for hydroxide ions at the annode?

Answer 14

4OH⁻ –> 2H₂O + O₂ + 4e⁻

Water does not always count as a product, therefore oxygen is the only product at the annode here

Question 15

What will be the two ionic half equations for the electrolysis of aqueous sodium sulphate as well as the overall chemical equation?

Answer 15

• 2H⁺ + 2e⁻ –> H₂
• 4OH⁻ –> 2H₂O + O₂ + 4e⁻
• 2H₂O –> 2H₂ (g) + O₂ (g)

• The hydrogen is attracted to the cathode because sodium is more reactive than hydrogen
• The hydroxide is attracted to the annode because it is simpler and loses electrons more readily than the sulphate
• Water is also a product in the overall equation, but it is not written
• Sodium sulphate is there, but is not written in the equation as it remains unchanged
• Bubbling will be observed at both electrodes

Question 16

What will be the two ionic half equations for the electrolysis of aqueous copper chloride as well as the overall chemical equation?

Answer 16

• Cu²⁺ + 2e⁻ –> Cu
• 2Cl⁻ –> Cl₂ + 2e⁻
• CuCl₂ –> Cu + Cl₂

• The copper is attracted at the cathode because it is less reactive than hydrogen
• The chloride is attracted at the annode because it is simpler and loses electrons more readily than hydroxide
• At the cathode, a red-brown metal will be deposited at the bottom, and at the annode bubbling will be observed

Question 17

At which electrodes do oxidation/reduction reactions happen and why?

Answer 17

• Oxidation occurs at the annode because the ions lose electrons
• Reduction occurs at the cathode because the ions gain electrons