Lead is an excellent roofing material. It is malleable and resistant to corrosion. Lead rapidly becomes coated with basic lead carbonate which protects it from further corrosion.
(a) Lead has a typical metallic structure which is a lattice of lead ions surrounded by a ‘sea’ of mobile electrons. This structure is held together by attractive forces called a metallic bond.
(i) Explain why there are attractive forces in a metallic structure.[2]
(ii) Explain why a metal, such as lead, is malleable.[2]
(b) Basic lead(II) carbonate is heated in the apparatus shown below. Water and carbon dioxide are produced.
(i) Silica gel absorbs water. Silica gel often contains anhydrous cobalt(II) chloride. When this absorbs water it changes from blue to pink.
Suggest a reason.[1]
(ii) Soda lime is a mixture of sodium hydroxide and calcium oxide. Why do these two substances react with carbon dioxide?[2]
(iii) Name two substances formed when soda lime reacts with carbon dioxide.[2]
(c) Basic lead(II) carbonate has a formula of the type xPbCO3.yPb(OH)2 where x and y are whole numbers.
Determine x and y from the following information.
PbCO3 → PbO + CO2
Pb(OH)2 → PbO + H2O
When heated, the basic lead(II) carbonate gave 2.112 g of carbon dioxide and 0.432 g of water.
Mass of one mole of CO2 = 44g
Mass of one mole of H2O = 18g
Number of moles of CO2 formed = [1]
Number of moles of H2O formed = [1]
x = ___ and y = ___
Formula of basic lead(II) carbonate is ___ [1] [Total: 12]
▶️ Answer/Explanation
(a) (i) Ans: Attractive forces exist due to electrostatic attraction between positive lead ions and the ‘sea’ of delocalized electrons.
In a metallic lattice, the positively charged metal ions are attracted to the negatively charged mobile electrons, creating strong metallic bonds.
(ii) Ans: Lead is malleable because its layers of ions can slide over each other without breaking the metallic bonds.
The non-directional nature of metallic bonding allows the lattice to deform under pressure, making lead malleable.
(b) (i) Ans: Anhydrous cobalt(II) chloride changes color because it forms a hydrated compound when it absorbs water.
The blue anhydrous CoCl2 turns pink as it becomes CoCl2·6H2O.
(ii) Ans: Sodium hydroxide and calcium oxide react with carbon dioxide because CO2 is acidic, and these are bases.
NaOH reacts to form Na2CO3, while CaO forms CaCO3.
(iii) Ans: Two possible products are sodium carbonate (Na2CO3) and calcium carbonate (CaCO3).
Other products may include water (H2O) or sodium bicarbonate (NaHCO3).
(c) Ans: x = 2, y = 1; Formula = 2PbCO3·Pb(OH)2
Moles of CO2 = 2.112/44 = 0.048 (from PbCO3). Moles of H2O = 0.432/18 = 0.024 (from Pb(OH)2). Ratio x:y = 2:1.
There are three types of giant structure – ionic, metallic and giant covalent.
(a) In an ionic compound, the ions are held in a lattice by strong forces.
(i) Explain the term lattice.[2]
(ii) Explain how the ions are held together by strong forces.[1]
(b) Describe the bonding in a typical metal.[3]
(c) The electrical conductivities of the three types of giant structure are given in the following table.
Explain the differences in electrical conductivity between the three types of giant structure and the difference, if any, between the solid and liquid states of the same structure.[5] [Total: 11]
▶️ Answer/Explanation
(a)(i) Ans: A lattice is a regular, repeating 3D arrangement of ions in an ionic compound.
This ordered structure maximizes electrostatic attraction between oppositely charged ions.
(a)(ii) Ans: Ions are held together by strong electrostatic forces between oppositely charged ions (cations and anions).
(b) Ans: In metals:
- Atoms lose outer electrons to form positive ions (cations).
- Electrons become delocalized and move freely throughout the structure.
- There is a strong electrostatic attraction between the positive ions and the delocalized electrons (metallic bonding).
(c) Ans:
Giant covalent: No conductivity (solid/liquid) because there are no charged particles free to move (all electrons are fixed in bonds).
Ionic: Conducts only when molten/dissolved because ions become mobile; solid ionic compounds don’t conduct as ions are fixed in the lattice.
Metallic: Conducts in both solid and liquid states due to delocalized electrons that can move freely and carry charge.