Carbon and silicon (group IVA)

The Group IVA elements are carbon C, silicon Si, germanium Ge, tin Sn and lead Pb. Down the group non-metallic character weakens and metallic character grows: carbon and silicon are non-metals, germanium is a semiconductor, and tin and lead are metals. Carbon is the basis of the whole organic world and of the main fuels (coal, oil, gas); silicon is the second most abundant element in the Earth’s crust after oxygen (sand, clay, granite, quartz) and the "heart" of modern electronics. In exams this topic is usually tested through problems on carbonates, CO₂ and alkalis, and on silicates.

§1. Carbon and its allotropes

In the excited state carbon has four unpaired electrons, and in compounds it is almost always tetravalent. Its allotropes are:

• Diamond — sp³, a giant covalent lattice in which each atom is bonded tetrahedrally to four others; the hardest natural substance, a non-conductor, strongly refracting light (cut diamonds).
• Graphite — sp², a layered structure: strong bonds within each layer, weak forces between layers; soft, marks paper, conducts electricity (electrodes, pencil "leads", lubricant).
• Carbyne (sp, linear chains), fullerenes (C₆₀, a ball-shaped molecule) and graphene (a single layer of graphite one atom thick).

Amorphous carbon (charcoal, coke, soot, activated charcoal) is a good adsorbent (gas masks, medicines) and a reducing agent: ZnO + C → Zn + CO, 2CuO + C → 2Cu + CO₂. With metals it forms carbides: CaC₂ + 2H₂O → Ca(OH)₂ + C₂H₂, Al₄C₃ + 12H₂O → 4Al(OH)₃ + 3CH₄.

Worked example

7.2 g of aluminium carbide was completely hydrolysed by water. What volume of methane (STP) is released?

1. M(Al₄C₃) = 4·27 + 3·12 = 144; n = 7.2/144 = 0.05 mol.
2. Al₄C₃ → 3CH₄: n(CH₄) = 0.15 mol.
3. V = 0.15·22.4 = 3.36 L.

§2. Oxides of carbon

Carbon monoxide CO is a colourless, odourless and highly toxic gas: it binds firmly to haemoglobin and stops oxygen transport. It forms when fuel burns incompletely in stoves and cars, which is why rooms must be ventilated. It is a neutral (non-salt-forming) oxide but a strong reducing agent:

Preparation: HCOOH → CO + H₂O (conc. H₂SO₄), CO₂ + C → 2CO (over hot coke).
Carbon dioxide CO₂ is a colourless, odourless gas that does not support combustion, is denser than air (M = 44) and as a solid is "dry ice". It is an acidic oxide: dissolving in water it gives weak, unstable carbonic acid CO₂ + H₂O ⇌ H₂CO₃. Plants absorb it in photosynthesis, and it is the main greenhouse gas. It is used in fire extinguishers and fizzy drinks.

With an alkali, depending on the ratio: CO₂ : NaOH = 1 : 2 → Na₂CO₃; 1 : 1 → NaHCO₃; in between, a mixture of both salts.

§3. Carbonates

The test for carbonate and hydrogencarbonate ions is the release of gas (CO₂) with a strong acid; the gas turns limewater milky: CaCO₃ + 2HCl → CaCl₂ + CO₂↑ + H₂O.
On heating: CaCO₃ → CaO + CO₂ (lime burning), 2NaHCO₃ → Na₂CO₃ + CO₂ + H₂O (baking soda making dough rise). Carbonates of the alkali metals (except lithium) do not decompose on heating.
In nature: limestone, chalk and marble (CaCO₃), dolomite (CaCO₃·MgCO₃), magnesite (MgCO₃). Carbonates are used in construction (lime, cement), glassmaking (sodium carbonate) and food (baking soda).

Worked example

Heating 18.4 g of a mixture of CaCO₃ and MgCO₃ released 4.48 L of CO₂ (STP). Find the composition of the mixture.

1. n(CO₂) = 4.48/22.4 = 0.2 mol → 0.2 mol of carbonates in total: x + y = 0.2.
2. By mass: 100x + 84y = 18.4.
3. 100x + 84(0.2 − x) = 18.4 → 16x = 1.6 → x = 0.1.
4. CaCO₃ 10 g, MgCO₃ 0.1·84 = 8.4 g.

§4. Silicon

Silicon is a grey crystalline solid with a metallic lustre and a diamond-like giant covalent lattice; it is a semiconductor, its conductivity rising with temperature. It never occurs free in nature, only in compounds (silica and silicates). Preparation: SiO₂ + 2Mg → Si + 2MgO (laboratory) or with coke in an electric furnace, SiO₂ + 2C → Si + 2CO. Ultra-pure silicon is used in solar cells, transistors and microchips.
At room temperature silicon is unreactive; when heated it burns in oxygen: Si + O₂ → SiO₂. It dissolves in alkalis, releasing hydrogen:

It does not dissolve in acids (except a mixture of HF and HNO₃).

Worked example

5.6 g of silicon dissolved in excess sodium hydroxide solution. What volume of hydrogen (STP) is released?

1. n(Si) = 5.6/28 = 0.2 mol.
2. Si → 2H₂: n(H₂) = 0.4 mol.
3. V = 0.4·22.4 = 8.96 L.

§5. Silicon dioxide, silicic acid and silicates

SiO₂ (sand, quartz, flint) is a hard, high-melting acidic oxide with a giant covalent structure; it neither dissolves in nor reacts with water. Fused with an alkali or sodium carbonate it forms silicates: SiO₂ + 2NaOH → Na₂SiO₃ + H₂O, SiO₂ + Na₂CO₃ → Na₂SiO₃ + CO₂, and with basic oxides SiO₂ + CaO → CaSiO₃. Of the acids, only hydrofluoric acid attacks it (etching glass).
Silicic acid H₂SiO₃ is a very weak acid, insoluble in water; any stronger acid displaces it from its salts as a jelly-like precipitate: Na₂SiO₃ + 2HCl → H₂SiO₃↓ + 2NaCl. A solution of sodium or potassium silicates is "water glass" (an adhesive and fireproof coating).
The silicate industry: ordinary glass (Na₂O·CaO·6SiO₂, made by melting sodium carbonate, limestone and sand), cement (limestone roasted with clay), ceramics, porcelain and brick. Adding metal oxides to glass gives coloured glass, and adding potassium and lead compounds gives lead crystal. Cement sets when mixed with water and is the basis of concrete and mortar.

Check yourself

1. 4.48 L of CO₂ (STP) was absorbed in a solution containing 0.3 mol of NaOH. Which salts form?

2. Why is CO poisonous?

3. What volume of CO₂ (STP) is released when 10 g of CaCO₃ reacts with excess hydrochloric acid?

4. Which acid etches glass?

5. By how many grams does the mass decrease when 16.8 g of NaHCO₃ is heated?

6. Why does graphite conduct electricity but diamond does not?

7. Why does the milkiness disappear when excess CO₂ is passed through limewater?

8. What is the formula of silicic acid?

Common mistakes

• Calling CO an acidic oxide: it is a neutral oxide.
• Writing that SiO₂ reacts with water to give silicic acid.
• Thinking only CaCO₃ forms even with excess CO₂.
• Assuming all carbonates decompose on heating: Na₂CO₃ and K₂CO₃ are stable.
• Writing that silicon dissolves in acids: it dissolves in alkalis.