Reactions of Group IV Elements: MDCAT Chemistry notes
Reactions of Group IV Elements for MDCAT: C to Pb trend, +4 and +2 states, inert pair effect, oxides, silicates and uses of lead compounds.
Group IV (Group 14) at a glance
Group IVA contains carbon, silicon, germanium, tin and lead. It is the one group that shows the full change from non-metal (C) through metalloids (Si, Ge) to metals (Sn, Pb). All have the valence configuration $ns^2np^2$, so they usually form four covalent bonds.
| Element | Nature | Common oxidation states | More stable state |
|---|---|---|---|
| C | Non-metal | +4 (also +2 in CO) | +4 |
| Si | Metalloid | +4 | +4 |
| Ge | Metalloid | +4, +2 | +4 |
| Sn | Metal | +4, +2 | +4 (Sn(II) is a reducing agent) |
| Pb | Metal | +2, +4 | +2 |
Inert pair effect
Down the group the $ns^2$ pair becomes more reluctant to take part in bonding. So the +2 state becomes more stable and the +4 state less stable. For lead, Pb(II) compounds are stable while Pb(IV) compounds such as $\mathrm{PbO_2}$ act as oxidizing agents. $\mathrm{PbCl_4}$ decomposes easily into $\mathrm{PbCl_2}$ and $\mathrm{Cl_2}$.
Typical reactions
- With oxygen: all form dioxides $\mathrm{MO_2}$. $\mathrm{CO_2}$ and $\mathrm{SiO_2}$ are acidic; $\mathrm{GeO_2}$, $\mathrm{SnO_2}$ and $\mathrm{PbO_2}$ are amphoteric. The monoxides $\mathrm{SnO}$ and $\mathrm{PbO}$ are amphoteric; $\mathrm{CO}$ is neutral. Acidic character of oxides decreases down the group.
- With halogens: tetrahalides $\mathrm{MX_4}$ are covalent. $\mathrm{CCl_4}$ is not hydrolysed, but $\mathrm{SiCl_4}$ is readily hydrolysed by water because silicon has vacant d-orbitals.
- With alkali: silicon dissolves in hot NaOH: $\mathrm{Si + 2NaOH + H_2O \rightarrow Na_2SiO_3 + 2H_2}$.
- With steam: red-hot carbon gives water gas: $\mathrm{C + H_2O \rightarrow CO + H_2}$.
- Hydrides: $\mathrm{CH_4, SiH_4, GeH_4, SnH_4, PbH_4}$; their stability falls sharply down the group. Carbon alone shows extensive catenation.
Important compounds and uses
| Compound | Use |
|---|---|
| Tetraethyl lead, $\mathrm{(C_2H_5)_4Pb}$ | Former anti-knock additive in petrol; phased out because lead is toxic |
| Sodium silicate, $\mathrm{Na_2SiO_3}$ (water glass) | Adhesive, fire-proofing, silica gel preparation |
| Red lead, $\mathrm{Pb_3O_4}$ | Protective paint that prevents rusting of iron |
| Litharge, $\mathrm{PbO}$ | Glass and rubber industry |
| $\mathrm{PbO_2}$ | Positive plate of the lead storage battery |
| Silicones | Water repellents, lubricants, insulators |
Silicates are not fertilizers. The mineral used to make phosphate fertilizers is calcium phosphate, $\mathrm{Ca_3(PO_4)_2}$ (phosphate rock), a compound of phosphorus. Treating it with sulphuric acid gives superphosphate.
Common MDCAT traps
- Group IIA, VIIA and IB do not contain all three kinds of element (non-metal, metalloid and metal). Group IVA does.
- The anti-knock compound is the lead alkyl, not the Sn, Ge or Si analogue.
- For lead the stable state is +2, not +4: the inert pair effect reverses the carbon pattern.
- A silicate in the options is not the fertilizer; the phosphate is.
- $\mathrm{CO_2}$ is acidic, but $\mathrm{SnO_2}$ and $\mathrm{PbO_2}$ are amphoteric.
Quick revision
- Group IVA: C non-metal, Si and Ge metalloids, Sn and Pb metals.
- Inert pair effect makes +2 more stable down the group.
- $\mathrm{PbO_2}$ is a strong oxidizing agent.
- $\mathrm{SiCl_4}$ hydrolyses, $\mathrm{CCl_4}$ does not.
- $\mathrm{Na_2SiO_3}$ is water glass; $\mathrm{Pb_3O_4}$ is red lead.