Groups 13 and 14 Compared: Quick Revision

A brief note on Groups 13 and 14 Compared. Read it before a test. Try each question at the end before you open its answer.

The inert pair effect

• The heavy elements prefer the state two lower: Tl$^+$, Pb$^{2+}$. Cause: poor shielding by $d$ and $f$ electrons, and weak bonds of large atoms.
• Tl(III), Pb(IV) oxidise. Sn(II), Ge(II) reduce. The lower state is more ionic.
• $\text{PbCl}_{4}$ loses chlorine. $\text{PbBr}_{4}$ and $\text{PbI}_{4}$ do not exist. TlI$_3$ is Tl$^+$ I$_3^-$.
• $\log K = nE^\circ/0.05916$ at 298 K. A reaction goes if $E^\circ_{cell}$ is positive.

Hydrides

• Electron deficient: $\text{B}_{2}\text{H}_{6}$, $(\text{AlH}_{3})\text{n}$. Electron precise: $\text{CH}_{4}$, $\text{SiH}_{4}$. Stability falls down the groups.
• $\text{LiAlH}_{4}$ and $\text{NaBH}_{4}$ hold tetrahedral $[\text{AlH}_{4}]^{-}$ and $[\text{BH}_{4}]^{-}$. $\text{LiAlH}_{4} + 4\text{H}_{2}\text{O} \to \text{LiOH} + \text{Al}(\text{OH})_{3} + 4\text{H}_{2}$.
• $\text{SiH}_{4}$ burns in air and is hydrolysed in alkaline water. $\text{Mg}_{2}\text{Si} + 4\text{HCl} \to 2\text{MgCl}_{2} + \text{SiH}_{4}$.

Halides

• Lewis acid strength: $\text{BF}_3 < \text{BCl}_3 < \text{BBr}_3 < \text{BI}_3$, because of back bonding.
• $\text{CCl}_{4}$ is not hydrolysed. $\text{SiCl}_{4} + 4\text{H}_{2}\text{O} \to \text{Si}(\text{OH})_{4} + 4\text{HCl}$. $\text{SiF}_{4}$ gives $[\text{SiF}_{6}]^{2-}$.
• $\text{Al}_{2}\text{Cl}_{6}$ has two-centre bridges. Diborane has three-centre bridges.

Oxides and carbides

• Acidic: $\text{B}_{2}\text{O}_{3}$, $\text{CO}_{2}$, $\text{SiO}_{2}$, $\text{GeO}_{2}$. Amphoteric: $\text{Al}_{2}\text{O}_{3}$, $\text{SnO}_{2}$, $\text{PbO}_{2}$, PbO. Basic: $\text{In}_{2}\text{O}_{3}$, $\text{Tl}_{2}\text{O}$. Neutral: CO.
• Red lead $\text{Pb}_{3}\text{O}_{4}$ is $2\text{PbO}\cdot \text{PbO}_{2}$. Dilute nitric acid gives $\text{PbO}_{2}$ and lead nitrate.
• $\text{CaC}_{2}$ gives acetylene, $\text{Al}_{4}\text{C}_{3}$ gives methane, $\text{Mg}_{2}\text{C}_{3}$ gives propyne. SiC and $\text{B}_{4}\text{C}$ are hard covalent carbides.

A worked example

Find $\log K$ for a reaction with a cell potential of $0.92\ \text{V}$ and two electrons.

1. Put in the numbers. $\log K = 2 \times 0.92 / 0.05916$.
2. Work out. $\log K = 31.1$, so the reaction runs to completion.

Practice with new numbers

A program to try

Mistakes to avoid

• Saying that Sn(IV) is a strong oxidiser. Pb(IV) is.
• Saying that BF$_3$ is the strongest of the boron halides as a Lewis acid. It is the weakest.
• Saying that CCl$_4$ is hydrolysed. It is not.
• Calling PbO$_2$ or SnO$_2$ acidic. They are amphoteric.

Check yourself

1. Why is thallium(I) more stable than thallium(III)?

2. Why is SiCl$_4$ hydrolysed but CCl$_4$ is not?

3. What gas comes from water and aluminium carbide?

4. Name the amphoteric oxides of group 14.

Sources

• OpenStax, Chemistry 2e, Chapter 18.
• NCERT Chemistry for Class 11, the chapter The p-Block Elements (older edition).
• Atkins and Shriver, Inorganic Chemistry, the chapters on the group 13 and group 14 elements.