Oxides, Chlorides and Hydrides of Group 14 and Their Thermal Stability – Trends in Chemical Properties of Group 14 Elements and Their Compounds (Senior 4 Chemistry) | Ganzaa

Trends in Chemical Properties of Group 14 Elements and Their Compounds

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Senior 4 · Chemistry

Trends in Chemical Properties of Group 14 Elements and Their Compounds

5 lessons

Lesson 3 of 5

Oxides, Chlorides and Hydrides of Group 14 and Their Thermal Stability

25 min

Same formula, different worlds

Carbon dioxide and silicon dioxide have the same type of formula, $\mathrm{MO_2}$ . Yet carbon dioxide is the gas you breathe out, while silicon dioxide is sand and quartz, so hard and high-melting that it is used to make glass and the bricks that line furnaces. In the same way, tetrachloromethane, $\mathrm{CCl_4}$ , can be shaken with water and nothing happens, but silicon tetrachloride, $\mathrm{SiCl_4}$ , reacts with the moisture in air and fumes. This lesson compares the oxides, chlorides and hydrides of the Group 14 elements, and asks which of them break down when heated.

Definition

Hydrolysis: the breaking of a compound by reaction with water. Example: $\mathrm{SiCl_4(l) + 2H_2O(l) \rightarrow SiO_2(s) + 4HCl(aq)}$ .

Thermal stability: how well a compound resists decomposition when heated. A compound that decomposes at a lower temperature is less thermally stable. Example: lead(IV) oxide decomposes at 290 °C, $\mathrm{2PbO_2 \rightarrow 2PbO + O_2}$ , while tin(IV) oxide melts unchanged at 1630 °C.

The oxides

Table 9.3: Oxides of the Group 14 elements

Oxide	Structure	Melting point	Acid–base character
$\mathrm{CO_2}$	simple linear molecules, O=C=O	sublimes at −78.5 °C	acidic
CO	simple molecules	gas	neutral; a reducing agent
$\mathrm{SiO_2}$	giant covalent	1713 °C	acidic (weakly)
$\mathrm{GeO_2}$	giant covalent	1115 °C	amphoteric, mainly acidic
$\mathrm{SnO_2}$ ; SnO	giant, partly ionic	1630 °C ( $\mathrm{SnO_2}$ )	amphoteric
PbO; $\mathrm{PbO_2}$	giant, largely ionic	888 °C (PbO); $\mathrm{PbO_2}$ decomposes at 290 °C	amphoteric, PbO mainly basic

Why $\mathrm{CO_2}$ is a gas and $\mathrm{SiO_2}$ a solid. Carbon is small enough for its 2p orbitals to overlap sideways with those of oxygen, forming strong π bonds, so each carbon atom can be satisfied by two C=O double bonds and the molecule is complete. Silicon is larger: its 3p orbitals overlap poorly with oxygen's 2p orbitals, so Si=O double bonds are weak. Silicon instead forms four single Si–O bonds to four different oxygen atoms, and each oxygen bridges two silicon atoms, building a giant network (Figure 9.5).

Figure 9.5: Molecular $\mathrm{CO_2}$ and giant covalent $\mathrm{SiO_2}$ .

Acid–base behaviour changes from acidic at the top to amphoteric lower down, and for any one element the +2 oxide is more basic than the +4 oxide:

\mathrm{CO_2(g) + 2NaOH(aq) \rightarrow Na_2CO_3(aq) + H_2O(l)} \qquad \mathrm{SiO_2(s) + 2NaOH(l) \rightarrow Na_2SiO_3(s) + H_2O(g)}

\mathrm{SnO_2(s) + 2NaOH(aq) \rightarrow Na_2SnO_3(aq) + H_2O(l)} \qquad \mathrm{PbO(s) + 2HNO_3(aq) \rightarrow Pb(NO_3)_2(aq) + H_2O(l)}

\mathrm{PbO(s) + 2NaOH(aq) \rightarrow Na_2PbO_2(aq) + H_2O(l)}

Silicon dioxide reacts only with hot, concentrated or molten alkali; it is also attacked by hydrofluoric acid, which is how glass is etched: $\mathrm{SiO_2 + 4HF \rightarrow SiF_4 + 2H_2O}$ .

The chlorides

Table 9.4: Chlorides of the Group 14 elements

Chloride	State at 25 °C	Boiling or melting point	With water	On heating
$\mathrm{CCl_4}$	colourless liquid	boils at 76.7 °C	no reaction	stable
$\mathrm{SiCl_4}$	colourless liquid	boils at 57.7 °C	hydrolysed rapidly; fumes of HCl	stable
$\mathrm{GeCl_4}$	colourless liquid	boils at 86.5 °C	hydrolysed	stable
$\mathrm{SnCl_4}$	colourless fuming liquid	boils at 114.2 °C	hydrolysed	stable
$\mathrm{PbCl_4}$	yellow oily liquid	stable only below about 0 °C	hydrolysed	decomposes near 50 °C: $\mathrm{PbCl_4 \rightarrow PbCl_2 + Cl_2}$
$\mathrm{SnCl_2}$	white solid	melts at 247 °C	partly hydrolysed	stable
$\mathrm{PbCl_2}$	white solid	melts at 501 °C	sparingly soluble; no hydrolysis	stable

The tetrachlorides are simple covalent molecules, so they are volatile liquids. The dichlorides of tin and lead are more ionic solids, in line with the inert pair effect: the +2 state is more ionic and more stable for the heavier elements.

Why $\mathrm{SiCl_4}$ is hydrolysed but $\mathrm{CCl_4}$ is not. Water attacks by donating a lone pair on its oxygen to the central atom (Figure 9.6). The silicon atom is large enough for a fifth group to approach between the chlorine atoms, and it has low-energy empty orbitals (3d) that can accept the pair. A chlorine then leaves as HCl, and the steps repeat:

\mathrm{SiCl_4(l) + 4H_2O(l) \rightarrow Si(OH)_4(s) + 4HCl(aq)} \qquad \text{then} \qquad \mathrm{Si(OH)_4 \rightarrow SiO_2{\cdot}2H_2O}

Carbon is small, so its four chlorine atoms shield it completely, and it has no 2d orbitals: there is no route for the water to attack, so $\mathrm{CCl_4}$ is not hydrolysed even though the reaction would release energy.

Figure 9.6: The first step in the hydrolysis of $\mathrm{SiCl_4}$ . The orange arrow shows the lone pair of water attacking silicon.

Tin(II) chloride is partly hydrolysed, giving a cloudy basic chloride: $\mathrm{SnCl_2 + H_2O \rightleftharpoons Sn(OH)Cl + HCl}$ . Its solutions are therefore made up in dilute hydrochloric acid, which pushes the equilibrium to the left.

The hydrides

All the elements form hydrides $\mathrm{MH_4}$ : methane, silane, germane, stannane and plumbane. Their thermal stability falls sharply down the group, because the M–H bond gets longer and weaker as the atom M gets bigger. Methane is stable to well above 500 °C; silane catches fire in air of its own accord; stannane decomposes slowly to tin and hydrogen even at room temperature; plumbane is so unstable that it has barely been detected. Carbon is unique in forming thousands of stable hydrides with chains and rings, the hydrocarbons of organic chemistry.

Thermal stability: the pattern

Oxides: $\mathrm{CO_2}$ , $\mathrm{SiO_2}$ and $\mathrm{SnO_2}$ are stable to heat; $\mathrm{PbO_2}$ loses oxygen at 290 °C to give PbO.
Halides: the tetrachlorides of C, Si, Ge and Sn are stable; $\mathrm{PbCl_4}$ decomposes to $\mathrm{PbCl_2}$ and chlorine near 50 °C.
Hydroxides: the white precipitates of tin(II) and lead(II) hydroxide lose water on gentle heating to give the oxides, $\mathrm{Pb(OH)_2 \rightarrow PbO + H_2O}$ ; "silicic acid" dehydrates to silica.
Hydrides: stability falls from $\mathrm{CH_4}$ to $\mathrm{PbH_4}$ .

In every case the +4 compounds of lead are the least stable: heat drives them to the +2 state, another face of the inert pair effect.

Worked example 1: acid from a fuming liquid

0.850 g of silicon tetrachloride is added to water and the solution is made up to 250 cm³. Calculate the concentration of hydrochloric acid in the solution. $A_{r}$ : Si 28.09, Cl 35.45.

Solution. $\mathrm{SiCl_4 + 2H_2O \rightarrow SiO_2 + 4HCl}$ : 1 mol $\mathrm{SiCl_4}$ gives 4 mol HCl. $M(\mathrm{SiCl_4}) = 28.09 + 4(35.45) = 169.89\ \mathrm{g\,mol^{-1}} \qquad n = \frac{0.850}{169.89} = 5.00 \times 10^{-3}\ \mathrm{mol}$ $n(\mathrm{HCl}) = 4 \times 5.00 \times 10^{-3} = 0.0200\ \mathrm{mol} \qquad c = \frac{0.0200\ \mathrm{mol}}{0.250\ \mathrm{dm^3}} = 0.0800\ \mathrm{mol\,dm^{-3}}$

Worked example 2: heating lead(IV) oxide

4.78 g of lead(IV) oxide is heated until no more gas is given off. Calculate (a) the loss in mass and (b) the volume of oxygen at room temperature and pressure (24.0 dm³ mol⁻¹). $M(\mathrm{PbO_2}) = 239.2\ \mathrm{g\,mol^{-1}}$ .

Solution. $\mathrm{2PbO_2(s) \rightarrow 2PbO(s) + O_2(g)}$ . $n(\mathrm{PbO_2}) = \frac{4.78}{239.2} = 0.01998\ \mathrm{mol} \qquad n(\mathrm{O_2}) = \frac{0.01998}{2} = 9.99 \times 10^{-3}\ \mathrm{mol}$ (a) Loss in mass $= 9.99 \times 10^{-3} \times 32.00 = 0.320\ \mathrm{g}$ . (b) $V = 9.99 \times 10^{-3} \times 24.0 = 0.240\ \mathrm{dm^3} = 240\ \mathrm{cm^3}$ .
Tin(IV) oxide treated the same way loses no mass: +4 is tin's stable state.

Warning

Common mistakes. (1) Saying $\mathrm{CCl_4}$ is not hydrolysed because the C–Cl bond is too strong: the reaction would be exothermic; it has no pathway (kinetic stability). (2) Drawing $\mathrm{SiO_2}$ as O=Si=O molecules. (3) Forgetting that the +2 oxides are more basic than the +4 oxides of the same element. (4) Writing the hydrolysis of $\mathrm{SiCl_4}$ without HCl as a product.

Safety. Silicon tetrachloride and tin(IV) chloride react with moisture to give hydrogen chloride fumes, which attack the eyes and lungs: use them only in a fume cupboard, with goggles and gloves. Hydrofluoric acid causes deep burns that may not hurt at first and is never used in school laboratories. If a chemical gets on your skin or into your eyes, rinse at once with plenty of running water and get medical help.

Tip

Three rules cover most Group 14 compound questions: (1) C forms π bonds, Si does not (gas $\mathrm{CO_2}$ , giant $\mathrm{SiO_2}$ ); (2) C cannot be attacked, Si can ( $\mathrm{CCl_4}$ inert, $\mathrm{SiCl_4}$ hydrolysed); (3) Pb(IV) falls to Pb(II) on heating ( $\mathrm{PbO_2}$ , $\mathrm{PbCl_4}$ ).

Dr Roe Chemistry: Group IV Chlorides, A2 Chemistry (20 min). Watch for how the bonding in the chlorides changes down the group and for the explanation of why silicon tetrachloride reacts with water while tetrachloromethane does not.

✦

Practice

Explain, in terms of bonding and structure, why carbon dioxide sublimes at −78.5 °C but silicon dioxide melts at 1713 °C.
Write equations to show that tin(IV) oxide is amphoteric.
Arrange $\mathrm{CO_2}$ , $\mathrm{SnO_2}$ and PbO in order of increasing basic character.
Describe what is seen when silicon tetrachloride is added to water, and write the equation.
Explain why tetrachloromethane is not hydrolysed by water.
Why are tin(II) chloride solutions made up in dilute hydrochloric acid?
Arrange $\mathrm{CH_4}$ , $\mathrm{SnH_4}$ and $\mathrm{SiH_4}$ in order of decreasing thermal stability, and explain.
A sample of lead(IV) oxide lost 0.160 g of mass when heated to constant mass. What mass of lead(IV) oxide was heated? $M(\mathrm{PbO_2}) = 239.2$ .

Show answersHide answers

$\mathrm{CO_2}$ is made of small separate molecules (C forms strong π bonds, so two C=O bonds complete it), held together only by weak London forces. $\mathrm{SiO_2}$ is a giant covalent lattice: Si forms four single bonds to O atoms that bridge to other Si atoms, so melting breaks strong covalent bonds.
$\mathrm{SnO_2 + 2NaOH \rightarrow Na_2SnO_3 + H_2O}$ (acting as an acid); $\mathrm{SnO_2 + 4HCl \rightarrow SnCl_4 + 2H_2O}$ with concentrated acid (acting as a base).
$\mathrm{CO_2}$ < $\mathrm{SnO_2}$ < PbO.
A vigorous reaction with steamy fumes of hydrogen chloride and a white solid (hydrated silica): $\mathrm{SiCl_4 + 2H_2O \rightarrow SiO_2 + 4HCl}$ .
The small carbon atom is surrounded by four chlorine atoms, so water cannot reach it, and carbon has no low-energy empty orbitals to accept a lone pair. The reaction has no pathway.
$\mathrm{SnCl_2 + H_2O \rightleftharpoons Sn(OH)Cl + HCl}$ : added HCl moves the equilibrium to the left, preventing the cloudy basic chloride forming.
$\mathrm{CH_4}$ > $\mathrm{SiH_4}$ > $\mathrm{SnH_4}$ : the M–H bond becomes longer and weaker as M gets larger.
$n(\mathrm{O_2}) = \frac{0.160}{32.00} = 5.00 \times 10^{-3}$ mol, so $n(\mathrm{PbO_2}) = 0.0100$ mol, and mass $= 0.0100 \times 239.2 = 2.39\ \mathrm{g}$ .

Quick check

Why is silicon dioxide a high-melting solid while carbon dioxide is a gas?

Key points

$\mathrm{CO_2}$ is molecular (C=O π bonds); $\mathrm{SiO_2}$ is giant covalent (four Si–O single bonds). The oxides change from acidic (C, Si) to amphoteric (Ge, Sn, Pb); the +2 oxides are more basic than the +4 oxides.
$\mathrm{MCl_4}$ are volatile covalent liquids; $\mathrm{SiCl_4}$ , $\mathrm{GeCl_4}$ and $\mathrm{SnCl_4}$ are hydrolysed, $\mathrm{CCl_4}$ is not (no pathway). $\mathrm{SnCl_2}$ and $\mathrm{PbCl_2}$ are more ionic solids.
Thermal stability of the hydrides falls from $\mathrm{CH_4}$ to $\mathrm{PbH_4}$ as the M–H bond weakens.
Lead(IV) compounds decompose on heating to lead(II): $\mathrm{2PbO_2 \rightarrow 2PbO + O_2}$ (290 °C) and $\mathrm{PbCl_4 \rightarrow PbCl_2 + Cl_2}$ (about 50 °C).

Sources and credits

Data from the Wikipedia articles Carbon dioxide, Silicon dioxide, Germanium dioxide, Tin(IV) oxide, Lead dioxide, Lead(II) oxide, Silicon tetrachloride, Germanium tetrachloride, Tin(IV) chloride, Lead(IV) chloride, Tin(II) chloride, Lead(II) chloride, Silane and Stannane (CRC Handbook values), CC BY-SA 4.0, adapted by Ganzaa.
First aid for chemical splashes: NIOSH Pocket Guide, first aid, CDC.
Video: Group IV Chlorides (A2 Chemistry), Dr Roe Chemistry, embedded from YouTube.
The text, structure diagrams, tables, worked examples and Practice items were written and drawn by Ganzaa.
This lesson adapts CC BY-SA material and is shared under CC BY-SA 4.0.

1Same formula, different worlds

2The oxides

3The chlorides

4The hydrides

5Thermal stability: the pattern

Same formula, different worlds

The oxides

The chlorides

The hydrides

Thermal stability: the pattern