Oxides, carbides and the reaction maps

A grey stone, a red paint and a grinding powder

A miner's carbide lamp drips water on a grey stone and burns the gas. Red lead paint coats iron to stop rust. Carborundum is a grey powder used to sharpen tools. All of them are compounds of group 13 or 14, and they belong to this lesson.

Oxides, from acidic to basic

Acid and base behaviour of an oxide changes in two ways.

• Down a group, the oxide becomes less acidic and more basic.
• In a higher oxidation state, the oxide is more acidic. A dioxide is more acidic than the monoxide of the same element.

Some reactions that show the amphoteric character of tin and lead: $\text{SnO}_{2} + 4\text{HCl} \to \text{SnCl}_{4} + 2\text{H}_{2}\text{O}$ and $\text{SnO}_{2} + 2\text{NaOH} + 2\text{H}_{2}\text{O} \to \text{Na}_{2}[\text{Sn}(\text{OH})_{6}]$. For lead(II) oxide, $\text{PbO} + 2\text{HNO}_{3} \to \text{Pb}(\text{NO}_{3})_{2} + \text{H}_{2}\text{O}$ and $\text{PbO} + 2\text{NaOH} \to \text{Na}_{2}\text{PbO}_{2} + \text{H}_{2}\text{O}$. Thallium(I) oxide is the basic one: $\text{Tl}_{2}\text{O} + \text{H}_{2}\text{O} \to 2\text{TlOH}$, and thallium(I) hydroxide is a strong base.

The oxides of lead. Yellow litharge, PbO, heated in air at about $750\ \text{K}$ gives red lead: $6\text{PbO} + \text{O}_{2} \to 2\text{Pb}_{3}\text{O}_{4}$. Red lead is a mixed oxide, $2\text{PbO}\cdot \text{PbO}_{2}$. With hot dilute nitric acid it splits into the two parts, $\text{Pb}_{3}\text{O}_{4} + 4\text{HNO}_{3} \to 2\text{Pb}(\text{NO}_{3})_{2} + \text{PbO}_{2} + 2\text{H}_{2}\text{O}$, and the brown solid that remains is lead(IV) oxide. Lead(IV) oxide is a strong oxidiser, and it is the active material in the lead storage battery: $\text{Pb} + \text{PbO}_{2} + 2\text{H}_{2}\text{SO}_{4} \to 2\text{PbSO}_{4} + 2\text{H}_{2}\text{O}$.

Carbides

A carbide is a compound of carbon with a less electronegative element. There are three types.

• Ionic carbides contain carbide ions and are hydrolysed by water. $\text{CaC}_{2}$ has the acetylide ion $\text{C}_{2}^{2-}$ and gives acetylene: $\text{CaC}_{2} + 2\text{H}_{2}\text{O} \to \text{Ca}(\text{OH})_{2} + \text{C}_{2}\text{H}_{2}$. $\text{Al}_{4}\text{C}_{3}$ has the methanide ion $\text{C}^{4-}$ and gives methane: $\text{Al}_{4}\text{C}_{3} + 12\text{H}_{2}\text{O} \to 4\text{Al}(\text{OH})_{3} + 3\text{CH}_{4}$. $\text{Mg}_{2}\text{C}_{3}$ gives propyne: $\text{Mg}_{2}\text{C}_{3} + 4\text{H}_{2}\text{O} \to 2\text{Mg}(\text{OH})_{2} + \text{C}_{3}\text{H}_{4}$.
• Covalent carbides are giant networks. Silicon carbide, SiC or carborundum, is made by $\text{SiO}_{2} + 3\text{C} \to \text{SiC} + 2\text{CO}$ in an electric furnace. It has the diamond structure, with silicon and carbon in turn, so it is hard, and it is used for grinding. Boron carbide, $\text{B}_{4}\text{C}$, is made by $2\text{B}_{2}\text{O}_{3} + 7\text{C} \to \text{B}_{4}\text{C} + 6\text{CO}$ and is harder still.
• Interstitial carbides such as tungsten carbide, WC, and titanium carbide, TiC, have carbon in the gaps of a metal. They are very hard and conduct electricity.

Following a chain of reactions

A reaction map joins compounds by arrows with their reagents. To work out a map, find the one step that you know, and go forwards and backwards from it. For these two groups, use the checks below.

1. What is the oxidation state at each stage? Does the step oxidise or reduce?
2. Is the oxide acidic or basic? Acids come from non-metals, bases from metals.
3. Does a gas come off? Hydrogen chloride, carbon dioxide and hydrogen are the usual ones.
4. Is there a colour or a solid that gives a clue? Brown lead(IV) oxide, red lead, blue borax bead, white boric acid.

Practise with new numbers

Try it in Python

The program counts the oxides by character, and reads the trend down group 14.

Common mistakes

• Calling CO an acidic oxide. It is neutral.
• Calling PbO$_2$ acidic. Both tin and lead dioxides are amphoteric.
• Writing the carbide of aluminium as $\text{AlC}_{3}$. It is $\text{Al}_{4}\text{C}_{3}$, with the methanide ion.
• Taking the wrong ratio from the equation of red lead. Six PbO give two Pb$_3$O$_4$, which is three to one.
• Forgetting that Pb$_3$O$_4$ is a mixed oxide of lead(II) and lead(IV).

Remember these

• Acidic character falls down a group and rises with the oxidation state.
• $\text{B}_{2}\text{O}_{3}$ acidic. $\text{Al}_{2}\text{O}_{3}$, $\text{SnO}_{2}$, $\text{PbO}_{2}$ amphoteric. $\text{Tl}_{2}\text{O}$ basic. CO neutral.
• Red lead is $2\text{PbO}\cdot \text{PbO}_{2}$. Nitric acid splits it, and the PbO$_2$ remains.
• $\text{CaC}_{2}$ 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.

Check yourself

Try each one on your own first. If you are stuck, open a hint. Then check with Show the answer.

1. Which is more acidic, SnO or SnO$_2$? Why?

2. What gas forms when water is added to aluminium carbide, and how many moles come from 1 mol?

3. How is red lead made, and what is its formula as a mixed oxide?

4. Silicon carbide is very hard. What does its structure look like?

References

• OpenStax, Chemistry 2e, Chapter 18 (Representative Metals, Metalloids and Nonmetals).
• NCERT Chemistry for Class 11, the chapter The p-Block Elements (older edition), the sections on oxides and carbides.
• Greenwood and Earnshaw, Chemistry of the Elements, the chapters on carbon and on germanium, tin and lead.