Chemistry 5070/21 — May/June 2017

Walkthrough

The question gives a list of oxides and asks for the one with a giant covalent structure. In 5070 terms, a giant covalent structure is a continuous lattice in which every atom is joined to its neighbours by covalent bonds, so there are no separate molecules.

Silicon dioxide ($\text{SiO}_2$) is the only oxide in the list with this structure. Each silicon atom is covalently bonded to four oxygen atoms, and each oxygen atom to two silicon atoms, giving one huge covalent network. The other oxides are either ionic solids (sodium oxide, calcium oxide, copper(II) oxide, zinc oxide) or simple molecular substances (carbon dioxide, sulfur dioxide, sulfur trioxide), not giant covalent.

Key Takeaways

A giant covalent substance has no discrete molecules; the whole crystal is bonded covalently. Common examples include diamond, graphite and silicon dioxide.

Common Mistakes

Choosing carbon dioxide because it is an oxide of a non-metal. Carbon dioxide is a simple molecular substance, not a giant covalent structure.
Thinking that all covalent solids are giant; many covalent compounds exist as separate molecules with weak intermolecular forces.

Things to Be Careful About

The formula $\text{SiO}_2$ is an atom ratio, not evidence of a small molecule. The word 'giant' means the bonding network extends through the entire sample.

Walkthrough

An oxide that reacts with both acids and alkalis is called an amphoteric oxide. At O level, the common amphoteric oxides are zinc oxide and aluminium oxide, so the answer here is zinc oxide.

Zinc oxide acts as a base with acids, for example:

$$\text{ZnO}(\text{s}) + 2\text{HCl}(\text{aq}) \rightarrow \text{ZnCl}_2(\text{aq}) + \text{H}_2\text{O}(\text{l})$$

It also acts as an acidic oxide with alkalis, for example:

$$\text{ZnO}(\text{s}) + 2\text{NaOH}(\text{aq}) \rightarrow \text{Na}_2\text{ZnO}_2(\text{aq}) + \text{H}_2\text{O}(\text{l})$$

The basic oxides in the list (sodium oxide, calcium oxide, copper(II) oxide) react with acids but not with alkalis. The acidic oxides (carbon dioxide, sulfur dioxide, sulfur trioxide) react with alkalis but not with acids.

Key Takeaways

Amphoteric oxides show both acidic and basic properties. Zinc oxide is a memorised example needed for 5070.

Common Mistakes

Choosing a basic oxide such as calcium oxide or sodium oxide because it reacts with acids; the question also requires reaction with alkalis.
Calling zinc oxide 'neutral'; neutral oxides such as carbon monoxide react with neither acids nor alkalis.

Things to Be Careful About

An amphoteric oxide must be able to react with both an acid and an alkali. One statement of reaction with acid alone is not enough.

Walkthrough

The oxides of non-metals are usually acidic oxides: they react with water to form acids. The question asks for a strong acid, so you need the oxide whose product is a strong acid.

Sulfur trioxide reacts with water to form sulfuric acid:

$$\text{SO}_3(\text{g}) + \text{H}_2\text{O}(\text{l}) \rightarrow \text{H}_2\text{SO}_4(\text{aq})$$

Sulfuric acid is a strong acid in 5070 terms because it is fully ionised in aqueous solution. By contrast, sulfur dioxide forms sulfurous acid, $\text{H}_2\text{SO}_3$, which is weak, and carbon dioxide forms carbonic acid, $\text{H}_2\text{CO}_3$, which is also weak.

Key Takeaways

• Acidic oxides react with water to form acids.
• Sulfur trioxide gives sulfuric acid; sulfur dioxide gives sulfurous acid. Remember the difference for questions on strong acids.

Common Mistakes

Choosing sulfur dioxide instead of sulfur trioxide. Sulfur dioxide forms a weak acid, not a strong acid.
Choosing carbon dioxide; carbonic acid is weak.
Omitting the equation in an 'explain' answer when the question asks why the oxide forms a strong acid.

Things to Be Careful About

Write $\text{SO}_3$, not $\text{SO}_2$. State symbols are useful: sulfur trioxide is a gas, water is liquid, and sulfuric acid is aqueous.
Strong here describes the acid formed, not the strength of the oxide as a base.

Walkthrough

Look at the metal oxides in the list and work out the charge on the metal cation. Sodium is in Group I, so it forms a +1 ion. Sodium oxide has the formula $\text{Na}_2\text{O}$: two $\text{Na}^+$ ions balance one $\text{O}^{2-}$ ion.

The other metal oxides contain cations with higher charges: calcium oxide has $\text{Ca}^{2+}$, copper(II) oxide has $\text{Cu}^{2+}$, and zinc oxide has $\text{Zn}^{2+}$. The remaining oxides (carbon dioxide, sulfur dioxide, sulfur trioxide, silicon dioxide) are covalently bonded and do not contain simple cations.

Key Takeaways

The charge on a cation can be predicted from the group number: Group I gives +1, Group II gives +2. The formula of an oxide must balance the + charge of the cation with the -2 charge of the oxide ion.

Common Mistakes

Choosing calcium oxide because calcium is a metal; calcium is in Group II and forms $\text{Ca}^{2+}$, not $\text{Ca}^+$.
Writing $\text{NaO}$ instead of $\text{Na}_2\text{O}$; the formula must balance charges.
Forgetting that non-metal oxides are covalent and do not contain metal cations.

Things to Be Careful About

The word 'cation' means a positive ion. The charge on the oxide ion is always $\text{O}^{2-}$, so count the number of metal ions needed to balance it.