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Practise 19 exam-style questions on Giant covalent structures for AQA GCSE Chemistry. Write your answer as you would in the exam and get it marked in seconds against the mark scheme, point by point, with an AI tutor on hand if you get stuck.
Here are 4 of the questions in this topic, each with a model answer, an explanation of why it earns the marks and the mark scheme an examiner would use.
State whether substances with giant covalent structures are usually solids, liquids or gases at room temperature.
Solids.
Giant covalent substances have very high melting points because strong covalent bonds must be broken to melt them, so they are solids at room temperature.
solids
In silicon dioxide, each silicon atom forms four covalent bonds to oxygen atoms, and each oxygen atom forms two covalent bonds to silicon atoms.
Explain why the formula of silicon dioxide is SiO₂.
Each silicon atom is bonded to four oxygen atoms, but each of those oxygen atoms is also bonded to another silicon atom, so it is shared between two silicon atoms. This means each silicon atom has a share of 4 × ½ = 2 oxygen atoms, so the ratio is 1 : 2 and the formula is SiO₂.
In a giant structure atoms are shared between neighbours, so you cannot simply count the bonds around one atom. Counting bonds from both sides gives the ratio of atoms in the formula.
each silicon atom is bonded to four oxygen atoms but each oxygen atom is shared between two silicon atoms
so there are (4 × ½ =) 2 oxygen atoms for every silicon atom / ratio Si : O = 1 : 2
A scientist describes the structure of quartz:
'Each silicon atom is covalently bonded to four oxygen atoms, and each oxygen atom is covalently bonded to two silicon atoms. The bonds continue in all directions throughout the crystal.'
What type of structure does quartz have? Explain how the description shows this.
Quartz has a giant covalent structure. The description shows that every atom is joined to other atoms by covalent bonds, and that the bonding continues in all directions throughout the crystal, so there are no separate molecules.
To recognise a giant covalent structure, look for atoms (not ions) joined by covalent bonds in a network that keeps going, with no separate molecules. Quartz is a crystalline form of silicon dioxide.
giant covalent (structure)
allow macromolecular / giant lattice of atoms
all the atoms are joined by covalent bonds
the bonds continue throughout the crystal, so there are no separate molecules
allow continuous network / bonds extend in all directions
The bar chart shows the melting points of three oxides of non-metals. Silicon dioxide has a giant covalent structure. Phosphorus oxide (P₄O₁₀) and sulfur dioxide (SO₂) consist of molecules.
Use the bar chart and your knowledge of structure and bonding to explain the differences in their melting points.
Silicon dioxide melts at 1710 °C, far higher than phosphorus oxide (340 °C) and sulfur dioxide (−72 °C). To melt silicon dioxide, many strong covalent bonds in its giant structure must be broken, which needs a lot of energy. Phosphorus oxide and sulfur dioxide are made of molecules, so only the intermolecular forces between molecules are overcome when they melt, which needs much less energy. P₄O₁₀ molecules are larger than SO₂ molecules, so their intermolecular forces are stronger and P₄O₁₀ has the higher melting point of the two.
All three oxides contain covalent bonds, so the big jump in melting point shows a change in structure, from giant covalent to molecular. Between the two molecular oxides, the difference is explained by the size of the molecules.
silicon dioxide has a much higher melting point (1710 °C) than the other two oxides
max 4
melting silicon dioxide requires (many) strong covalent bonds to be broken
melting P₄O₁₀ or SO₂ only requires weak intermolecular forces to be overcome
much more energy is needed to break covalent bonds than to overcome intermolecular forces
P₄O₁₀ has a higher melting point than SO₂ because its molecules are larger, so the intermolecular forces are stronger
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