AQA GCSE Chemistry 4.2.2 How Bonding and Structure Are Related to Properties Exam Questions

About AQA GCSE Chemistry 4.2.2 How Bonding and Structure Are Related to Properties

Section 4.2.2 of AQA GCSE Chemistry links the type of bonding in a substance to how it behaves. You use the particle model to explain changes of state, then explain the melting points, conductivity and other properties of ionic compounds, small molecules, polymers, giant covalent structures, metals and alloys. This content is examined on Paper 1 and supports later topics including electrolysis (4.4.3) and materials (4.10.3). Exam questions usually give data for an unknown substance and ask you to identify its structure, or ask you to explain a property using the correct forces, so precise wording about bonds versus intermolecular forces matters.

What you need to know

• States of matter Solid, liquid and gas are modelled as small solid spheres. The amount of energy needed to change state depends on the strength of the forces between particles. Higher tier only: limitations of the model include that particles are not solid spheres, the model ignores forces between particles and does not show the space between them accurately.
• State symbols (s) solid, (l) liquid, (g) gas and (aq) aqueous, meaning dissolved in water.
• Ionic compounds High melting and boiling points because lots of energy is needed to overcome the strong electrostatic forces in the lattice. They conduct electricity when molten or dissolved because the ions are free to move, but not when solid.
• Small molecules Usually gases or liquids at room temperature with low melting and boiling points because only weak intermolecular forces are overcome, not the covalent bonds. Intermolecular forces increase with the size of the molecule. They do not conduct because molecules have no overall charge.
• Polymers and giant covalent structures Polymers are very large molecules with strong covalent bonds and relatively strong intermolecular forces, so are solid at room temperature. Giant covalent structures such as diamond, graphite and silicon dioxide have very high melting points because many strong covalent bonds must be broken.
• Metals and alloys Metals have high melting points and are good conductors because delocalised electrons carry charge and energy. Pure metals are soft because layers of atoms slide; alloys are harder because different-sized atoms distort the layers and stop them sliding.

Common mistakes in 4.2.2 How Bonding and Structure Are Related to Properties

• Saying covalent bonds break when a simple molecular substance melts or boils. Only the weak intermolecular forces are overcome.
• Saying ionic compounds conduct because 'electrons move'. In ionic compounds it is the ions that move and carry the charge.
• Stating that solid ionic compounds conduct electricity. Ions are fixed in the lattice so they cannot move.
• Explaining that alloys are harder simply because they are 'mixed'. Explain that different-sized atoms distort the regular layers so they cannot slide over each other easily.
• Using 'strong bonds' for every high melting point without saying which type, for example 'strong electrostatic forces between oppositely charged ions'.

Try three 4.2.2 How Bonding and Structure Are Related to Properties exam-style questions