2.6 Properties of ionic compounds
5 exam questions worth 11 marks. Write your answer, open the mark scheme, then give yourself the marks you earned. Your answers are saved on this device.
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Why does solid sodium chloride not conduct electricity?
Choose one answer.
Correct Not quite: the answer is Its ions cannot move
Mark scheme
C
Model answer
C: the ions are held in fixed positions in the lattice, so they cannot move.
Explain why sodium chloride has a high melting point.
Mark scheme
- giant (ionic) lattice (1)
- strong electrostatic forces between oppositely charged ions (1)
- a lot of energy needed to overcome them (1)
Model answer
Sodium chloride is a giant ionic lattice with strong electrostatic forces between oppositely charged ions in all directions. A lot of energy is needed to overcome these forces.
Explain why molten sodium chloride conducts electricity.
Mark scheme
- ions are free to move (1)
- and carry charge (to the electrodes) (1)
Model answer
When it melts, the ions are no longer held in the lattice and can move, carrying charge through the liquid.
Magnesium oxide has a much higher melting point than sodium chloride.
Suggest why.
Mark scheme
- the ions in magnesium oxide have greater charges (2+ and 2−) (1)
- so the electrostatic forces are stronger and need more energy to overcome (1)
Model answer
Magnesium oxide contains 2+ and 2− ions, while sodium chloride contains 1+ and 1− ions. The attraction between the more highly charged ions is stronger, so more energy is needed to overcome it.
A student tests whether solid potassium iodide and potassium iodide solution conduct electricity, using a bulb in a circuit.
Predict the results and explain them.
Mark scheme
- solid: bulb does not light / does not conduct (1)
- solution: bulb lights / conducts (1)
- in solution the ions are free to move; in the solid they are fixed (1)
Model answer
The solid does not conduct because its ions are fixed in the lattice. The solution conducts, so the bulb lights, because the ions are free to move and carry charge.
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