Electrolysis

Question 1

Explain why magnesium chloride must be molten or dissolved in water to be electrolysed. [2]

Answer 1

• melted / dissolved ions can move…
• …to the electrodes
• …and carry charge

Question 2

Explain how magnesium is produced at the negative electrode. [3]

Answer 2

• magnesium (ions) get attracted to the electrode

- …so magnesium ions gain 2 electrons

Question 3

Why must zinc chloride be molten for electrolysis? [1]

Answer 3

so ions can move (and carry charge)

Question 4

Davy first tried to electrolyse a solid potassium salt to produce potassium. Explain why this electrolysis did not work. [2]

Answer 4

• current / charge couldn’t flow…
• …because the ions / particles
  couldn’t move
• (salt) needs to be molten /
  dissolved (to conduct electricity)…
• …so that the ions / particles can
  move

Question 5

Humphrey Davy’s experiment to produce this new element was quickly accepted by
other scientists.
Suggest why. [1]

Answer 5

• he had status

- he had evidence / proof

Question 6

A student dissolved some potassium chloride in water.
The student tried to electrolyse the potassium chloride solution to produce potassium.
The student expected to see potassium metal at the negative electrode, but instead saw bubbles of gas.
- Name the gas produced at the negative electrode.
- Explain why this gas was produced at the negative electrode and why potassium was not produced. [3]

Answer 6

• hydrogen / H₂
• the ions are positive
• potassium is more reactive (than hydrogen)
  
  • potassium ions are less easily…
  • …discharged / reduced

Question 7

Potassium metal was produced at the negative electrode.

Describe how potassium atoms are formed from potassium ions. [2]

Answer 7

• potassium ions gain…

- …one electron

Question 8

Complete and balance the equation for the reaction at the positive electrode. [1]
___Cl⁻ → Cl₂ + ____

Answer 8

2Cl⁻ → Cl₂ + 2e⁻