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Electrolysis

Worksheet Overview

QUESTION 1 of 10

Electrolysis is a way of separating and purifying substances using electricity. It works because the ions in an ionic compound are electrically charged, so different ions will move towards differently charged electrodes. The substance being separated needs to be liquid- either molten or in a solution, so that the ions can move. The liquid is called the electrolyte.

First example: electrolysis of molten compounds, like sodium chloride

The simplest types of electrolysis to understand are where the electrolyte is a melt. Let's take sodium chloride as an example. The liquid contains Na+ ions and Cl- ions.

Because of the battery, electrons move through the wires from the anode through the battery to the cathode. The anode electrode becomes positively charged, and the cathode electrode becomes negatively charged. The charges on the anode and cathode then attract the ions in the molten NaCl. The positively charged sodium cations are attracted to the negatively charged cathode, because opposite electrical charges attract. The negatively charged chloride anions are attracted to the positively charged anode for the same reason.

Once the ions reach the electrodes, they interact with electrons:

At the anode, the chloride anions lose their excess electrons, and are turned into chlorine atoms. Loss of electrons is called oxidation. The half-equation for this process is Na+ + e- → Na. We call it a half-equation because this is only half the process, and can't happen by itself.

At the cathode, the sodium cations gain electrons, and are turned into sodium atoms. Gain of electrons is called reduction. The half-equation for this process is 2 Cl- → Cl2 + 2e-. (We can't write Cl- → Cl + e- because chlorine atoms are only found in pairs, not as individual atoms). 

The electrons that the chloride loses when it becomes chlorine are equal to the electrons gained by the sodium ions when they become sodium atoms.

Second example: electrolysis of solutions, like copper sulfate in water

The copper sulfate example is more complicated, but it is a common experiment, so it's useful to know what happens in outline.

The setup is the same as before, but there are now four types of ion in the solution. The copper sulfate gives us Cu2+ and SO42-. The water breaks down into H+ and OH-. Like before, the positive cations are attracted to the cathode and the negative anions are attracted to the anode.

At the anode, the hydroxide anions lose their excess electrons, and the oxygen from the hydroxide becomes oxygen gas. This is oxidation, and the half-equation is 4 OH- → O2 + H2O + 4e-.

At the cathode, the copper cations gain electrons, and are turned into copper atoms. Gain of electrons is called reduction. The half-equation is Cu2+ + 2e- → Cu.

If you're wondering what happens to the sulfate- well done, that's a really good question! It's also a tricky question, so you will need to look at the activity "Electrolysis of solutions" to find out the rules for what happens.

Although a lot of the details of electrolysis get complicated- especially once there are more than two ions involved- the main idea is pretty easy. Melt or dissolve an ionic compound so the ions can move, then use charged electrodes to pull differently charged ions in different directions.

Fill in the gaps in this paragraph about electrolysis. Use these words:

anions

anode

cathode

cations

current

electric

negative

positive

Which of these substances would be a suitable electrolyte?

solid NaCl

solid Al

NaCl dissolved in water

molten Al2O3

If we do electrolysis on molten lead bromide, which of these statements describes the anode? Tick all the correct answers.

attracts lead ions

attracts bromide ions

makes lead atoms

makes bromine gas molecules

oxidation

reduction

If we do electrolysis on molten lead bromide, what happens to the lead ions? Tick all the correct answers.

attracts lead ions

attracts bromide ions

makes lead atoms

makes bromine gas molecules

oxidation

reduction

If we do electrolysis on molten lead bromide, what is the correct pair of half-equations for the reaction?

attracts lead ions

attracts bromide ions

makes lead atoms

makes bromine gas molecules

oxidation

reduction

If we do electrolysis on molten iron chloride, FeCl2, which of these statements describe the cathode? 

iron is oxidised

iron is reduced

Fe2+ + 2 e- → Fe

Fe2+ → Fe + 2 e-

chloride is oxidised

chloride is reduced

If we do electrolysis on iron sulfate dissolved in water, FeSO4 (aq), which of these statements describe the anode?

attracts sulfate ions

attracts iron ions

4 OH- - 4 e- → O2 + H2O

Fe2+ + 2 e- → Fe

attracts hydroxide ions

attracts hydrogen ions

If we do electrolysis on copper chloride dissolved in water, CuCl2 (aq), which of these statements describe the anode? 

reduction

oxidation

Cu2+ + 2 e- → Cu

2 Cl- → Cl2 + 2 e-

attracts hydroxide ions

attracts hydrogen ions

If we do electrolysis on copper chloride dissolved in water, CuCl2 (aq), which of these statements describe the cathode? 

reduction

oxidation

Cu2+ + 2 e- → Cu

2 Cl- → Cl2 + 2 e-

attracts hydroxide ions

attracts hydrogen ions

Complete the gaps in this sentence.

reduction

oxidation

Cu2+ + 2 e- → Cu

2 Cl- → Cl2 + 2 e-

attracts hydroxide ions

attracts hydrogen ions

  • Question 1

Fill in the gaps in this paragraph about electrolysis. Use these words:

anions

anode

cathode

cations

current

electric

negative

positive

CORRECT ANSWER
EDDIE SAYS
This is the key paragraph to learn to define electrolysis. Remember that the names of the electrodes and the ions match (cation/cathode) but the charges are opposite (cation is positive, cathode is negative).
  • Question 2

Which of these substances would be a suitable electrolyte?

CORRECT ANSWER
NaCl dissolved in water
molten Al2O3
EDDIE SAYS
Sodium chloride dissolved in water can conduct electricity, because the ions can move. Molten aluminium oxide can conduct electricity for the same reason. Solid aluminium can conduct electricity, but there aren't cations and anions to separate. Solid sodium chloride has ions, but they can't move.
  • Question 3

If we do electrolysis on molten lead bromide, which of these statements describes the anode? Tick all the correct answers.

CORRECT ANSWER
attracts bromide ions
makes bromine gas molecules
oxidation
EDDIE SAYS
The anode attracts the anions, which are the negatively charged ions (so the non-metal, which is bromide). These ions lose their excess electrons, which is oxidation (some people remember this as OILRIG; oxidation is loss of electrons, reduction is gain of electrons).
  • Question 4

If we do electrolysis on molten lead bromide, what happens to the lead ions? Tick all the correct answers.

CORRECT ANSWER
EDDIE SAYS
Lead ions are positively charged cations, so they are attracted to the negative electrode, which is the cathode. Once they get there, the voltage of the battery forces electrons onto them, turning ions back into atoms.
  • Question 5

If we do electrolysis on molten lead bromide, what is the correct pair of half-equations for the reaction?

CORRECT ANSWER
EDDIE SAYS
Lead starts as 2+ ions, so it needs to gain two electrons to be turned into an atom. Bromine starts as 1- ions (bromide), but the atom is always found in pairs as Br2 molecules, so we need two ions and two electrons.
  • Question 6

If we do electrolysis on molten iron chloride, FeCl2, which of these statements describe the cathode? 

CORRECT ANSWER
iron is reduced
Fe2+ + 2 e- → Fe
EDDIE SAYS
The cathode (negative charge) attracts cations (positively charged); in this case that's the iron. At the cathode, iron ions are forced to take electrons to become iron atoms.
  • Question 7

If we do electrolysis on iron sulfate dissolved in water, FeSO4 (aq), which of these statements describe the anode?

CORRECT ANSWER
attracts sulfate ions
4 OH- - 4 e- → O2 + H2O
attracts hydroxide ions
EDDIE SAYS
The anode (positive charge) attracts anions (negatively charged); in this case that's sulfate and hydroxide. The hydroxides are converted to water and oxygen by removing electrons.
  • Question 8

If we do electrolysis on copper chloride dissolved in water, CuCl2 (aq), which of these statements describe the anode? 

CORRECT ANSWER
oxidation
2 Cl- → Cl2 + 2 e-
attracts hydroxide ions
EDDIE SAYS
The anode (positive charge) attracts anions (negatively charged); in this case that's chloride and hydroxide. The chlorides are oxidised (they lose electrons) to become chlorine gas.
  • Question 9

If we do electrolysis on copper chloride dissolved in water, CuCl2 (aq), which of these statements describe the cathode? 

CORRECT ANSWER
reduction
Cu2+ + 2 e- → Cu
attracts hydrogen ions
EDDIE SAYS
The cathode (negative charge) attracts cations (positively charged); in this case that's copper and hydrogen. The copper is reduced (they gain electrons) to become copper metal.
  • Question 10

Complete the gaps in this sentence.

CORRECT ANSWER
EDDIE SAYS
Yes, it\'s confusing that the gain of electrons is called reduction! There\'s a good reason, to do with the history of extracting metals from rocks, but that\'s explained in another activity.
---- OR ----

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