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Electrolysis of Solutions

Worksheet Overview

QUESTION 1 of 10

Electrolysis is a way of separating and purifying substances using electricity. If the substance is a molten liquid, the process is simpler; the positive metal ions go to one electrode and the negative ions go to the other. This activity is about the harder type of electrolysis, where the substance is dissolved in water. Even if you don't normally try the level 1 and level 2 activities, it is a good idea to do the activities on "Electrolysis" first, to make sure you are able to use the basics confidently.

For electrolysis of solutions, the basic setup is the same as for electrolysis of molten substances.

The difference between molten compounds and dissolved compounds is that the solution has four types of ion in the solution. Two come from the solute (for example, copper sulfate has Cu2+ and SO42-) and two come from the water; H2O breaks down into H+ and OH-. Like all examples of electrolysis, the positive cations are attracted to the cathode and the negative anions are attracted to the anode. So the anode attracts SO42- and OH-, and the cathode attracts Cu2+ and H+

Now comes the super-tricky part. Each electrode is attracting two different ions, so what happens when they reach the electrodes?

What happens at the cathode?

The behaviour of the cathode depends on something like the reactivity series. Remember that the best definition of the reactivity series is that more reactive metals push unwanted electrons onto less reactive metals. This means that the electrons at the cathode go on to the ion of the less reactive element. Hydrogen is between copper and iron in the reactivity series, so:

If the metal ion is an unreactive element (copper or below): the metal takes the electrons, and is reduced from metal ions to metal atoms. Hydrogen stays as an ion in the solution.  The half-equation is something like Cu2+ + 2 e- → Cu.

If the metal ion is a reactive element (iron or above): the hydrogen ions take the electrons, and are reduced from hydrogen ions to hydrogen atoms in H2 molecules.  The half-equation is 2 H+ + 2 e- → H2.

What happens at the anode?

A similar rule applies to the ions at the anode. The halogens are more reactive than hydoxide, and other anions are less reactive than hydroxide.

If the anion is a halide (for example chloride): the halide gains electrons, and is oxidised from a halide ion to halogen atoms. These atoms become molecules. The half-equation is something like 2 Cl- → Cl2 + 2 e-.

If the anion is anything other than a halide (for example sulfate): the hydorxide gains electrons, and is oxidised to form water (which stays in the solution) and oxygen gas. The half-equation is 4 OH- → O2 + 2 H2O + 4 e-.

Putting it all together

Suppose we have sodium chloride dissolved in water; NaCl (aq). What happens when we electrolyse this?

1. Work out what the ions are. From the water, we have H+ and OH-. From the sodium chloride, we have Na+ and Cl-.

2. Decide what is attracted to the cathode, and which ion is reduced. H+ and Na+ go to the cathode. Sodium is more reactive than hydrogen, so the hydrogen is reduced. 2 H+ + 2 e- → H2 and hydrogen gas is produced.

2. Decide what is attracted to the anion, and which ion is oxidised. OH- and Cl- go to the cathode. Chloride is a halide, so the chloride is oxidised. 2 Cl- → Cl2 + 2 e- and chlorine gas is produced.

And that's it! This isn't easy; there are lots of steps to think about, and the answers aren't always obvious. But if you think through this step-by-step, you will be able to do it, which will make you officially good at chemistry.

 

Which of these would be attracted to the anode in an electrolysis experiment? Tick all the correct possibilities.

anions

cations

hydrogen ions

hydroxide ions

copper ions

chloride ions

Which of these would be attracted to the cathode in an electrolysis experiment? Tick all the correct possibilities.

anions

cations

hydrogen ions

hydroxide ions

copper ions

chloride ions

When we do electrolysis on a solution of potassium nitrate, what would happen at the anode?

nitrate is attracted

oxygen gas forms

potassium is attracted

hydrogen is attracted

hydroxide is attracted

nitrogen is formed

When we do electrolysis on a solution of potassium fluoride, what would happen at the anode?

fluoride is attracted

oxygen gas forms

potassium is attracted

hydrogen is attracted

hydroxide is attracted

fluorine is formed

When we do electrolysis on a solution of potassium fluoride, what would happen at the cathode?

fluoride is attracted

potassium metal forms

potassium is attracted

hydrogen is attracted

hydroxide is attracted

hydrogen gas is formed

If we do electrolysis on potassium fluoride dissolved in water, what are the half-equations at each electrode?

fluoride is attracted

potassium metal forms

potassium is attracted

hydrogen is attracted

hydroxide is attracted

hydrogen gas is formed

If we do electrolysis on zinc nitrate dissolved in water, what are the half-equations at each electrode?

fluoride is attracted

potassium metal forms

potassium is attracted

hydrogen is attracted

hydroxide is attracted

hydrogen gas is formed

If we do electrolysis on pure water, what would be produced at each electrode?

oxygen at the cathode, hydrogen at the anode

oxygen at the anode, hydrogen at the cathode

hydroxide at the anode, hydrogen at the cathode

nothing

What is produced at each electrode if we do electrolysis on silver fluoride dissolved in water? Name the elements. 

oxygen at the cathode, hydrogen at the anode

oxygen at the anode, hydrogen at the cathode

hydroxide at the anode, hydrogen at the cathode

nothing

If we do electrolysis on silver fluoride dissolved in water, what are the half-equations at each electrode?

oxygen at the cathode, hydrogen at the anode

oxygen at the anode, hydrogen at the cathode

hydroxide at the anode, hydrogen at the cathode

nothing

  • Question 1

Which of these would be attracted to the anode in an electrolysis experiment? Tick all the correct possibilities.

CORRECT ANSWER
anions
hydroxide ions
chloride ions
EDDIE SAYS
The anode attracts anions (negative ions). These include hydroxide and non-metal ions.
  • Question 2

Which of these would be attracted to the cathode in an electrolysis experiment? Tick all the correct possibilities.

CORRECT ANSWER
cations
hydrogen ions
copper ions
EDDIE SAYS
The cathode attracts cations (positive ions). These include hydrogen and metal ions.
  • Question 3

When we do electrolysis on a solution of potassium nitrate, what would happen at the anode?

CORRECT ANSWER
nitrate is attracted
oxygen gas forms
hydroxide is attracted
EDDIE SAYS
The anode attracts the anions; hydroxide and nitrate. Since the nitrate isn\'t a halogen, the hydroxide is oxidised instead of the nitrate, and this releases oxygen gas.
  • Question 4

When we do electrolysis on a solution of potassium fluoride, what would happen at the anode?

CORRECT ANSWER
fluoride is attracted
hydroxide is attracted
fluorine is formed
EDDIE SAYS
The anode attracts the anions; hydroxide and fluoride. Since the fluoride is a halogen, the fluoride is oxidised instead of the hydroxide, and this releases fluorine gas.
  • Question 5

When we do electrolysis on a solution of potassium fluoride, what would happen at the cathode?

CORRECT ANSWER
potassium is attracted
hydrogen is attracted
hydrogen gas is formed
EDDIE SAYS
The cathode attracts the cations; potassium and hydrogen. Since potassium is more reactive than hydrogen, the the potassium stays as ions in solution. The hydrogen ions are reduced to hydrogen atoms, which form a gas.
  • Question 6

If we do electrolysis on potassium fluoride dissolved in water, what are the half-equations at each electrode?

CORRECT ANSWER
EDDIE SAYS
At the cathode, we make hydrogen; potassium is reactive enough to stay as an ion in the solution. Hydrogen gas has H2 molecules, not H atoms. At the anode, we make fluorine, because it\'s a halogen. Again, the fluorine atoms make molecules with two atoms- called diatomic- so we have to adjust the half-equation to show this.
  • Question 7

If we do electrolysis on zinc nitrate dissolved in water, what are the half-equations at each electrode?

CORRECT ANSWER
EDDIE SAYS
At the cathode, we make hydrogen; zinc is reactive enough to stay as an ion in the solution. Hydrogen gas has H2 molecules, not H atoms. At the anode, we make convert hydroxide to oxygen and water, because nitrate is not a halogen ion.
  • Question 8

If we do electrolysis on pure water, what would be produced at each electrode?

CORRECT ANSWER
oxygen at the anode, hydrogen at the cathode
EDDIE SAYS
This might sound pointless- it's electrolysis without the substances we have been thinking about separating, but it is actually useful. Hydrogen is being researched as an energy store to replace fossil fuels. The idea is to use renewable energy resources (like solar power) to do electrolysis on water, then use the hydrogen produced to store the energy for times when it isn't sunny.
  • Question 9

What is produced at each electrode if we do electrolysis on silver fluoride dissolved in water? Name the elements. 

CORRECT ANSWER
EDDIE SAYS
Silver is very unreactive- definitely less reactive than hydrogen- so it forms as a metal at the cathode. Fluorine is a halogen, so it forms at the anode.
  • Question 10

If we do electrolysis on silver fluoride dissolved in water, what are the half-equations at each electrode?

CORRECT ANSWER
EDDIE SAYS
At the cathode, we make silver; silver is not reactive enough to stay as an ion in the solution. At the anode, we make convert fluoride to fluorine, because fluoride is not a halogen ion. This makes diatomic molecules.
---- OR ----

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