Topic 4a- Electrolysis (paper 1)

Question 1

What are electrolytes?

Answer 1

Ionic compounds in the molten state or dissolved in water.

Question 2

What is electrolysis?

Answer 2

The process of passing an electrical current through an electrolyte, causing the ionic substance to decompose.

Question 3

Where will the positive ions (cations) go in electrolysis?

Answer 3

To the negatively charged electrode (cathode).

Question 4

Where will the negative ions (anions) go in electrolysis?

Answer 4

To the positively charged electrode (anode).

Question 5

What type of reaction always occurs in an electrolysis?

Answer 5

An oxidation reaction and a reduction reaction.

Question 6

Explain what reaction is occurring at the cathode in electrolysis

Answer 6

• The positive cations are gaining electrons
• Reduction is occuring

Question 6

Explain what is occuring at the anode in electrolysis

Answer 6

• The negative anions loose electrons to the positive cathode
• Oxidation has occured

Question 7

Why are dissolved ionic substances able to conduct electricity?

Answer 7

They contain free positively and negatively charged ions that are able to carry a charge.

Question 8

What are binary compounds?

Answer 8

Ionic compounds that contain two elements- a positive metal ion and a negative non-metal ion.

Question 9

In the electrolysis of molten ionic substances, where does the metal go?

Answer 9

The metal ions go to the negatively charged cathode to gain electrons and become neutral.

Question 10

In the electrolysis of molten ionic substances, where does the non-metal go?

Answer 10

The non metal ions go to the positively charged anode to loose electrons and become neutral.

Question 11

Why can solid ionic substances not be electrolysed?

Answer 11

The ions are in fixed positions and are unable to move.

Question 12

What do half equations do?

Answer 12

They show how electrons are transferred (for example at the electrodes).

Question 13

Why do you need to be careful when predicting the products of electrolysis in an aqueous solution?

Answer 13

You need to remember the H+ and OH- ions present in the water.

Question 14

What happens if a metal in a solution being electrolysed is less reactive than hydrogen?

Answer 14

The metal will form at the cathode since the more reactive substance (hydrogen) remains as ions.

Question 15

When will hydrogen NOT be produced at the cathode?

Answer 15

When the metal is less reactive than it.

Question 16

When will hydrogen be produced at the cathode?

Answer 16

When the metal is more reactive than it.

Question 17

When will OH- (hydrogen monoxide) be produced at the anode?

Answer 17

If NO halides are present.

Question 18

When will OH- (hydrogen monoxide) NOT be produced at the anode?

Answer 18

If halides are present.

Question 19

How would you set up an experiment for the electrolysis of copper sulphate solution using inert electrodes?

Answer 19

1. [Draw diagram as displayed on pg 139]
2. Set up equipment shown as in the diagram above.
3. Turn on the power supply to allow a potential difference to flow through the solution.
4. At the cathode, copper metal is produced, since it is less reactive than hydrogen, and coats the electrode.
5. At the anode, oxygen and water molecules are produced since there aren’t any halide ions present meaning the OH- molecules are oxidised and loose electrons.

Question 20

How do you set up an experiment for the electrolysis of copper sulphate using non-inert electrodes?

Answer 20

1. [Draw diagram as shown on pg140]
   * Anode and cathode are made from copper and in a CuSO4 solution.
2. Set up equipment as shown in the diagram.
3. Turn on the power supply and allow the potential difference to flow thorugh the solution.
4. The cell should be left for around 30 minutes to ensure a measurable change in mass.
5. The mass of the anode will decrease and the mass of the cathode will increase.
6. This is because copper is transferred from the anode to the cathode.

Question 21

Write a method for the electrolysis of a molten ionic substance.

Answer 21

1. Put your solid ionic substance in a crucible.
2. Heat the crucible with a Bunsen burner until the solid melts.
3. Once the solid is melted, put two clean, inert electrodes in the electrolyte.
4. Then, connect the electrodes to a power supply using wires and clips.
5. [Draw the diagram on pg141 if you’re feeling like it or they give you space to]

Question 22

What happens when a substance is reduced?

Answer 22

The substance gains electrons.

Question 23

What happens when a substance is oxidised?

Answer 23

The substance looses electrons.

Question 24

OIL RIG stands for…

Answer 24

Oxidation is loss
Reduction is gain

Question 25

Explain how copper is purified using electrolysis.

Answer 25

1. Using two copper electrodes. The anode starts off as a big lump of impure copper, the cathode as a thin sheet of pure copper.
2. Copper ions from the impure anode dissolve in the electrolyte.
3. The copper ions move to the pure copper cathode, and react to form a layer of pure copper.
4. Any impurities from the impure anode sink to the bottom of the cell, forming a sludge.

Question 26

What does increasing the current do to the rate of electrolysis?

Answer 26

An increased current increases the rate at which electrolysis occurs.