Topic 3.22-3.31 Electrolytic processes

Question 1

What are electrolytes?

Answer 1

Ionic compounds in the molten state or dissolved in water

Question 2

Describe electrolysis

Answer 2

A process in which electrical energy (from a direct current supply) decomposes electrolytes

Question 3

Explain the movement of ions during electrolysis

Answer 3

Positively charged cations migrate to the negatively charged cathode.
And
Negatively charged anions migrate to the positively charged anode

Question 4

What is oxidation?

Answer 4

Loss of electrons

Question 5

Where does oxidation occur?

Answer 5

The Anode

Question 6

What is reduction?

Answer 6

Gain of electrons

Question 7

Where does reduction occur?

Answer 7

The cathode

Question 8

Why can only ionic compounds be electrolysed?

Answer 8

Ionic compounds contain free-moving ions that can carry electrical current when molten or dissolved in solution, making electrolysis possible.

Question 9

Explain the formation of the products in the electrolysis of copper sulfate solution, using copper electrodes, and how this electrolysis can be used to purify copper
​​ ​​​​ ​​​​ ​​ ​​​​ ​​ ​​ ​​​​
​​ ​​ ​​​​ ​​​​

Answer 9

set​ ​up:
○ anode​ ​is​ ​made​ ​of​ ​impure​ ​copper​ ​(that​ ​you​ ​are​ ​purifying)
○ cathode​ ​is​ ​made​ ​of​ ​pure​ ​copper
○ the​ ​solution​ ​is​ ​copper​ ​sulfate
● what​ ​happens:
○ Cu2​+​​​ions​ ​from​​ the​​ anode​​ move​​ to​​ the​​ cathode,​​ where​​they​​ gain​​ electrons and​ ​are​ ​discharged​ ​as​ ​pure​ ​copper
○ impurities​ ​form​ ​as​ ​sludge​ ​below​ ​the​ ​anode
● the​ ​cathode​ ​will​ ​increase​ ​in​ ​mass​ ​as​ ​it​ ​gains​ ​pure​ ​copper,​ ​whilst​ ​the​ ​anode​ ​will lose​ ​mass​ ​as​ ​copper​ ​ions​ ​are​ ​lost​ ​(they​ ​replace​ ​the​ ​ones​ ​from​ ​the​ ​CuSO​4​​ ​solution that​ ​go​ ​to​ ​the​ ​cathode)​ ​and​ ​so​ ​are​ ​impurities

Question 9

How can you predict where the products of electrolysis will go?

Answer 9

the​ ​+​ ​ions​ ​will​ ​go​ ​to​ ​the​ ​cathode
● the​ ​-​ ​ions​ ​will​ ​go​ ​to​ ​the​ ​anode

Question 10

How do you form the half equation for an electrolysis?

Answer 10

negative​ ​electrode:​ ​X​+​​ ​->​ ​X, so
X​+​​ ​+​ ​e-​ ​​ ​->​ ​X,​ ​electrons​ ​gained,​ ​so​ ​positive​ ​ions​ ​are​ ​reduced

positive​ ​electrode:​ ​X​-​​ ​->​ ​X,​ ​so​
X​-​​ ​->​ ​e-​ ​​ ​+​ ​X,​ ​electrons​ ​are​ ​lost,​ ​so​ ​negative​ ​ions​ ​are​ ​oxidised

Question 11

A student conducts an investigation to find out what is produced during the electrolysis of sodium sulfate.
Describe how the student could carry out an investigation, and show (using ions) what is given off at each electrode.

Answer 11

Wearing safety glasses, add sodium sulfate solution to a beaker, and connect two inert electrodes (make sure they do not touch) to a power supply.
Completely fill two small test tubes with sodium sulfate solution and position a test tube over each electrode, then turn on the power supply and observe what happens at each electrode.
Any gases produced can be collected in the test tubes and tested (squeaky pop test - hydrogen, relight a glowing splint - oxygen).

hydrogen gas will form at the negative electrode/cathode: 2H+(aq) + 2e- → H2(g)
oxygen gas will form at the positive electrode/anode: 4OH-(aq) → 2H2O(l) + O2(g) + 4e-

Question 12

What are the rules for production at the cathode from aqueous solutions?

Answer 12

• The metal will be produced at the cathode if it is less reactive than hydrogen
  and hydrogen will be produced if the metal is more reactive than it

Question 13

What metals are less reactive than hydrogen?

Answer 13

Copper, silver, gold and platinum are less reactive than hydrogen

Question 14

What are the rules for production at the anode for aqueous solutions?

Answer 14

• A halogen will be produced if the electrolyte contains a halide ion. Otherwise, oxygen is produced.

(For non-aqueous solutions just the