C6 electrolysis

Question 1

What is electrolysis?

Answer 1

Electrolysis is a process in which electrical energy, from a
direct current (dc) supply, breaks down electrolytes. The free moving ions in electrolytes are attracted to the oppositely charged
electrodes which connect to the dc supply.

Question 2

What are electrolytes?

Answer 2

Electrolytes
are
ionic compounds
that are:

in the
molten
state (heated so they become liquids), or

dissolved
in water
Under these conditions, the
ions
in electrolytes are free to move within the liquid or solution.

Question 3

What is an anode and a cathode in electrolysis?

Answer 3

The negatively charged electrode in electrolysis is called the cathode
. Positively charged ions move towards the cathode.

The positively charged electrode in electrolysis is called the anode. Negatively charged ions move towards the anode.

Question 4

What happens to ions at electrolysis?

Answer 4

Products of electrolysis
When ions reach an electrode, they gain or lose
electrons. As a result, they form atoms or molecules of elements:

positive ions gain electrons from the negatively charged cathode
negative ions lose electrons at the positively charged anode
Molten lead bromide, PbBr2(l), is an electrolyte. During electrolysis:

Pb2+ ions gain electrons at the cathode and become Pb atoms
Br- ions lose electrons at the anode and become Br atoms, which pair up to form Br2 molecules

Question 5

What is the balanced equation for water?

Answer 5

2H2O(l) → 2H2(g) + O2(g)

Question 6

What does the electrolyte dissolved in a solution of water have?

Answer 6

An electrolyte
formed by dissolving an
ionic compound
contains:

hydrogen ions from the water, and positive ions from the compound
hydroxide ions from the water, and negative ions from the compound
The ions compete at each
electrode
to gain or lose electrons.

Question 7

What are half equations?

Answer 7

A half equation is used to represent the reaction that happens at an electrode during electrolysis. It shows what happens when ions gain or lose electrons. In half equations:

-electrons are shown as e-
-the numbers of atoms
of each element
must be the same on both sides
-the total
charge
on each side must be the same (usually zero)

Question 8

Give example of cathode half equations

Answer 8

Positively charged ions gain electrons at the
cathode
Na+ + e- → Na

Pb2+ + 2e- → Pb

2H+ + 2e- → H2

Question 9

Give examples of anode reactions

Answer 9

Negatively charged ions lose electrons at the
anode
2Cl- → Cl2 + 2e-

2O2- → O2 + 4e-

Question 10

Write a balanced half equation for hydroxide

Answer 10

4OH- → 2H2O + O2 + 4e-

Question 11

What is the order of discharge for the anode?

Answer 11

halide ion>hydroxide>other negatively charged ions

Question 12

What metals are produced at the cathode?

Answer 12

Any metal less reactive than hydrogen like copper.

Question 13

Explain the process of extracting aluminium

Answer 13

-Collect aluminium ore (bauxite)
-Purify it to aluminium oxide or alumina
-Alumina has a high melting point so dissolve it molten cryolite as it acts as a solvent and lowers the melting point
-Then it is electrolysed where aluminium gains 3 electrons to form aluminium
-The molten aluminium sinks to the bottom of the cell, where it is tapped off.

Question 14

What are the three gas tests for electrolysis

Answer 14

-hydrogen - lighted splint goes out with a squeaky pop
-oxygen - a glowing splint relights
-chlorine - damp blue litmus paper turns red and is then bleached white

Question 15

Why do graphite cathodes require replacing?

Answer 15

The graphite (carbon) anode reacts with the oxygen produced resulting in them being burnt away over time
c + o2 –> co2

Question 16

What is the test for copper?

Answer 16

A pink brown solid formed at the cathode

Question 17

What is a chemical cell?

Answer 17

Chemical cells use chemical reactions to transfer energy by
electricity. The voltage
of a cell depends upon a number of factors, including what the electrodes are made from, and the substance used as the electrolyte.
A simple cell can be made by connecting two different metals in contact with an electrolyte. A number of cells can be connected in series to make a
battery
, which has a higher voltage than a single cell.

In non-rechargeable cells, eg alkaline cells, a
voltage
is produced until one of the
reactants
is used up. When this happens, we say the battery ‘goes flat’.

In rechargeable cells and batteries, like the one used to power your mobile phone, the chemical reactions can be reversed when an
external circuit
is supplied.