C4 Electrolysis of Aqueous Solutions (page 136)

Question 1

When you electrolyse an aqueous solution, you also have to factor in what?

Answer 1

factor in the ions in the water

Question 2

In aqueous solutions, as well as the ions from the ionic compound, what other ions will be in the solution?

Answer 2

There will also be Hydrogen Ions (H+) and Hydroxide Ions (OH-) from the water

H2O(l) &raquo_space;»»> H+(aq) + OH- (aq)
«««<

Question 3

At the cathode, if H+ ions and metal ions are present, what will be produced?

Answer 3

Hydrogen gas will be produced.

If the metal ions form an elemental metal that is more reactive than hydrogen (e.g. sodium ions).

If the metal ions form an elemental metal that is less reactive than hydrogen (e.g. copper ions), a solid layer of the pure metal will be produced instead

Question 4

At the anode, if OH- and halide ions (CL-, Br-, I-) are present, what will be formed?

Answer 4

molecules of chlorine, bromine or iodine will be formed.

If no halide ions are present, then the OH- ions are discharged and oxygen will be formed.

Question 5

A solution of Copper(II) sulfate (CUSO4) contains four different ions:

CU2+, SO42-, H+ and OH-

explain the electrolysis of the aqueous solutuion?

Answer 5

Copper metal is less reactive than hydrogen. So at the cathode, copper metal is produced and coats the electrode

CU2+ + 2e- &raquo_space;» Cu

There aren’t any halide ions present So at the anode Oxygen and water are produced. The oxygen can be seen as bubbles.

4OH- &raquo_space;» O2 + 2H2O + 4e-

(See diagram on page 136)

Question 6

A solution of sodium chloride (NaCl) contains four different ions:

Na+, Cl-, OH- and H+

explain the electrolysis of the aqueous solutuion?

Answer 6

Sodium metal is more reactive than hydrogen, so at the cathode, hydrogen gas is produced

2H+ + 2e- &raquo_space;» H2

Chloride ions are present in the solution, so at the anode chlorine gas is produced

2Cl- &raquo_space;> Cl2 + 2e-

see diagram on page 136

(if you are drawing the appratus for an electrolysis experiment, remember to include a d.c. power supply, with wires and labels for the anode and the cathode. The anode is the electrode on the same side as the longer line of the d.c. power supply symbol, see diagram on page 136)

Question 7

Chlorine bleaches damp litmus paper what colour?

Answer 7

turns it white

Question 8

Hydrogen makes a squeaky pop with a what?

Answer 8

lighted splint

Question 9

Oxygen will relight what?

Answer 9

a glowing splint

You can set up electrolysis experiments in the lab like the set up on page 236. Once the experiment is finished you can test any gaseous products to work ou hat was produced.

(for more tests for gasses, turn to page 155)

Question 10

What do Half Equations show?

Answer 10

they show the reactions at the electrodes.

Question 11

When you’re combining half equations you need to remember what?

Answer 11

that the number of electrons needs to be the same for each half equation.

for the electrolysis of aqueous sodium chloride the half equations are:

Negative Electrode: 2H+ + 2e- &raquo_space;» H2
Positive Electrode: 2Cl- &raquo_space;» Cl2 + 2e- or 2Cl- &raquo_space;» H2 + Cl2

(the electrons on each side of the half equation balance, so they can be cancelled out in the full ionic equation.

When a halide isn’t present in the aqueous solutuion, the half equation for the anode is:

4OH- &raquo_space;» O2 + 2H2O + 4e- or 4OH- - 4e- &raquo_space;» O2 + 2H2O

Question 12

An aqueous solutuion of copper bromide, CuBr2, is eletrolysed using inert electrodes.

state what is produced at:

a) the anode
b) the cathode

Answer 12

a) bromine gas Br2
b) copper atoms Cu