Electrolysis of Aqueous solutions

Question 1

In molten of molten compounds explain what happens to the positive and negative ions

Answer 1

Positive ions are attracted to and move to the cathode (negative electrode) because opposite charges attract. At the cathode, the positive ions gain electrons to form atoms. This is an example of a reduction reaction

Negative ions are attracted to and move to the anode (positive electrode) because opposite charges attract. At the anode, the negative ions lose electrons to form atoms. This is an example of a oxidation reaction

Question 2

How many ions are involved in the electrolysis of molten compounds

Answer 2

2 ions - from the molten compound

Question 3

What do water molecules ionise to form

Answer 3

Water molecules ionise (split) forming hydrogen ions and hydroxide ions

Question 4

write the equation showing the ionisation of water

Answer 4

H2O ⇌ H+ + OH-
hydrogen ions hydroxide ions

Question 5

Ions present in electrolysis of CuSO4 (aq)

Answer 5

Cu 2+ SO4 2-
H + OH - (because the cuso4 is dissolved in water)

Question 6

In aqueous electrolysis, which element will be produced at the cathode

Answer 6

Hydrogen is produced at the cathode, if the metal is more reactive than hydrogen

Question 7

In the electrolysis of Copper Sulfate solution what element will be produced at the cathode

Answer 7

Copper is less reactive than hydrogen therefore copper will be produced at the cathode

Question 8

In aqueous electrolysis, which element will be produced at the anode

Answer 8

At the positive electrode (anode), oxygen is produced unless the
solution contains halide ions when the halogen is produced.

If the aqueous solution contains halide ions, then the halogen will be produced at the anode. If the aqueous solution does not contain halide ions then oxygen is produced at the anode instead.

Question 9

In the electrolysis of Copper Sulfate solution what element will be produced at the anode

Answer 9

The aqueous solution does not contain a halide ion therefore oxygen gas is produced at the anode

Question 10

Write the half-equations for the reactions at the cathode for the electrolysis of aqueous copper sulfate

Answer 10

Cu 2+ SO4 2-
H+ OH -

Cu2+ +2e- —> Cu (copper ions are gaining to electrons to become copper atoms - reduction reaction)

Question 11

Write the half-equations for the reactions at the anode for the electrolysis of aqueous copper sulfate

Answer 11

4OH - —-> O2 + 2H2O + 4e-

4 hydroxide ions react to form oxygen gas and water, losing a total of 4 electrons in the process - this is an oxidation reaction

or

4OH- - 4e- —–> O2 + 2H2O

Question 12

State what can be used as an electrode in electrolysis

Answer 12

Platinum / graphite - as it is inert (unreactive)

Question 13

In the electrolysis of sodium chloride solution what element will be produced at the cathode

Answer 13

Sodium is more reactive than hydrogen (gas) therefore hydrogen will be produced at the cathode

Question 14

In the electrolysis of sodium chloride solution what ions will be attracted to the cathode

Answer 14

2 different ions will be attracted to the cathode - sodium ion and hydrogen ion

Question 15

example of halide ions

Answer 15

chloride ions
fluoride ions
bromide ions
iodide ions

Question 16

In the electrolysis of sodium chloride solution what ions will be attracted to the anode

Answer 16

2 different ions will be attracted to the anode - chloride ion and hydroxide ion

Question 17

In the electrolysis of sodium chloride solution what element will be produced at the cathode

Answer 17

The aqueous solution contains the chloride ion, which is a halide ion therefore chlorine gas will be produced at the anode

Question 18

Write the half-equations for the reactions at the cathode for the electrolysis of aqueous sodium chloride

Answer 18

2H + + 2e- —–> H2
Each hydrogen ion is gaining one electron to from hydrogen gas (H2 gas molecule)
This is a reduction reaction

Question 19

Write the half-equations for the reactions at the anode for the electrolysis of aqueous sodium chloride

Answer 19

2Cl - —-> 2Cl + 2e-

Each chloride ion is losing one electron to form chlorine gas (Cl2 molecule) This is a oxidation reaction

Question 20

describe how to investigate what happens when aqueous solutions are electrolysed using
inert electrodes.

Answer 20

In this practical, we are going to carry out electrolysis on two different aqueous solutions and explain the different products formed

2 different solutions Copper (II) chloride and sodium chloride

1) First pour approximately 50cm^3 of copper (II) chloride solution into a beaker

2) Place a plastic petri dish over the beaker. The beaker should have 2 holes

3) Insert a carbon graphite rod into each hole. These are our electrodes.

Carbon graphite is used because it is unreactive, therefore the electrodes are inert

The two electrodes must not touch each other as that would produce a short circuit

4) Attach crocodile leads to the rods, and then connect the rods to the terminals of a low-voltage power supply

5) Select 4V on the power supply and turn it on

6) At the negative electrode (the cathode), we can see that it is being coated with copper - because copper is less reactive than hydrogen, copper is produced (discharged) at the cathode

7) At the positive electrode (the anode), we can see bubbles of a gas and we might notice the smell of chlorine in the air- copper (II) remember chloride is being electrolysed -
chloride is a halide ion.

Whenever we carry out electrolysis on an aqueous solution containing a halide ion, then the halogen is discharged at the anode - this means we a producing chloride gas at the anode

If we hold a piece of damp blue litmus paper near the anode, it will turn pink then it becomes bleached and turn white. This proves that the gas is chlorine

Repeat the experiment but electrolyse sodium chloride solution

Place 50cm^3 of sodium chloride solution into the beaker. Turn on the low voltage power supply and look at the two electrodes

At the anode, we can see bubbles of gas being produced. This gas bleaches damp blue litmus paper and that tells us that the gas is chlorine
(Sodium chloride contains the chloride ion which is a halide. If we carry out electrolysis of a solution containing a halide ion, then the halogen is discharged at the cathode

We can also see gas bubbles at the cathode and this gas is hydrogen
(this is because we are electrolysing sodium chloride solution - because sodium is more reactive than hydrogen, hydrogen gas is produced (discharged) at the cathode

To prove that this gas is hydrogen, by collecting it and testing it with a lit splint. Hydrogen gas produces a squeaky pop.

Collect the gas in a test tube.
Light a split. Put the lit splint in the test tube. If the gas is hydrogen, then a squeaky pop would be heard (it gave/produced a squeaky pop)