Wednesday, 21 January 2015

6.3.2 - Laboratory Activity : Concentration of ions in electrolyte solution


Laboratory Activity 6.3.2:
Concentration of ions in electrolyte solution
Aim: : To study the effect of the concentration of ions in a solution on the choice of ions for discharge on electrolysis
Problem statement: How does ion concentration influence the choice of ion for discharge?
Hypothesis: Ion that is more concentrated will be preferably discharged at the electrode.
Variable:
» Fixed variable : Electrolyte and types of electrodes
» Manipulated variable : Concentration of ions
» Responding variable : Types of ions discharged at the anode and the cathode

Material:
» Potassium bromide solution 2 mole dm-3 and 0.001 mole dm-3

Apparatus:
» Electrolytic cell
» Test tube
» Carbon electrodes
» Switch
» Connecting wires
» Crocodile clip
» Wooden splinter
» Batteries
Procedure:

The animation below shows the arrangement and results of the experiment.

1. Electricity is passed through the potassium bromide solution 2 mole dm-3 for 15 minutes.
2. The gases collected at the electrodes are tested, and observation are recorded..
3. The experiment is repeated by using a dilute potassium bromide solution 0.001 mole dm-3
Results:

Potassium bromide solution 2 mole dm-3
Electrode Observation Inference
Anode Brown gas produced turns blue litmus to red Bromine gas is released.
Cathode Colourless gas released. Give a 'pop' sound with burning splinter. Hydrogen gas is released

Dilute potassium bromide solution 0.001 mole dm-3
Electrode Observation Inference
Anode Colourless gas released. Rekindle glowing glowing splinter. Oxygen gas is released.
Cathode Colourless gas released. Give a 'pop' sound with burning splinter. Hydrogen gas is released
Discussion:

Potassium bromide solution 2 mole dm-3
Aqueous solution At anode(+) At cathode(-)
Movements of ion to electrode Br-, OH- K+, H+
Ion selected for discharged Ion Br- selected for discharged as it has a higher concentration than hydroxide ion.. Ion H+ selected for discharged as it is lower position in electrochemical series.
Half-equation 2Br- → Br2 + 2e-
Bromine gas is collected.
H+ + 2e- → H2
Hydrogen gas is collected.

Dilute potassium bromide solution 0.001 mole dm-3
Aqueous solution At anode(+) At cathode(-)
Movements of ion to electrode Br-, OH- K+, H+
Ion selected for discharged Ion OH- selected for discharged as it is lower position in electrochemical series. Ion H+ selected for discharged as it is lower position in electrochemical series.
Half-equation 4OH- → 2H2 + O2 + 4e-
Oxygen gas is collected.
H+ + 2e- → H2
Hydrogen gas is collected.
Conclusion:

The more concentrated a type of ion in an aqueous solution, the more readily it is discharged during electrolysis.

The hypothesis is accepted.


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