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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2 comments:

  1. I used this one exercise book and i have a question ,, if 0.001 mole of MgCl2solution , the product form at cathode is Hydrogen and when 2mole of MgCl2 solution is use why doesn't the Mg form at the cathode? Could any of these type of experiment,, the concentration of ion affect the product form at cathode?or maybe when using carbon rod only hydrogen can be form??

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