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.

↶ 6.3 Electrolysis of aqueous solutions

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