Aim :To investigate the effect of concentration of ions on selective discharge of ions at the electrodes
Problem statement :How does the concentration of ions in hydrochloric acid, HCl affect the discharge of ions at the anode?
Hypothesis :When the concentration of chloride ion is higher, then the chloride ion will be selectively discharged at anode.
Variables :Manipulated variable : Concentration of chloride ion
Responding variable : Ion discharged
Constant variable : Type of electrolyte, type of electrode, duration of electrolysis
Materials 1.0 mol dm-3 hydrochloric acid, 0.001 mol dm-3 hydrochloric acid
Apparatus batteries, carbon electrodes, connecting wires with crocodile clips, ammeter, electrolytic cell, test tubes, litmus paper and splinter
Procedure 1. A electrolytic cell is filled with 1.0 mol dm-3 hydrochloric acid until it is half full.
2. The switch is turned on.
3. Gas produced at anode is collected and tested with moist litmus paper and glowing splinter.
4. Observations are recorded.
5. Steps 1 to 4 are repeated by using 0.001 mol dm-3 hydrochloric acid to replace 1.0 mol dm-3 hydrochloric acid.
Observation at anode
1.0 mol dm-3 hydrochloric acid
A greenish-yellow gas with pungent smell is released.
The gas turns the blue litmus paper to red then to white.
0.001 dm-3 hydrochloric acid
Gas bubbles are released.
A colourless gas relight a glowing splinter.
1. the aqueous solution of hydrochloric acid consists of hydrogen ions, H+, chloride ions, Cl- and hydroxide ions, OH- tat move freely.
2. During the electrolysis, the Cl- ions and OH- ions move to the anode.
a) Electrolysis of 0.001 dm-3 hydrochloric acid
The OH- ions are selectively discharged at anode to form oxygen and water. This is because OH-ion is lower than Cl- ion in the electrochemical series.
4OH- → O2 + 2H2O + 4e
b) Electrolysis of 1.0 mol dm-3 hydrochloric acid
The Cl- ions are selectively discharged at anode to form chlorine gas. This is because the concentration of Cl- ions are higher than OH- ions.
2Cl- → Cl2 + 2e
Conclusion : During electrolysis of 1.0 mol dm-3 hydrochloric acid, the Cl- ions are selectively discharged at anode instead of OH-.to form chlorine gas due tohigher concetration of Cl-.The hypothesis is accepted.