Chemistry : Electrolysis

Electrolysis is the reaction that occurs when electricity pass through the electrolyte in aqueous or molten state. Electrolytic cell is where electrolysis occurs.

Note that in electrolytic cell, negative terminal is cathode while positive terminal is anode (interchanged with chemical cell), also usually inert electrodes is used.

Dissociation of water: H₂O→H⁺+OH^- (reversible reaction)

The three factor affecting preferential discharge of aqueous ions:

1)       Position of ions in the E.C.S. (Note that NO₃^- and SO₄^2- is even lower than Cl^-.)

Lower position of cations in E.C.S. is easier to be discharged, while higher position of anions in E.C.S. is easier to be discharged.

e.g. Acidified water (H⁺ used to conduct electricity): In cathode, there’re only one species of ion (H⁺) so H₂ is released. In anode, there’re two species of ion: SO₄^2- and OH^-, since OH^- has higher position in E.C.S., O₂ is given off. Overall reaction: 2H₂O→2H₂+O₂

e.g.2: Very dilute NaCl (~0.1M), H⁺ is preferentially discharged than Na⁺, and OH^- is preferentially discharged than Cl^-, Overall reaction: 2H₂O→2H₂+O₂.

In both cases, water continuously dissociates to H⁺ and OH^-, at cathode there’re excess OH^- so it’s alkaline, while there’re excess H⁺ at anode so it’s acidic around the anode.

e.g.3: CuSO₄, OH^- is preferentially discharged, and Cu²⁺ is preferentially discharged, so there’s Cu formed on cathode. Blue colour fades due to loss of Cu²⁺.

2)       Concentration factor: when the two ions has similar position in E.C.S., ions with high concentration are preferentially discharged.

e.g. Conc. NaCl: In anode, though Cl^- is lower than OH^-, but it has a high concentration so it’s discharged and give Cl₂ (since Cl₂ attacks Pt, Pt electrode can’t be used). I^- is preferentially discharged at any concentration, while Br^- and Cl^- preferentially discharge only when it’s concentrated. At anode, halides dissolves in water to form acidic solution, so solution around anode is acidic.

3)       Electrodes: In conc. solution, Na⁺ is preferentially discharged and dissolves in Hg to give stable sodium amalgam (Na/Hg). e.g. in conc. NaCl with Hg cathode, at cathode Na⁺ is preferentially discharged: Na⁺+e^-+Hg→Na/Hg, then the amalgam reacts with water: 2Na/Hg+2H₂O→2NaOH+H₂+2Hg, overall reaction: 2Na⁺+2Cl^-+2Hg→2Na/Hg+Cl₂. The solution become more dilute.

Applications

-          Refining Cu: impure Cu at anode and pure Cu at cathode in CuSO₄; The impure copper changes to ions, but only Cu²⁺ is preferentially discharged to form Cu at cathode.

-          Electroplating: Plating metal at anode, object at cathode, aq. Solution with salt of plating metal as electrolyte. Factor affecting its quality: lower concentration, acidic solution, brighteners and leveling agents, low current, similar shape with the object, etc.

Environmental concerns on electroplating: acids/alkalis, heavy metal, CN^- in water.

Electrochemistry is now finished.