Types of electrolytic cells

inert electrode. e.g zinc chloride. cathode: Zn2+ anode: Cl-

molten

water dissociates abit to produce ions, acid added to increase ion conc in electrolyte to increase conductivity. inert electrode. cathode: H+ anode: OH-

water

undergoes preferential discharge, less reactive cation is preferentially discharged. anions always discharged: OH- > I- > Br- > Cl- > NO3- > SO42-

dilute

cathode: Zn2+ and H+, H+ less reactive and preferentially discharged 2H+ + 2e- -> H2 (g). anode: Cl- and OH-, OH- preferentially discharged 4OH- -> 2H2O(l) + O2(g) + 4e-. Eff observed, solution gets more concentrated over time as water is removed

dilute electrolyte of ZnCl2

cathode: Cu2+ and H+, Cu2+ less reactive and preferentially discharged Cu2+ + 2e- -> Cu(s) anode: Cl- and OH-, OH- preferentially discharged 4OH- -> 2H2O(l) + O2(g) + 4e-. Eff observed, soln also changes colour from blue to colourless as Cu2+ ions are discharged, reddish brown solid forms. pH of solution increases as OH- ions are discharged

dilute electrolyte of CuCl2

higher conc of electrolyte affects preferential discharge of anions. Halogens become more preferentially discharged over OH-. E.g conc ZnCl2. anode: Cl- and OH-, Cl- preferentially discharged 2Cl- -> Cl2(g) + 2e-

concentrated

reactive electrodes take part in electrolysis by oxidation. e.g copper electrodes and dilute CuCl2. cathode: Cu2+ and H+, Cu2+ preferentially discharged to form reddishbrown solid. anode: Cl- and OH-, neither are discharged, instead Cu electrode oxidises to produce Cu2+ ions, replenishing ions in solution Cu(s) -> Cu2+ + 2e-. Soln remains blue as conc of Cu2+ ions remains unchanged. anode decreases in size, cathode increases in size as copper solidifies on cathode

dilute with reactive electrode

used to separate impurities. e.g using impure copper anode and pure copper cathode, copper(II)sulfate. cathode: Cu2+(from anode) and H+, reddishbrown solid forms and adds on to pure copper cathode, increasing size and mass. anode: SO42- and OH-, anode ionises, dissolves to form Cu2+, decreasing size and mass, impurities fall to bottom of cell. Soln remains blue as conc of Cu2+ remains same, no net loss of Cu ions as ions from anode go to cathode

electrolytic purification

used to beautify objects or prevent rusting. plating metal anode, object to be plated cathode, aqueous soln of salt of plating metal as electrolyte. e.g aqueous dilute copper(II)sulfate and copper anode. cathode: Cu2+ ion from anode solidifies, reddishbrown solid deposited on cathode(object to be plated) anode: anions from soln not discharged, anode ionises and dissolves releasing Cu2+ ions into soln. blue solution of CuSO4 unchanged, no net loss of Cu2+ ions

electroplating