ELECTROCHEMSTRY POWER POINT

ELECTROLYSIS conduct platinum positive negative compound aqueous molten compound electric current Positive cathode Cu 2+ Negative anode Cl - release positive OH - TERM MEANING Conductor A substance that can ……………… electricity without chemical changes, usually in solid form. Example ………… ….., and carbon. Anode A conductor which is connected to the ……… … terminal of batteries in electrolytic cell. Cathode A conductor which is connected to the ………… terminal of batteries in electrolytic cell Electrolyte Chemical ……........ that can conduct electricity either in ………… or …………. state because of the presence of free moving ions. Electrolysis A process whereby a ……………. is decomposed to its constituent elements when …………… ……… ………. passes through an electrolyte. Cation ………… .. ion attracted to the … ……… …., for example ……….. Anion ………… .. Ion attracted to the ……………., for example ………… and ………….. ions Electrochemical series An arrangement of metals based on the tendency to………………….. electron to form a ………………….. ion.

The important basics to master electrolysis ; Able to write ionization equation to determine the ions present in molten or aqueous electrolyte : : NaCI (l) Na 2 O (l) AI 2 O 3 (l) -NaCI (aq) H 2 O -CuSO 4 (aq) H 2 O -H 2 SO 4 (aq) Example of aqueous electrolyte : H 2 O H + + OH - H + + OH - H + + OH - Cu 2+ + SO 4 2- Na + + CI - Na + + CI - 2H + + SO 4 2- 2Na + + O 2- 2AI 3+ + 3O 2- Example of molten electrolyte

Able to write half equation at the anode and cathode . The table below shows common discharge equations at the anode and cathode. Complete the table : Cl 2 + 2e 2e H 2 2H 2 O + O 2 + 4e + e Cu 2+ + 2e + 2e Cu I 2 + 2e Ag + + e - Half equation at the anode [ negative ion @ metal releases electron] Half equation at the cathode [positive ion receives electrons] 2Cl - -> 2H + + -> ( Very common cation discharge equation because hydrogen ion present in all aqueous solution . Hydrogen ion is always selected in dilute solution because it is less electropositive than most cation ) 4OH - -> ( Very common anion discharge equation because hydroxide ion present in all aqueous solution .Hydroxide ion is always selected because it is the least electronegative ion in the electro negativity series of anion . Ag + -> Ag Cu -> Cu 2+ -> 2I - -> - Ag -> -

(iii)Able to identify factor that affect selective discharge of ions for aqueous electrolyte : Factor When to apply How to choose the ion 1.Position of ion in the electrochemical series Dilute solution Inert Electrode e.g carbon / platinum The less electropositive ion is selected i.e H + /Cu 2+ /Ag + ion at the cathode The less electronegative ion is selected i.e hydroxide ion at the anode 2. Concentration Concentrated solution Inert Electrode e.g carbon / platinum The concentrated ion is selected at the anode but this is only true for halide ions, which are chloride, bromide and iodide H + is always chosen because K + /Na + ions are very electropositive 3. Type of electrode Metal electrode eg copper and silver. Metal atom of metal anode releases electron to form metal ion, metal anode becomes thinner. The less electropositive ion than hydrogen ion will be discharged . The concentration of electrolyte is unchanged because the number of metal atom become metal ion at the anode is the same the number metal ion become metal atom at the cathode.

DILUTE [0.0001 Moldm -3 ] CONCENTRATED [ 2 Moldm -3 ] NaCI (aq) H 2 O Na + (aq) + CI - H + + OH - OH - , Cl - H +, Na + OH - , Cl - H +, Na + 4OH - 2H 2 O + O 2 + 4e 2H + + 2e H 2 2Cl - Cl 2 + 2e Bubbles are released , place glowing wooden splinter near the mouth of test tube, glowing wooden splinter lighted up 2H + + 2e H 2 Bubbles are released . Place a lighted wooden splinter near the mouth of test tube, “pop” sound is produced A Greenish-yellow gas released. Place a damped blue litmus at the mouth of the test tube, the gas changed the damp blue litmus paper to red and bleached it Concentration of sodium chloride solution increases Hydrogen gas is released at the cathode and oxygen gas is given off at the anode. Water decomposed to oxygen gas and hydrogen gas. Hydrogen gas released at the cathode and chlorine gas given off at the anode. Concentration of chloride ion decreases. Concentration of sodium chloride solution decreases Bubbles are released . Place a lighted wooden splinter near the mouth of test tube, “pop” sound is produced Carbon electrode ANODE(+) CATHODE(-) ANODE(+) CATHODE(-) IONS ATTRACTED H. EQN OBSVN/ CONFIRM. TEST CONC OF ELECTROLYTE AFTER A CERTAIN PERIOD

SO 4 2- , OH - Cu 2+ , H + SO 4 2- , OH - Cu 2+ , H + 4OH - 2H 2 O + O 2 + 4e Cu 2+ + 2e Cu Cu Cu 2+ + 2e Cu 2+ + 2e Cu Bubbles are released , place glowing wooden splinter near the mouth of test tube, glowing wooden splinter is lighted up Brown solid deposited Copper electrode becomes thinner Brown solid deposited Concentration of copper(II) sulphate solution remains unchanged. The intensity of blue colour of copper(II) sulphate solution remains unchanged. The number of copper atoms form copper(II) ions at the anode is equal to the number of copper(II) ions form copper atoms at the cathode. Concentration of copper(II) sulphate solution decreases The intensity of blue colour of copper(II) sulphate decreases. Copper(II) ions discharge at the cathode H 2 O H + + OH - CARBON ELECTRODE COPPER ELECTRODE CuSO 4 ( aq ) Cu 2+ (aq) + SO 4 2- CARBON ELECTRODE ANODE(+) CATHODE(-) ANODE(+) CATHODE(-) IONS ATTRACTED HALF EQUATION OBSRN/ CON TEST CONCENTRATION OF ELECTROLYTE AFTER A CERTAIN PERIOD

Position of ion in electrochem series H + , SO 4 2- , OH - 2H + + 2e -> H 2 Bubbles are released 4OH - -> 2H 2 O + O 2 + 4e Bubbles are released Concentration H + , Cl - , OH - Position of ion in electrochem series Position of ion in electrochem series Position of ion in electrochem series Concentration Type of electrode 2H + +2e -> H 2 Bubbles are released 2H + + 2e -> H 2 Bubbles are released 2H + + 2e -> H 2 Bubbles are released 2H + + 2e -> H 2 Bubbles are released Ag + + e -> Ag Shiny grey solid deposited Ag + + e -> Ag Shiny grey solid deposited 4OH - -> 2H 2 O + O 2 + 4e Bubbles are released 4OH - -> 2H 2 O + O 2 + 4e Bubbles are released 4OH - -> 2H 2 O +O 2 + 4e Bubbles are released 2Cl - -> Cl 2 + 2e A greenish-yellow gas released 2I - -> I 2 + 2e Brown solution is formed Ag + , NO 3 - , H + , OH - Ag + , NO 3 - , H + , OH - , K + , H + , I - , OH - K + , H + , I - , OH - H + , Na +, OH - , SO 2 - Ag -> Ag + + e Silver electrode becomes thinner ELECTROLYTE ELECTRODE FACTOR THAT AFFECT ELECTROLYSIS IONS PRESENT HALF EQUATION AT THE ANODE AND OBSERVATION HALF EQUATION AT THE CATHODE & OBSERVATION Dilute sulphuric acid Carbon Concentrated hydrochloric acid Carbon Silver nitrate solution Carbon Silver nitrate solution Silver Potassium iodide solution Carbon Concentrated potassium iodide solution Carbon Sodium sulphate solution Carbon

ELECTROLYSIS IN INDUSTRY Electroplating Example : Silver electroplating Anode: Silver Ag -> Ag + + e Cathode : Spoon Ag + + e -> Ag

Silver atom the anode releases electrons to form silver ions ,Ag + and moves into the silver nitrate solution: Ag (s) Ag + (aq) + e - The anode becomes thinner/ smaller Silver ion, Ag + move to the cathode receives electron to form silver atom : Ag + (aq) + e - Ag (s) Silver is deposited onto the surface of the object as the object becomes silver plated. The rate at which the silver atoms become silver ions at the anode is the same as the rate at which the silver ions become silver atoms at the cathode The concentration of the silver nitrate solution therefore remains unchanged.

2. Purification of metal Anode is impure copper Cathode is pure copper Electrolyte is copper(II) sulphate solution Copper atom the anode releases electrons to form copper ions ,Cu 2+ and moves into the copper sulphate solution. Cu(s) Cu 2+ (aq) + e - The anode becomes thinner/ smaller Copper ion receives electrons to form copper atom at the cathode Cu 2+ (aq) + 2e - Cu (s) The pure copper is deposited at the cathode

Silver electroplating Anode is silver Ag -> Ag+ + e Silver anode becomes thinner Ag+ + e -> Ag Shiney Grey solid deposited Purification of copper Anode is impure copper Cu -> Cu 2+ + 2e Impure copper becomes thinner Cathode is pure copper Cu 2+ + 2e -> Cu Pure copper becomes thicker Extraction of Aluminium Anode is with carbon 2O 2- -> O 2 + 4e Colorless gas given off. Cathode is steel container is coated with carbon Al 3+ + 3e -> Al Grey liquid metal formed Metal Extraction Purification of metal Electroplating CATHODE/ HALF EQUATION / OBSERVATION ANODE / HALF EQUATION / OBSERVATION EXAMPLE APPLICATION

A cell that produces electrical energy when chemical reactions occur in it . Chemical energy -> Electric Energy Produced when two different metals are dipped in an electrolyte and connected by an external circuit . The voltage of chemical cell depends on the distance between the two metals in the electrochemical series , where the further the distance between them, the higher is the voltage . VOLTAIC CELL / CHEMICAL CELL

More electropositive metal Metal atom releases electrons Electrons flow through the external circuit Metal atom becomes metal ion (becomes thinner) OXIDATION Less electropositive metal The electrons that flow from the external circuit flow through the positive terminal. Positive ions in the electrolyte receive electrons to form atom/molecule (gas bubbles releases/ metal deposited) REDUCTION NEGATIVE TERMINAL POSITIVE TERMINAL - + Electrolyte v

EXAMPLE - + Negative terminal: Mg -> Mg 2+ + 2e Magnesium atom releases electron to form magnesium ion, Mg 2+ . Electrons flow through external circuit to the positive terminal. Magnesium electrode becomes thinner/dissolve. Positive terminal: 2H + + 2e -> H 2 Hydrogen ions receive electrons to form hydrogen molecule. Gas bubbles are released. v H 2 SO 4 Magnesium Copper

DANIELL CELL Each metal is dipped in its aqueous solution. The two solutions are separated by a salt bridge or porous pot. The function of the salt bridge/porous pot is to allow the flow of the ions. Copper(II) suphate Zinc sulphate V Zinc Copper Porous pot V Salt bridge Zinc suphate Copper Copper(ii) sulphate Zinc

ZINC ELECTRODE : …………………… terminal because ………………………………………….. Half equation : ……………………………………………………………… Observation : ……………………………………… COPPER ELECTRODE : ……………… terminal because ………………………………………. Equation : ……………………………………………………………….. Observation : ………………………………………………………………… If the zinc metal is replaced with a magnesium metal , the voltage reading increases because magnesium is further from copper in the Electrochemical Series Negative zinc is more electropositive than copper Zn -> Zn 2+ + 2e Zinc electrode becomes thinner Positive copper is less electropositive than zinc (i)Brown solid deposited. (ii) The intensity of blue colour of copper(II) sulphate decreases. Cu 2+ + 2e -> Cu

Metal Displacement Reaction. The metal which is situated at a higher position ( higher tendency to release electron) in the electrochemical series is able to displace a metal below it from its salt solution . Copper Silver nitrate solution Grey solid Blue solution

thinner grey colourless Copper(II) ion grey silver silver silver Copper(II) ion silver Cu(NO 3 ) 2 + 2Ag more above Copper atom releases electrons to form ………….. The ………… solid is ……… . ……… ion receives electron to form ……….. atom The blue solution is……………….. Copper has displaced …………from silver nitrate solution : Cu + 2AgNO 3 ->............................. Copper is………….electropositive than silver// Copper is …………silver in the Electrochemical Series. Copper strip becomes…………. A………. solid deposited The …………… solution turns blue. Explanation/ Inference/Conclusion Observation Experiment Silver nitrate solution Copper strip

thinner brown colourless magnesium ion brown copper. Copper(II) copper. magnesium sulphate copper. MgSO 4 + Cu more above Copper(II) sulphate solution Magnesium strip Magnesium atom releases electrons to form……………….. The………… solid is ……….. ……………ion receives electrons to form ………….. atom The colourless solution is ……………………. Magnesium has displaced ………………from copper(II) sulphate solution : Cu + 2AgNO 3 ->............................... Magnesium is ………… electropositive than copper // Magnesium is …………copper in the electrochemical series of metal Magnesium strip becomes ……….. The …………solid deposited The blue solution turn ………………

ELECTROCHEMSTRY POWER POINT

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