ELECTROCHEMSTRY POWER POINT

ELECTROLYSIS

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.

conduct platinum

positive

negative

compound aqueous

molten

compound

electric current

Positive cathode Cu2+

Negative anode Cl-

release

positive

OH-

The important basics to master electrolysis ;(i) Able to write ionization equation to determine the ions present in molten or aqueous electrolyte :

:NaCI (l)

Na2O (l)

AI2O3 (l)

-NaCI (aq) H2O

-CuSO4 (aq)

H2 O

-H2SO4 (aq)

Example of aqueous electrolyte :

H2O

H+ + OH-

H+ + OH-

H+ + OH-

Cu2+ + SO42-

Na+ + CI-

Na+ + CI-

2H+ + SO42-

2Na+ + O2-

2AI3+ + 3O2-

Example of molten electrolyte

(ii) 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 :

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 → Cu2+ →

2I- → -

Ag → -

Cl2 + 2e 2e H2

2H2O + O2 + 4e + e

Cu2+ + 2e + 2e Cu

I2 + 2e

Ag+ + e

-

(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+/Cu2+/Ag+ ion at the cathodeThe 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.

ANODE(+) CATHODE(-) ANODE(+) CATHODE(-)

IONS ATTRACTED

H. EQN

OBSVN/ CONFIRM. TEST

CONC OF ELECTROLYTE AFTER A CERTAIN PERIOD

DILUTE [0.0001 Moldm-3] CONCENTRATED [ 2 Moldm-3]

NaCI (aq)

H2O

Na+ (aq) + CI-

H+ + OH-

OH-, Cl- H+, Na+ OH-, Cl- H+, Na+

4OH- 2H2O + O2+ 4e 2H+ + 2e H22Cl- Cl2 + 2e

Bubbles are released , place glowing wooden splinter near the mouth of test tube, glowing wooden splinter lighted up

2H+ + 2e H2

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

HALF EQUATION

OBSRN/ CON TEST

CONCENTRATION OF ELECTROLYTE AFTER A CERTAIN PERIOD

SO42-, OH- Cu2+, H+ SO4

2-, OH- Cu2+, H+

4OH- 2H2O + O2 + 4e Cu2+ + 2e Cu Cu Cu2+ + 2e Cu2+ + 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

H2O H+ + OH-

CARBON ELECTRODE COPPER ELECTRODE

CuSO4 (aq) Cu2+ (aq) + SO42-

CARBON ELECTRODE

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

Position of ion in electrochem series

H+, SO4

2-, OH-

2H+ + 2e → H2

Bubbles are released4OH- → 2H2O + O2 + 4eBubbles 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 → H2 Bubbles are released

2H+ + 2e → H2

Bubbles are released

2H+ + 2e → H2

Bubbles are released

2H++ 2e → H2Bubbles are released

Ag+ + e → AgShiny grey solid deposited

Ag+ + e → AgShiny grey solid deposited

4OH- → 2H2O + O2+ 4eBubbles are released

4OH- →2H2O + O2 + 4eBubbles are released

4OH- → 2H2O +O2+ 4eBubbles are released

2Cl-→ Cl2 + 2e A greenish-yellow gas released

2I- → I2 + 2eBrown solution is formed

Ag+, NO3

-, H+, OH-

Ag+, NO3

-, H+, OH-,

K+, H+, I-, OH-

K+, H+, I-, OH-

H+, Na+, OH-, SO2

-

Ag →Ag+ + eSilver electrodebecomes thinner

ELECTROLYSIS IN INDUSTRY

1. 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 ,Cu2+and moves into the copper sulphate solution. Cu(s)              Cu2+(aq) + e-

The anode becomes thinner/ smaller• Copper ion receives electrons to form copper atom at the cathode Cu2+

(aq) +  2e-                 Cu (s)

The pure copper is deposited at the cathode

APPLICATION EXAMPLEANODE / HALF EQUATION /OBSERVATION

CATHODE/ HALF EQUATION / OBSERVATION

Electroplating

Purification of metal

Metal Extraction

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 → Cu2+ + 2e

Impure copper becomes thinner

Cathode is pure copper

Cu2+ + 2e → Cu

Pure copper becomes thicker

Extraction of Aluminium

Anode is with carbon

2O2- → O2 + 4e

Colorless gas given off.

Cathode is steel container is coated with carbon

Al3+ + 3e → Al

Grey liquid metal formed

•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

v

More electropositive metalMetal atom releases electronsElectrons flow through the external circuitMetal atom becomes metal ion (becomes thinner)

OXIDATION

Less electropositive metalThe 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

EXAMPLE

- +

v

H2 SO4

Magnesium Copper

Negative terminal: Mg → Mg2+ + 2e

•Magnesium atom releases electron to form magnesium ion, Mg2+.

•Electrons flow through external circuit to the positive terminal.

•Magnesium electrode becomes thinner/dissolve.

Positive terminal: 2H+ + 2e → H2

•Hydrogen ions receive electrons to form hydrogen molecule.

•Gas bubbles are released.

Copper(II) suphate

Zinc sulphate

V

Zinc Copper

Porous pot

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.

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 → Zn2+ + 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.

Cu2+ + 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

Experiment Observation Explanation/ Inference/Conclusion

-Copper strip

becomes………….

A………. solid

deposited

-The ……………

solution turns blue.

-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 + 2AgNO3 →.............................

-Copper is………….electropositive than

silver// Copper is …………silver in the

Electrochemical Series.

Silver nitratesolution

Copper strip

thinner

grey

colourless

Copper(II) ion

grey silver

silver

silver

Copper(II) ion

silver

Cu(NO3)2 + 2Ag

more

above

Copper(II) sulphatesolutionMagnesium strip

-Magnesium strip

becomes ………..

The …………solid

deposited

-The blue solution

turn ………………

-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 + 2AgNO3 →...............................

-Magnesium is …………

electropositive than copper //

Magnesium is …………copper in the

electrochemical series of metal

thinner

brown

colourless

magnesium ion

brown copper.

Copper(II)

copper.

magnesium sulphate

copper.

MgSO4+ Cu

more

above