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7/31/2019 Electrolysis 090618180154 Phpapp01

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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.

conductplatinum

positive

negative

compoundaqueous

molten

compound

electric current

Positive cathode Cu2+

Negative anodeCl-

release

positive

OH-
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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 ofaqueous electrolyte:

H2O

H+ + OH-

H+ + OH-

H+ + OH-

Cu2+ + SO42-

Na+ + CI-

Na+ + CI-

2H+ + SO42-

2Na+ + O2-

2AI3+ + 3O2-

Example ofmolten electrolyte


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(ii) Able to write half equation at the anode and cathode. The table below showscommon 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 equationbecause hydrogen ion present in all aqueoussolution. Hydrogen ion is always selected indilute solution because it is less electropositive

than most cation)4OH-

(Very common anion discharge equation becausehydroxide 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

-
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(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 inthe

electrochemicalseries

Dilute solution

Inert Electrode e.g

carbon / platinum

The less electropositive ion isselected i.e H+/Cu2+/Ag+ ion at the

cathodeThe less electronegative ion isselected i.e hydroxide ion at the anode

2. Concentration Concentrated

solutionInert Electrode e.gcarbon / platinum

The concentrated ion is selected at

the anode but this is only true forhalide ions, which are chloride,bromide and iodide

H+ is always chosen because K+/Na+

ions are very electropositive

3. Type of electrode Metal electrode egcopper and silver.

Metal atom of metal anodereleaseselectron to form metal ion, metal

anode becomes thinner.The less electropositive ion thanhydrogen ion will be discharged.

The concentration of electrolyte isunchanged because the number ofmetal atom become metal ion at theanode is the same the number metalion become metal atom at the cathode.


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

IONSATTRACTED

H. EQN

OBSVN/CONFIRM. TEST

CONCOFELECTROLYTEAFTER ACERTAINPERIOD

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 H2 2Cl- Cl2 + 2e

Bubbles arereleased , placeglowing woodensplinter near themouth of test tube,

glowing woodensplinter lighted up

2H+ + 2e H2

Bubbles arereleased . Place alighted woodensplinter near themouth of test

tube, pop soundis produced

A Greenish-yellow gasreleased. Place adamped blue litmus atthe mouth of the testtube, the gas changedthe damp blue litmus

paper to red andbleached it

Concentration of sodium chloridesolution increasesHydrogen gas is released at the cathodeand oxygen gas is given off at theanode.

Water decomposed to oxygen gas andhydrogen gas.

Hydrogen gas released at thecathode and chlorine gas given off atthe anode.

Concentration of chloride iondecreases.

Concentration of sodium chloridesolution decreases

Bubbles arereleased . Place alighted woodensplinter near themouth of testtube, popsound isproduced

Carbon electrode
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ANODE(+) CATHODE(-) ANODE(+) CATHODE(-)

IONSATTRACTED

HALFEQUATION

OBSRN/CONTEST

CONCENTRATION OFELECTROLYTEAFTER ACERTAINPERIOD

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 woodensplinter near themouth of test tube,glowing woodensplinter is lighted up

Brown soliddeposited

Copperelectrodebecomesthinner

Brown soliddeposited

Concentration of copper(II) sulphatesolution remains unchanged.

The intensity of blue colour of copper(IIsulphate solution remains unchanged.

The number of copper atoms formcopper(II) ions at the anode is equal to

the number of copper(II) ions form coppatoms at the cathode.

Concentration of copper(II)sulphate solution decreases

The intensity of blue colour ofcopper(II) sulphate decreases.

Copper(II) ions discharge at thecathode

H2O H+ + OH-

CARBON ELECTRODE COPPER ELECTRODE

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

CARBON ELECTRODE
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ELECTROLYTE

ELECTRODE

FACTOR THATAFFECT

ELECTROLYSIS

IONSPRESENT

HALF EQUATION ATTHE ANODE AND

OBSERVATION

HALF EQUATION AT THECATHODE &

OBSERVATION

Dilutesulphuric acid

Carbon

Concentratedhydrochloric

acid

Carbon

Silver nitratesolution

Carbon

Silver nitratesolution

Silver

Potassiumiodide

solution

Carbon

Concentrated

potassiumiodide

solution

Carbon

Sodiumsulphatesolution

Carbon

Position ofion inelectrochem

series

H+,SO4

2-,OH-

2H+ + 2e H2Bubbles are released

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

Concentration H+, Cl-,

OH-

Position ofion inelectrochemseries



Concentration

Type ofelectrode

2H+ +2e H2Bubbles are released



2H++ 2e H2Bubbles are released

Ag+ + e AgShiny grey soliddeposited

Ag+ + e AgShiny grey soliddeposited

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

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

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

2Cl- Cl2 + 2eA greenish-yellowgas released

2I- I2 + 2eBrown solutionis 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


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ELECTROLYSIS IN INDUSTRY

1. Electroplating

Example : Silver electroplating

Anode: Silver

Ag Ag+ + e

Cathode : Spoon

Ag+ + e Ag


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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 anodeis 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.
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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
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APPLICATION EXAMPLEANODE / HALF EQUATION /OBSERVATION

CATHODE/ HALFEQUATION /OBSERVATION

Electroplating

Purification ofmetal

Metal

Extraction

Silverelectroplating

Anode is silver

Ag Ag+ + eSilver anodebecomes thinner

Ag+ + e Ag

Shiney Greysolid deposited

Purification ofcopper

Anode is impurecopper

Cu Cu2+ + 2e

Impure copperbecomes thinner

Cathode is purecopper

Cu2+ + 2e CuPure copperbecomes thicker

Extraction ofAluminium

Anode is with carbon

2O2- O2 + 4e

Colorless gas givenoff.

Cathode is steel

container is coatedwith carbon

Al3+ + 3e Al

Grey liquid metalformed


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A cell that produces electrical energy when chemical

reactions occur in it.

Chemical energy Electric Energy

Produced when two different metals are dipped in anelectrolyte and connected by an external circuit.

The voltage of chemical cell depends on the distancebetween the two metals in the electrochemical series,where the further the distancebetween them, the higher isthe voltage.

VOLTAIC CELL / CHEMICAL CELL
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More electropositive metal.

Metal atom(electrode) releaseselectrons & dissolve to form

metal ions.

Electrons flow through the

external circuit to positive

terminal.

Metal atom becomes metal ion

(becomes thinner)

Less electropositive metal

The electrons that flow from the

external circuit flow through the

positive terminal.

Positive ions in the electrolyte

accept electrons to form neutral

atom/molecule (gas bubbles

releases/ metal deposited)

NEGATIVE TERMINAL (anode)POSITIVE TERMINAL (cathode)

- +

Electrolyte

v


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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 H2Hydrogen ions receive electrons to form hydrogen molecule.

Gas bubbles are released.


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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


16/20


17/20

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

Positivecopper is less electropositive than zinc

(i)Brown solid deposited. (ii) The intensity of blue colour of

copper(II) sulphate decreases.

Cu2+ + 2e Cu


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Metal Displacement Reaction.The metal which is situated at a higher position ( higher tendency torelease 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


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Experiment Observation Explanation/ Inference/Conclusion

-Copper strip

becomes.A. solid

deposited

-The

solution turns blue.

-Copper atom releases electrons toform ..

-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.

Silvernitratesolution

Copperstrip

thinner

grey

colourless

Copper(II) ion

grey silver

silver

silverCopper(II) ion

silver

Cu(NO3)2 + 2Ag

more

above


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Copper(II)

sulphatesolution

Magnesiumstrip

-Magnesium strip

becomes ..

The solid

deposited

-The blue solution

turn

-Magnesium atom releases

electrons to form..

-Thesolid 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

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