Chapter # 08 [Electrochemistry]

 

UNIT# 08

ELECTROCHEMISTRY

Electrochemistry is the branch of the chemistry that deals with the study of relationship between electricity and chemical reaction i.e. the conversion of electrical energy into chemical energy and vice versa.

Q#	Questions	Year
Q# 01	Define Ampere.	[2010][2007][2006]
Q# 02	Define Coulomb.	[2010][2007][2006]
Q# 03	Define Faraday.	[2015] [2007] [2006]
Q# 04	Define Electrolysis.	[2014]
Q# 05	Define Electrolyte.	[2005]
Q# 06	Define Strong electrolytes.	[2005]
Q# 07	Define Weak electrolytes.	[2005]
Q# 08	Define Non electrolytes.	[2005]
Q# 09	Define Electrochemical equivalent.	[2014][2014][2010]
Q# 10	Define Electroplating.	[2018] [2010][2008]
Q# 11	State Faraday’s 1st law of electrolysis.	[2017][2012][2011] [2010] [2009] [2006]
Q# 12	State Faraday’s 2nd law of electrolysis.	[2017][2012][2011] [2009][2010][2007] [2006]
Q# 13	Explain Nickel platting with diagram.	[2018][2010]
Q# 14	Explain Lead storage battery.	[2009][2008]
Q# 15	Explain how would you electroplate an iron spoon with nickel?	[2008]
Q# 16	Explain Dry cell.	[2011][2007][2006]
Q# 17	With the help of labeled diagram, explain the construction of working of Lead storage battery.	[2013]

 

Q. Define Electrolytes and non-electrolytes.

ELECTROLYTES

The chemical compounds which conduct electricity in molten conditions or through the aqueous solution are called electrolyte.

 

FOR INSTANCE

·        HCl

·        H₂SO₄

·        NaOH

·        CuSO₄

NON ELECTROLYTES

The chemical compounds which do not conduct electricity in molten or aqueous solution are called non electrolytes.

Non electrolytes are also called non polar covalent compound.

FOR INSTANCE

·        Sugar [C₁₁H₁₂O₁₁]

·        Benzene [C₆H₆]

·        Petrol

 

Q. What is electrolysis? Give the application of electrolysis.

ELECTROLYSIS [ELECTROLYTIC CONDUCTION]

Movement of ions of an electrolyte towards their respective electrodes and their deposition or liberation as neutral specie under the influence of electric current is called Electrolysis.

 

APPLICATIONS OF ELECTROLYSIS

·        To extract certain metals from their ores

·        For electroplating

 

Q. Describe the electrolysis of molten Sodium chloride.

ELECTROLYSIS OF MOLTEN SODIUM CHLORIDE

Electricity can never passed from solid NaCl. To accomplish this task, it can be possible by forming an aqueous solution. For extraction of sodium from its salt, molten NaCl is required. Molten salt contains Na⁺ ion and chloride ion (Cl^-). Ionization reaction is as follows:

NaCl   ⟶       Na⁺ + Cl^-

By passing electric current, electrolysis starts to begin.

 

 

AT CATHODE (REDUCTION)

Positive charged sodium atom move towards cathode to get neutralize by gaining electron.

Na⁺ + e^-          ⟶       Na

 

AT ANODE (OXIDATION)

Negative charged chlorine ions (Cl^-) move towards anode to lose extra valence e^- as a result neutral chlorine gas is discharged.

Cl^-                   ⟶       Cl + e^-

Cl + Cl            ⟶       Cl₂

2Cl^-                 ⟶       Cl₂ + 2e^-

 

OVERALL REACTION

2 Na⁺ + 2e^-+    ⟶       2Na

2Cl^-                 ⟶       Cl₂ + 2e^-

2 Na⁺ + 2Cl^-   ⟶       2Na + Cl₂

 

Q. Describe the electrolysis of molten Water.

ELECTROLYSIS OF WATER

Pure water doesn’t conduct electricity. Although by adding few drops of acids or bases, it can be made electrolyte. When H₂O is electrolyzed, hydrogen gas it produced at cathode and oxygen at anode.

Let us consider, two electrodes are dipped in acidified water which results in its dissociation as:

2H₂O              ⟶       H₃O⁺ + OH^-

 

AT CATHODE

Positive charged hydronium ion or oxonium ion (H₃O⁺) move towards cathode and gain electrons. Hydrogen gas also liberated out.

2H₃O⁺ + 2e^-   ⟶       2H₂O + H₂

 

AT ANODE

Hydroxide ions (OH^-) move towards anode. OH^- ion lose electrons at anode. Oxygen liberated out.

4OH^- + 4e^-     ⟶                   2H₂O + O₂⬆

 

OVERALL REACTION:

To balance overall reaction, multiply cathode reaction by ‘2’

2[2H₃O⁺ + 2e^-           ⟶                   2H₂O + H₂]

4 H₃O⁺ + 4e^-  ⟶                  4H₂O + 2H₂               . . . eq (i)

 

Add anode reaction in eq (i)

4OH- + 4e-     ⟶                   2H2O + O2⬆

4 H3O+ + 4e- ⟶                   4H2O + 2H2

8H2O              ⟶                   6H2O + 2H2 + O2

6H2O              ⟶                   2H2+  O2

 

Q. State and explain Faraday’s first law of electrolysis.

FARADAY’S 1^st LAW OF ELECTROLYSIS

STATEMENT

“The amount of any substance deposited or liberated at an electrode during the electrolysis is directly proportional to the quantity of current passed through the electrolyte and the time.”

OR

“Mass of an element discharged during electrolysis is directly proportional to the time and magnitude of electric current.”

EXPLANATION

Consider ‘W’ is the amount or weight of a substance which is deposited or liberated when ‘A’ ampere of current is passed for ‘t’ seconds, then;

W α A X t

OR

W = ZAt

Where; ‘Z’ is electrochemical equivalent,

W= mass of element deposited/ liberated

A = amount of electric current

t= time in seconds

 

Q. State and explain Faraday’s second law of electrolysis.

FARADAY’S SECOND LAW OF ELECTROLYSIS

STATEMENT

“The masses of the different substances deposited or liberated when same quantity of current is passed through the different electrolytes, connected in series are directly proportional; to their chemical equivalent masses.’’

 

EXPLANATION

If exactly 96500C of electric charge is passed than the mass of Ag deposited would be equal to the 108 gm that of copper is 31.75gm and Al is 9gm which are their equivalent masses respectively.

 

Equivalent mass of the element = Atomic mass of the element

                                                         Valence of the element

 

Q. Define electric current, ampere, Coulomb, Faraday, Electrochemical equivalent.

ELECTRIC CURRENT

Flow of charges or electrons is called electric current. Its unit is ampere (Amp).

 

AMPERE

Ampere can be defined as the unit of electric current equals to a flow of one coulomb per second.

Mathematically, it can be written as:

COULOMB

Coulomb is the SI unit of electric charge, equal to the quantity of electricity conveyed in one second by a current of one ampere.

1 Coulomb = 1 Amp  x 1 sec

Coulomb = Amp x sec

 

FARADAY

A unit of electric charge equal to Faraday's constant which can be stated as:

1 Faraday = 96500 Coulomb

 

ELECTROCHEMICAL EQUIVALENT

The electrochemical equivalent, sometimes abbreviated Eq or Z, of a chemical element is the mass of that element transported by 1 coulomb of electric charge. The electrochemical equivalent of an element is measured with a volt meter.

OR

The weight of a substance (such as an element) deposited or evolved during electrolysis by the passage of a specified quantity of electricity and usually expressed in grams per coulomb is called Electro chemical equivalent.

Its SI Unit is Kg/ C

Q. What is Electroplatting? Describe the conditions of electroplating.

ELECTROPLATTING

Electroplating is the process which is used for the coating of one metal onto another.

CONDITIONS OF ELECTROPLATING

·        Object to be electroplated should be connected at cathode. It is also called basar element like Fe.

·        Metal which is to be coated on the object is connected with anode. It is also called coat metal like Ag, Au, Ni, Cr etc

Q. What is Nickel plating? Describe the mechanism of nickel plating.

NICKEL PLATING [SPOON PLATTING]

For the purpose of nickel plating, the object which is to be plated is cathode while the pure nickel is anode. NiSO₄ solution is used as electrolyte in electrolytic cell. Both the electrodes joined together with battery. On passing electric current, Ni loss its electrons (Oxidation) and forms Ni⁺². These Ni⁺² gets attraction from cathode, here they gain e^- (Reduction) and forms Ni which deposits on spoon. The description of reaction can be understood by:

ANODE                 Ni                    →        Ni⁺²+2e^-         [Oxidation]
CATHODE                      Ni⁺²+ 2e^-           →          Ni                    [Reduction]

To get the net reaction, simply we add the reaction and get:

Ni      →       Ni                                                          [Overall reaction]

 

Q. What is Chromium plating? Describe the mechanism of Chromium plating.

CHROMIUM PLATING

Chromium metal can be coated over baser metal by electrolysis.

The object which is to be chrome is cathode while the anode is the chromium metal. Chromium sulphate [Cr(SO₄)₃] is used as electrolyte. Both the electrodes joined together with battery.

By passing electric current, chromium losses its electrons and forms Cr⁺³ (Oxidation). These Cr⁺³ get attraction from cathode, here they gain e^- (Reduction) and forms Cr which deposits on the object. Description of reaction can be understand by:

 

ANODE                 Cr                    →        Cr⁺³ + 3e^-       [Oxidation]
CATHODE          Cr⁺³ + 3e^-          →          Cr                    [Reduction]

Cr                    →        Cr                    [Overall]

 

Q. What are electrochemical cells?

ELECTROCHEMICAL CELLS

The device which produces electrical energy (current) through chemical energy is called electrochemical cell.

 

TYPES OF ELECTROCHEMICAL CELLS

·        Galvanic cell / Voltaic cell

·        Dry cell

 

Q. Give the construction and working of Daniell cell.

DANIELL CELL

Simplest of Galvanic cell / voltaic cell which is used to convert chemical energy into electrical energy. It is an example of Galvanic cell or Voltaic cell..

 

CONSTRUCTION
Cell consists of two half cells i.e. Zinc rod dipped in 1 M ZnSO₄ and other is copper dipped in 1M CuSO₄. Two half cell join together to form complete cell. Salt bridge is used as a partition wall. It prevents the mixing of two solutions i.e. ZnSO₄ and CuSO₄. Both the electrodes are connected with the voltmeter. By producing electric current, Zn losses its electron (Oxidation) and forms Zn⁺². Free electrons of Zn electrode move towards this process. ZnSO₄ reduced its size on the other hand Cu grows its size by accepting the free electrons.

 

 

CELL REACTION

ANODE                             Zn        →         Zn⁺² + 2e^-

CATHODE                      Cu⁺² +2e^-    →      Cu

Sum of the reaction can be obtained by:

ANODE                 Zn               →        Zn⁺² + 2e^-

CATHODE        Cu⁺² +2e^-   →        Cu

Zn + Cu⁺²   →        Zn⁺² + Cu

 

 

FUNCTION OF SALT BRIDGE

The function of salt bridge is to prevent the mixing of two solutions and allows the transport of ions freely.

 

GALVANIC CELL

Cell which contains H₂O as solvent is galvanic cell. It is primary as well as irreversible cell.

 

DRY CELL

Cell which contains viscous mixture instead of H₂O as solvent is called dry cell. It is primary cell as well as irreversible cell.

OR

The primary cell which is used to convert chemical energy into electrical energy is called dry cell.

 

USES

It is used in flash lights, calculators, clocks, transistors and in portable electronic devices.

 

NATURE

It is an irreversible cell. It is called as dry cell as there is no free flow of liquid.

 

CONSTRUCTION

There is an outer zinc vessel which acts an anode and carbon (graphite) rod acts as cathode. Graphite rod is surrounded by MnO₂ (manganese di oxide) and carbon powder. Paste of ZnCl₂ (zinc chloride) and NH₄Cl (ammonium chloride is also placed). Concentrated electrolytic solution is thickened into a gel like substance by an agent like starch. The top position of cell is sealed with wax and a copper is fitted on the top of carbon rod to make the electrical current contact. The whole cell is covered by a safety cover.

 

WORKING

When Zinc and graphite electrodes are connected by a metallic wire Zn oxidized to form Zn⁺² ions and the electrons move from electrode to carbon electrode through external circuit. Cell reactions are complex.

 

BATTERY

Battery is a connection of two or more galvanic or voltaic cell. Batteries are secondary as well as reversible cells.

 

Q. Give the construction and working of Lead storage battery.

LEAD STORAGE BATTERY

It is a secondary cell which can be restored to its original condition.

 

USES

It is used in automobiles.

 

NATURE

It is a reversible cell.

 

 

 

CONSTRUCTION

There are several anodes of lead (Pb) alloy and several cathodes of lead oxide (PbO₂) which are connected together in series; about six cells are connected together. Each cell has a potential difference of 2 V and overall voltage of six cells would be 12V. The electrolyte is dilute sulphuric acid (30%).

 

WORKING

As the cell reaction precedes PbSO₄ (lead sulphate) precipitates and partially coats both the electrodes, the water dilutes the sulphuric acid and the battery said to be discharged. By connecting the battery to external circuit source, the electrons flow in opposite direction i.e. the net reaction cell can be reversed and the battery is recharged.