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Grade: 11

                        

sir..what determines the discharge potential of an ion..?

6 years ago

Answers : (4)

Rinkoo Gupta
askIITians Faculty
80 Points
							

Electrolysis and Electrolytic cell

Preferential Discharge Theory

If an electrolytic solution consists of more than two ions and the electrolysis is done, it is observed that all the ions are not discharged at the electrodes simultaneously but certain ions are liberated at the electrodes in preference to others. This is explained by preferential discharge theory. It states that if more than one type of ions are attracted towards a particular electrode, then the one discharged is the ion which requires least energy. The potential at which the ion is discharge or deposition potential. The values of discharge potential are different for different ions. For example, the discharge potential of H+ ions is lower than Na+ ions when platinum or most of the other metals* are used as cathodes. Similarly, discharge potential of Cl-ions is lower than that of OH-ions. This can be explained by some examples given below:

(i) Electrolysis of sodium chloride solution:

The solution of sodium chloride besides Na+and Cl-ions possesses H+and OH-ions due to ionization of water. However, the number is small as water is a weak electrolyte. When potential difference is established across the two electrodes, Na+and H+ions move towards cathode and Cl-and OH-ions move towards anode. At cathode H+ions are discharged in preference to Na+ions as the discharge potential of H+ions is lower than Na+ions. Similarly at anode, Cl-ions are discharged in preference to OH-ions.

NaCl <--> Na++ Cl-

H2O <--> H++ OH-

At cathode At Anode

H++ e---> H Cl---> Cl + e-

2H --> H2 2Cl --> Cl2

Thus, Na+and OH-ions remain in solution and the solution when evaporated yields crystals of sodium hydroxide.

(ii) Electrolysis of copper sulphate solution using platinum electrodes:

CuSO4<--> Cu2++ SO42-

H2O <--> H++ OH-

At cathode At Anode

Cu2++ 2e---> Cu 2OH---> H2O + O + 2e-

O + O --> O2

Copper is discharged at cathode as Cu2+ions have lower discharge potential than H+ions. OH-ions are discharged at anode as these have lower discharge potential than ions. Thus, copper is deposited at cathode and oxygen gas is evolved at anode.

(iii) Electrolysis of sodium sulphate solution using inert electrodes:

Na2SO4<--> 2Na++ SO42-

H2O <--> H++ OH-

At cathode At Anode

H++ e---> H 2OH---> H2O + O + 2e-

2H --> H2 O + O --> O2

Hydrogen is discharged at cathode as H+ ions have lower discharge potential than Na+ions. OH-ions are discharged at anode as these have lower discharge potential than ions. Thus, hydrogen is evolved at cathode and oxygen is evolved at anode, i.e., the net reaction describes the electrolysis of water. The ions of Na2SO4conduct the current through the solution and take no part in the overall chemical reaction.

The decreasing order of discharge potential or the increasing order of deposition of some of the ions is given below:

For cations:K+, Na+, Ca2+, Mg2+, Al3+, Zn2+, H+, Cu2+, Hg2+, Ag+

For anions:SO42-, NO3-, OH-, Cl-, Br-, I-

(iv) Electrolysis of copper sulphate solution using copper electrodes:

CuSO4<--> Cu2++ SO42-

At cathode, copper is deposited.

Cu2++ 2e-Cu

At anode, the copper of the electrode is oxidised to Cu2+ions or ions solution dissolve equivalent amount of copper of the anode.

Cu Cu2++ 2e-

or Cu + CuSO4+ 2e-

Thus, during electrolysis, copper is transferred from anode to cathode.

(v) Electrolysis of silver nitrate solution using silver electrodes:

AgNO2<--> Ag++ NO42-

At cathode, silver is deposited.

Ag++ e---> Ag

At anode, the silver of the electrode is oxidised to Ag+ions which go into the solution or ions dissolve equivalent amount of silver of the electrode.

Ag --> Ag++ e-

Ag + NO3---> AgNO3+ e-

Table 12.1 Some more examples of electrolysis

Electrolyte

Electrode

Cathodic reaction

Anodic reaction

Aqueous acidified CuCl2solution

Molten PbBr2

Sodium chloride solution

Silver nitrate solution

Sodium nitrate solution

Pt

Pt

Hg

Pt



Pt

Cu2++ 2e---> Cu

Pb2++ 2e---> Pb

2Na++ 2e---> 2Na


Ag++ e---> Ag



2H++ 2e---> H2

2Cl---> Cl2+ 2e-



2br---> Br2+ 2e-

2Cl---> Cl2+ 2e-


2OH--->
1/2 O2+ H2O + 2e-

2OH--->
1/2 O2+ H2O + 2e-



Thanks & Regards

Rinkoo Gupta

AskIITians Faculty

6 years ago
JAY
13 Points
							
The discharge potential order of Cations  some of which is (Na+>H+>Cu2+>Ag+)
is also dependent on Reducing Potential Value as we move towards right of above order of discharge potential of cations their Reducing Potential Value increase due to which as we move towards right of the a ove order of discharge potential of cations element become more stronge reducing agent and need less energy and time to do reduction and get (electron it need) and get back to its native state or origenal state than other element and reduction potentia values of cations are:-
Cations:-(Na+>H+>Cu2+>Ag+)
Reduction Potential values:-(-2.71
2 years ago
Kushagra Madhukar
askIITians Faculty
605 Points
							
Dear student
The discharge potential of any ion is the potential or voltage required to discharge the ion and mainly it depends on three factors :
 
(a) The relative position of ions in the electrochemical (activity) series.
 
Cations
The Cations will get discharged when they gain electrons (or get reduced), therefore standard reduction potential comes into play and the cations with Higher reduction potential (high tendency to get reduced) will get preferentially discharged and will have lower discharge potential, while those with lower reduction potential (low tendency to get reduced) will  have higher discharge potential.
 Anions
Anions get discharged when they lose electrons, therefore standard oxidation potential comes into play and anions with higher oxidation potential(high tendency to get oxidised) will have lower discharge potential while those with lower oxidation potential(low tendency to get oxidised) will have higher discharge potential
 
(b) The relative concentration of ions.
The ions with higher concentration are more preferably discharged. For example, in concentrated Brine solution, even though OH- is present and has lesser value of Discharge potential, Cl- will get preferably discharged due to its higher concentration
 
(c) The nature of the electrodes.
The electrode may be either inert or can take part in electrochemical reaction in which case it will affect the discharge potential.
 
Hope it helps
Regards,
Kushagra
4 months ago
Yash Chourasiya
askIITians Faculty
246 Points
							Dear Student

The discharge potential of any ion is the mainly depends on three factors :
(i)The relative position of ions in the electrochemical (activity) series
(ii)The relative concentration of ions.
(iii)The nature of the electrodes.

I hope this answer will help you.
Thanks & Regards
Yash Chourasiya
3 months ago
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