Electrochemistry_Notes (Electrolysis+Galvanic +Electronic cell+Faraday's Law of Electrolysis) ~ CHEMISTRY

This is the cover on PDF on notes on Chapter "Electrochemistry". This PDF will be published for download very close to the completion of this chapter. Basically, the notes include each and every part of the NEB syllabus of the chapter.

I strongly recommend going through this chapter in detail so that you can crack each NEB questions. Also, The collection, solution of questions will also be available. Blog on Collection Of Formulae will definitely aid you to do numerical. One can obtain PPT & PDF file on parts of Electrochemistry Chapter at the end of every composition.

ELECTROCHEMISTRY

Introduction:

Electrochemistry is defined as the branch of chemistry that studies chemical and physical processes at which either electricity is consumed or produced.

Types Of Substance:

A) On the basis of a tendency to produce ions in water

On the basis of a tendency to produce ions, Substances are of two types. They are i) Electrolyte and ii) Non-electrolyte.

i) Non-electrolyte

Non- electrolyte is the substance that cannot generate ions in an aqueous medium. For example, Methane, Chloroform, etc

ii) Electrolyte

An electrolyte is a substance that generates ions in an aqueous medium. For example, Acetic acid, HCl, NaCl, KCl, etc. The electrolyte is further classified as a) True electrolyte and b) Potential electrolyte.

a) True electrolyte

The true electrolyte is an electrolyte that exists as ions in a pure state for example NaCl, KCl etc.

b) Potential electrolyte.

The potential electrolyte is an electrolyte that doesn't exist as ions in a pure state, however, produces ions in an aqueous medium. For example, Acetic acid, HCl, Ammonia, etc.

B) On the basis of Electrochemistry

On the basis of Electrochemistry, Substances are of two types namely:

Insulator:

Those substances through which electricity cannot pass are insulators. Or, simply, Insulator are those substances that do not conduct electricity.

Conductor:

Those substances through which electricity can pass are conductors. Or, simply, conductors are those substances that do not conduct electricity. Conductors are of two types. They are:

Metallic conductor:

It is defined as those in which electrons conduct electricity. In this type of Conductor, there are no chemical changes. There is no transfer of matter. On an increase in temperature, conductance decreases in the metallic conductor. This is because the vibration of Kernels takes place and electrons cannot freely move.

Electrolytic conductor:

It is defined as those in which ions conduct electricity. In this type of Conductor there are chemical changes. There is a transfer of matter. On an increase in temperature, conductance increases in an electrolytic conductor because it increases the mobility of ions.

Electrolysis:

Definition

When electrolytes either in solution or in the molten state are connected with Source of electricity, cations migrate to cathode and anion move to the anode. They undergo chemical change and deposit at respective electrodes. This phenomenon is called Electrolysis.

Hence electrolysis can be defined as the process of chemical decomposition of electrolyte when electricity is passed through its solution or in molten state.

Electrolysis is a redox process

When electrolytes are subjected to potential differences cation migrates to the cathode and gets reduced at the cathode. Similarly, anion migrates to the anode and gets an oxidized ad deposit at the anode.

This concludes Electrolysis involves oxidation and reduction simultaneously so it is the redox process.

Experimental Set-Up For Electrolysis

Experimental Set up For electrolysis is shown in figure 1: Electrolysis is carried out in a special apparatus called an electrolytic cell. It consists of containers made of glass or plastic. It contains an electrolytic solution where anode and cathode are placed. These electrodes are connected with the source of electricity (cell). The positive terminal of the cell is connected to an anode and the negative terminal of the cell is connected to the cathode.

Examples Of Electrolysis

Electrolysis of Molten Sodium chloride:

When Sodium chloride is heated, the following transformation occurs.

When Molten Sodium Chloride is connected with a source of electricity. Cation (Na⁺) migrate to cathode gets electron and deposits at the cathode.

Similarly, anion (Cl^-) migrate to anode, loose electron, and deposits at the anode.

Ions involve in a reaction as:

Electrolysis of aqueous Sodium chloride:

In aqueous solution, H₃O⁺ (H₂O.H⁺) and OH^-are also available due to autoionization of water which is:

When Aqueous Sodium Chloride is connected with a source of electricity. Both Na⁺and H⁺migrate to cathode, however, H⁺ gets reduced over Na⁺ and hydrogen gas is deposited at the cathode. This is because Hydrogen is less electropositive than sodium. Whereas, anion (Cl^-) migrate to anode, loose electron, and Chlorine gas deposits at the anode.

Ions involve in a reaction as:

Electrolysis of Hydrochloric acid solution

Hydrogen chloride is a potential electrolyte that produces ions H₃O⁺ and Cl^- in aqueous solution. When electricity is passed, cation H₃O⁺ migrates to the cathode, gets reduced and Hydrogen gas is deposited. Whereas, anion Cl^- migrates to the cathode, gets oxidized to Cl_2, and Cl₂ gas is deposited at the anode.

Ions involve in a reaction as:

Electrolysis of Copper Sulphate solution:

Ionization of Aqueous solution of Copper sulphate takes place as:

When Copper Sulphate solution is electrolyzed using the Platinum electrode. Cu⁺⁺ migrate to cathode and Cu metal deposits at cathode due to reduction. Similarly, SO₄^-- along with OH^-migrate to the anode, however, SO₄^--is difficult to oxidize, OH^- is oxidized to Oxygen molecule so oxygen gas gets deposited at the anode.

Ions involve in a reaction as:

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Second Lecture On Electrochemistry:

Factors affecting nature of Product in Electrolysis

Nature of electrolyte
State of electrolyte
Nature of electrode
Standard electrode potential

1. Nature of electrolyte

The nature of electrolyte also affects the nature of the product formation. A strong electrolyte, which almost completely ionizes, electrolysis is rapid due to ready availability of ions, For example, HCl, NaCl, KCl etc

On the other hand, weak electrolyte ionizes feebly causes slow electrolysis and often requires a high voltage to occur. For example HCN, CH₃COOHetc

2. State of electrolyte:

The electrolyte in solid-state is unfavorable for electrolysis as ions are not in the free sate, however in molten and aqueous state favors electrolysis.

Also, the product varies depending on the state of electrolysis. For example electrolysis of NaCl in molten state deposits Na at cathode and Cl₂at the anode.

Reaction involved:

At cathode:

$N{a^ + } + 1{e^ - } \to Na$

At anode:

$C{l^ - } \to \frac{1}{2}C{l_2} + 1{e^ - }$

Overall reaction:

$N{a^ + } + C{l^ - } \to Na + \frac{1}{2}C{l_2}$

Whereas electrolysis of NaCl in aqueous state deposits H₂at cathode and Cl₂ at the anode.

Reaction involved:

At cathode:

${H^ + } + 1{e^ - } \to \frac{1}{2}{H_2}$

At anode:

$C{l^ - } \to \frac{1}{2}C{l_2} + 1{e^ - }$

Overall reaction:

${H^ + } + C{l^ - }\frac{1}{2}{H_2} + \frac{1}{2}C{l_2}$

Nature of electrode:

The nature of the electrode also affects the product in electrolysis.

If electrolysis is carried out in the presence of an inert electrode, which is unattackable by ions in solution, then the nature of the product completely depends on reduction potential Cation with high reduction potential undergoes reduction and one with low reduction potential undergoes oxidation.

While the use of active electrodes shows the polarisation so anion of electrolyte dissolves metal ion into solution. This is the principle behind electroplating.

For example:

Comparision of electrolysis of Copper sulphate solution by using Copper electrode and Platinum electrode

Example 1:

In Electrolysis of Copper sulphate by using Platinum electrode,

Copper deposit at cathode and Oxygen deposits at anode.

Reaction involved:

At cathode:

$C{u^{ + + }} + 2{e^ - } \to Cu$

At anode:

$2O{H^ - } \to {H_2}O + \frac{1}{2}{O_2} + 2{e^ - }$

Overall reaction:

$C{u^{ + + }} + 2O{H^ - }Cu + {H_2}O + \frac{1}{2}{O_2}$

In Electrolysis of Copper sulphate by using Copper electrode,

Copper deposit at the cathode and At anode, $SO{_4^ - ^{}}^ -$ polarizes Cu and dissolve as $CuS{O_4}$ into solution. Reaction involved:

At Cathode:

$C{u^{ + + }} + 2{e^ - } \to Cu$

At anode:

$\begin{array}{l} Cu(s) \to C{u^{ + + }} + 2{e^ - }\\ C{u^{ + + }} + SO{_4^ - ^ - } \to CuS{O_4} \end{array}$

Example 2:

Electrolysis Of Silver nitrate solution using Platinum electrode and silver electrode.

A) In electrolysis of Silver nitrate using Platinium electrode,

silver deposits at cathode and Oxygen gas evolve at anode.

At cathode

$2A{g^ + } + 2{e^ - } \to 2Ag$

At anode

$2O{H^ - } \to {H_2}O + \frac{1}{2}{O_2} + 2{e^ - }$

Overall reaction

$2A{g^ + } + 2O{H^ - }2Ag + {H_2}O + \frac{1}{2}{O_2}$

B) In electrolysis of Silver nitrate using Silver electrode as anode,

silver deposits at cathode and with this nitrate ion polarized Ag metal and silver nitrate goes to solution.

At cathode

$A{g^ + } + 1{e^ - } \to Ag$

At anode

$\begin{array}{l} Ag \to A{g^ + } + 1{e^ - }\\ A{g^ + } + NO_3^ - \to AgN{O_3} \end{array}$

Example:3

Electrolysis Of Sodium chloride solution using A)Platinum electrode and B) mercury electrode.

A) In electrolysis of Sodium chloride solution using Platinium electrode,

Hydrogen gas evolves at cathode and Chlorine gas evolve at anode.

B) In electrolysis of Sodium chloride solution using Mercury electrode as a cathode,

Sodium is reduced and combine with mercury to form sodium amalgum at cathode and Chlorine gas evolves at the anode.

Standard electrode potential:

When different cations and ions are available, the oxidation and reduction depend on Standard Electrode Potential. Cation with high value reduces and deposits at the cathode. White Anion with low-value oxidizes and deposits at the anode.

Fig: Electrochemical series

For example, In electrolysis of Copper sulphate solution, Cu⁺⁺ & H⁺ migrate to the cathode. Cu reduces and deposits at it due to a high reduction potential than hydrogen. Similarly, SO₄^{- -} & OH^-migrate to anode, OH^-oxidises and deposits at it due to low reduction potential than sulphate.

Laws Of Electrolysis

A)Faradays First Law Of Electrolysis

Statement:

Mass of substance deposited at the electrode is directly proportional to the quantity of charge passed through the electrolytic solution.

Mathematically,

$m \propto q.........................(i)$

Where,

m=mass of a substance deposited

q=quantity of charge

Illustration:

When the quantity of charge "q" is passed into CuSO₄ solution, the mass of copper deposited (m) is directly proportional to the quantity of charge "q". i.e. ${m_{Cu}} \propto q$

Also,

q=It where I = current in Ampere and t= time in the second

Substituting q in equation (i), equation (i) becomes:

$m \propto It$

Where z is proportionality constant called Electro Chemical Equivalent (E.C.E)

When, I=1A and t=1second,

z=m

Thus, Electro-Chemical Equivalent (E.C.E) is defined as the mass of substance deposited when 1-ampere current is passed to electrolytic solution for 1 second, or it is defined as the mass of substance deposited by 1Coloumb charge.

95600C charge deposits 1gram equivalent (E) of any substance.

1C charge deposits E/96500C of any substance.

Therefore, Electrochemical equivalent (E.C.E) is related to Chemical Equivalent as:

$E.C.E(z) = \frac{E}{{96500}}$ ......................................... (iii)

Replacing z in equation (ii)

$m = \frac{{EIt}}{{96500}}$

Also, Atomic mass (A) is related to Equivalent mass (E) as:

$E = \frac{A}{V}$

Replacing E in equation (iii),

$m = \frac{{AIt}}{{96500V}}$ ...........................................................(iv)

Collection Of Formulas for decoding numerical problems on Faraday First Law Of Electrolysis

$\begin{array}{l} m = zq\\ m = zIt\\ m = \frac{{EIt}}{{96500}}\\ Also,\\ m = Desity(\delta ) \times Area \times Thickness \end{array}$

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