Important Key Points and Questions & Answer of Electrochemistry

Electrochemistry

Differences between electrochemical cell and electrolytic cell

Electrochemical cell (Galvanic or Voltaic cell)	Electrolytic cell
It is a device which converts chemical energy into electrical energy.	It is a device which converts electrical energy into chemical energy.
It is based upon the redox reaction which is spontaneous. i.e., ΔG = –ve	The redox reaction is non-spontaneous and takes place only when electrical energy is supplied. i.e., ΔG = +ve
Two electrodes are usually set up in two separate beakers.	Both the electrodes are suspended in the solution or melt of the electrolyte in the same beaker.
The electrolytes taken in the two beakers are different.	Only one electrolyte is taken.
The electrodes taken are of different materials.	The electrodes taken may be of the same or different materials.
The electrode on which oxidation takes place is called the anode (or –ve pole) and the electrode on which reduction takes place is called the cathode (or +ve pole)	The electrode which is connected to the –ve terminal of the battery is called the cathode; the cations migrate to it which gain electrons and hence, a reduction takes place, the other electrode is called the anode.
To set up this cell, a salt bridge/porous pot is used.	No salt bridge is used in this case.

Nernst equation : For a reduction reaction,

$\displaystyle {{M}^{{n+1}}}_{{(aq)}}+n{{e}^{-}}\to {{M}_{{\left( s \right)}}}$

$\displaystyle {{E}_{{cell}}}=E_{{cell}}^{0}-\frac{{2.303RT}}{{nF}}\log \frac{1}{{\left[ {M_{{\left( {aq} \right)}}^{{n+}}} \right]}}$

At 298 K

$\displaystyle {{E}_{{cell}}}=E_{{cell}}^{0}-\frac{{0.0591}}{n}\log \frac{1}{{\left[ {M_{{\left( {aq} \right)}}^{{n+}}} \right]}}$

Kohlrausch’s law of independent migration of ions : It states that limiting molar conductivity of an electrolyte can be represented as the sum of the individual contributions of the anion and cation of the electrolyte.

$\displaystyle \Lambda _{m}^{0}={{v}_{+}}\lambda _{+}^{0}+{{v}_{-}}\lambda _{-}^{0}$ ; where $\displaystyle \lambda _{+}^{0}$ and $\displaystyle \lambda _{-}^{0}$ are the

limiting molar conductivities of the cation and anion respectively and $\displaystyle {{v}_{+}}$ and $\displaystyle {{v}_{-}}$ are stoichiometric number of cations and anions respectively in one formula unit of the electrolyte.

Applications of Kohlrausch’s law :

Calculation of molar conductivity of weak electrolytes:

$\displaystyle \Lambda _{m}^{0}\left( {C{{H}_{3}}COOH} \right)=\lambda _{{C{{H}_{3}}CO{{O}^{-}}}}^{0}+\lambda _{{{{H}^{+}}}}^{0}$

Electrolysis : It is the process of decomposition of an electrolyte by passing electricity through its aqueous solution or molten state.

Faraday’s first law of electrolysis : The amount of chemical reaction which occurs at any electrode during electrolysis by a current is proportional to the quantity of electricity passed through the electrolyte (solution or melt).

Faraday’s second law of electrolysis : The amounts of different substances liberated by the same quantity of electricity passing through the electrolytic solution are proportional to their chemical equivalent weights.

Primary cells : Cells once exhausted cannot be used again e.g., dry cell and mercury cell.

Secondary cells : Rechargeable cell which can be used again and again e.g., nickel cadmium cell and lead storage battery.

Fuel cells : Cells which can convert the energy of combustion of fuels such as H₂, CO, CH₄, etc. into electrical energy, e.g., H₂ – O₂ fuel cell.

Corrosion : The slow eating away of metals when exposed to the atmosphere is called corrosion.

Corrosion of iron (Rusting) : It is an electrochemical phenomenon which occurs in the presence of moisture and oxygen.

At anode : $\displaystyle 2F{{e}_{{\left( s \right)}}}\to 2Fe_{{\left( {aq} \right)}}^{{2+}}+4{{e}^{-}}$

At cathode : $\displaystyle {{O}_{2}}+4{{H}^{+}}+4{{e}^{-}}\to 2{{H}_{2}}{{O}_{{\left( l \right)}}}$

Overall reaction : $\displaystyle 2F{{e}_{{\left( s \right)}}}+{{O}_{2}}_{{\left( g \right)}}+4{{H}^{+}}\to 2Fe_{{\left( {aq} \right)}}^{{2+}}+2{{H}_{2}}{{O}_{{\left( l \right)}}}$

An electrochemical cell behave like an electrolytic cell when

(a) E_cell = E_external (b) E_cell = 0

Ans (c) E_external > E_cell

Give two points of differences between electrochemical and electrolytic cells.

Electrochemical cell

Electrolytic cells

It is a device to convert chemical energy into electrical energy, i.e., electrical energy is produced as a result of the redox reaction.

It is a device to convert electrical energy into

chemical energy, i.e., electrical energy is supplied to the electrolytic solution to bring about the redox reaction.

It is based upon the redox reaction which is spontaneous i.e., ΔG = –ve

The redox reaction is non-spontaneous and takes place only when electrical energy is supplied i.e., ΔG = +ve.

What is the necessity to use a salt bridge in a Galvanic cell?

The salt bridge allows the movement of ions from one solution to the other without mixing of the two solutions. Moreover, it helps to maintain the electrical neutrality of the solutions in the two half cells.

Give reason: Conductivity of CH₃COOH decreases on dilution.

Conductivity of CH₃COOH (weak electrolyte) decreases with dilution because the number of current carrying particles i.e., ions present per cm3 of the solution becomes less and less on dilution.

Define limiting molar conductivity. Why conductivity of an electrolyte solution decreases with the decrease in concentration?

The limiting molar conductivity of an electrolyte is defined as its molar conductivity when the concentration of the electrolyte in the solution approaches zero.

Conductivity of an electrolyte decreases with dilution because the number of current carrying particles i.e., ions present per cm3 of the solution becomes less and less on dilution.

State Kohlrausch’s law of independent migration of ions. Write its one application.

If $\displaystyle \lambda _{{N{{a}^{+}}}}^{0}$ and $\displaystyle \lambda _{{C{{l}^{-}}}}^{0}$ are limiting molar conductivities of the sodium and chloride ions respectively then the limiting molar conductivity for sodium chloride is given by

$\displaystyle \Lambda _{{m\left( {NaCl} \right)}}^{0}=\lambda _{{N{{a}^{+}}}}^{0}+\lambda _{{C{{l}^{-}}}}^{0}$

Kohlrausch’s law helps in the calculation of degree of dissociation of weak electrolyte like acetic acid. The degree of dissociation α is given by

α = $\displaystyle \frac{{{{\Lambda }_{m}}}}{{\Lambda _{m}^{0}}}$

Define Molar conductivity ( $\displaystyle {{{\Lambda }_{m}}}$ )

Molar Conductivity : Molar conductivity of a solution at a dilution V is the conductance of all the ions produced from one mole of the electrolyte dissolved in V cm3 of the solution when the electrodes are one cm apart and the area of the electrodes is so large that the whole solution is contained between them.

$\displaystyle {{\Lambda }_{m}}=kV$

It units is S cm²mol^–1

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Important Key Points and Questions & Answer of Electrochemistry

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