Chemical Effects of Current

 Definition

Current can produce or speed up chemical change, this ability of current is called chemical effect.

When current is passed through an electrolyte, it dissociates into positive and negative ions. This is called the chemical effect of current.

$\textbf{Some Terms}$

$\to$ $\textbf{Electrolysis}$

Electrolysis is a process by which electric current is passed through a substance to effect a chemical change.

$\to$ $\textbf{Electrolyte}$

The liquids which allow the current to pass through them and also dissociates into ions on passing current through them are called electrolytes. E.g. solutions of salts, acids and bases in water, etc.

$\to$ $\textbf{Electrodes}$

Two metal rods or plates which are partially dipped in the electrolyte for passing the current through the electrolyte.

        Anode: Connected to positive terminal of battery  

        Cathode: Connected to negative terminal of battery  

$\to$ $\textbf{Ionisation}$

The process of decomposition of a compound into its constituent ions is called ionization.

$\to$ Anions

The negatively charged ions which move towards the anode during electrolysis is called anions.

$\to$ Cations

The positively charged ions which move towards the cathode during electrolysis is called cations.

REMEMBER: $A \rightarrow A$ (i.e. anions to anode): $C \rightarrow C$ (i.e. cations to cathode)

$\to$ Chemical equivalent

The ratio of the atomic weight of an element to its valency is defined as its equivalent weight.

$\to$ $\textbf{Voltameter}$

The vessel in which the electrolysis is carried out is called a voltmeter. It is also known as an electrolytic cell.

$\textbf{Theory of Electrolysis}$

$\to$ Arrhenius explained the process of electrolysis by his theory of ionic dissociation ($1887$).

$\to$ When an electrolyte is dissolved in a liquid then some molecules of electrolyte dissociate into oppositely charged ions.

$\to$ When no current is passed through the solution the ions move randomly and the solution is electrically neutral.

$\to$ When an electric current is passed then anions and cations move towards their respective electrodes under influence of potential difference.

$\to$ On reaching electrodes the ions get discharged (becomes neutral) and appear as gas molecules or are deposited as thin layers on the electrode.

$\to$ The electrolytes conductivity is very low $10^{-6}$ times) than that of a good conductor because ions are heavier than electrons.

Common Types of Voltameters

Copper Voltameter

$\to$ It consists of $CuSO_4$ as electrolyte and two $Cu$ plates which work as electrodes.

$\to$ Reaction : $CuSO_4$ ionises in its aqueous solution as

$CuSO_4↔Cu^{++}+SO_4^—$

At the cathode:

$Cu^{++}+2e^- \rightarrow Cu$

The copper atoms are deposited on cathode.

At the anode:

$Cu\rightarrow Cu^{++}+2e^-$

$\to$ $Cu$ is lost from anode and deposited on cathode.

$\to$ The $Cu^{++}$ and $SO_4^{--}$ ions carry current from anode to cathode in the electrolyte. In external circuit the current is due to electrons.

$\to$ The concentration of $CuSO_4$ remains constant.

Silver Voltameter

$\to$ It consists of $AgNO_3$ as electrolyte and two $Ag$ plates which work as electrodes.

$\to$ Reaction : $AgNO_3$ ionises in its aqueous solution as

$AgNO_3↔Ag^+ + NO_3^+$

At the cathode:

$Ag^+ + e^- \rightarrow Ag$

	The silver atoms are deposited on cathode.

At the anode:

$Ag \rightarrow Ag^+ + e^-$

$\to$ Ag is lost from anode and deposited on cathode.

$\to$ The $Ag^+$ and $NO_3^-$ ions carry current from anode to cathode in the electrolyte. In external circuit the current is due to electrons.

$\to$ The concentration of $AgNO_3$remains constant.

$\textbf{Important Points}$

$\to$ Back emf(polarization) : The emf set up in water voltameter which opposes the external dc supply to the voltameter.

$\to$ Electrolysis is possible for dc and low frequency AC as at high frequency due to inertia ions cannot follow frequency of ac.

$\to$ In electrolysis electrical energy is converted to chemical energy.

$\to$ In electrolyte the current is due to directed motion of ions. Current due to positive and negative ions are not equal due to different mobilities.

$\to$ Insoluble electrode voltameters mass of cathode increases while that of anode decreases and concentration of electrolyte remains constant.

$\to$ The conductivity of electrolytes increases with rise in temperature.

$\to$ Mercury is a liquid which conducts electricity but does not dissociate into ions.

Faraday’s Law of Electrolysis

Published by Michael Faraday in $1834$ .

Gives the quantitative (mathematical) relationships that describe the above electrolysis.

$\textbf{First Law of Faraday}$

$\to$ It states that the mass of substance deposited or liberated at the electrode during electrolysis is directly proportional to the quantity of electricity (total charge) passed through the electrolyte.

i.e. m ∝q

or, $m= Zq=ZIt$ where,

Z is the electrochemical equivalent (ECE) of substance.

$\to$ If $q =1$ coulomb, then we have m = z \times 1 or $z = m$ . Hence, the electrochemical equivalent of substance may be defined as the mass of its substance deposited or liberated at the electrode, when one coulomb of charge passes through the electrolyte.

$\to$ S.I. unit ECE is $kilogram coulomb^{-1} (kg-C^{–1})$ but generally expressed in $ram coulomb^{-1} (g-C^{–1})$.

$\to$ Dimension of ECE is $M^1 A^{-1} T^{-1}$.

$\textbf{Second Law of Faraday}$

$\to$ If the same quantity of electricity is passed through different electrolytes, masses of the substance deposited at the respective cathodes are directly proportional to their chemical equivalents.

i.e. $m ∝E$

or,$\dfrac{m}{E}=constant$

$\textbf{Important Points}$

$\to$ $E=FZ$

Where F is faraday’s constant. Faraday is charge of $1 g$ equivalent ions.

If $p$ is the valency and $N$ is Avogadro’s number then,

$F=Charge in an ion \times no.of ions in 1g equivalent=(p \times) \times (\dfrac{N}{p}=Ne$.

or,$F=6.023×10^23×1.6 \tiimes10^{-19} ≈ 96500 Cmol^{-1}$

NOTE: $96500C$ charge is required to liberate or deposit $1.008g$ of hydrogen or $31.5g$ of Cu or $108g$ of silver on cathode during electrolysis.

$\to$ In general Faraday’s law can be written as $m=Zq=\dfrac{E}{q} Q=E(\dfrac{Q}{F})=\dfrac{M}{p}.\dfrac{Q}{F}=\dfrac{atomic mass}{valency} \times charge in faraday$