40Chemical effect of Current

📚
CHEMICAL EFFECT OF CURRENT
▢ Basic Terms:

Table 1: Electrolysis Terms

Term
Meaning
Voltameter
Vessel containing electrodes and electrolyte where electrolysis is carried out
Electrolyte
Solution containing oppositely charged ions
Cation
Positive ion moving towards cathode
Anion
Negative ion moving towards anode
Current in electrolyte
Due to flow of positive and negative ions
▢ Faraday's First Law of Electrolysis:
❖ Statement: Mass of substance liberated or deposited at an electrode is directly proportional to quantity of charge passed through electrolyte

Table 1: First Law Formulae

Quantity
Formula
Mass-charge relation
\(m\propto q\)
Mass deposited
\(m=zq\)
Using current
\(m=zIt\)
Using power and voltage
\(m=z\frac{P}{V}t\)
❖ Electrochemical Equivalent:
◉ Symbol: \(z\)
◉ Definition: Mass of substance liberated or deposited when 1 coulomb charge passes through electrolyte
◉ Also: Mass deposited by 1 ampere current in 1 second
◉ SI Unit: \(kg\ C^{-1}\)
◉ Common Unit: \(g\ C^{-1}\)
❖ Values:

Table 1: ECE Values

Substance
ECE
Hydrogen
\(0.0000105\ gC^{-1}\)
Copper
\(0.0003294\ gC^{-1}\)
▢ Faraday's Second Law of Electrolysis:
❖ Statement: When same charge passes through different electrolytes, masses deposited are proportional to their chemical equivalents

Table 1: Second Law Formulae

Quantity
Formula
Mass-equivalent relation
\(m\propto E\)
Chemical equivalent
\(E=\frac{Atomic\ weight}{Valency}\)
Mass ratio
\(\frac{m_1}{m_2}=\frac{E_1}{E_2}\)
ECE relation
\(z\propto E\)
Combined relation
\(\frac{m_1}{m_2}=\frac{E_1}{E_2}=\frac{z_1}{z_2}\)
▢ Faraday Constant:
❖ Symbol: \(F\)
❖ Definition: Charge required to liberate or deposit one gram equivalent of a substance during electrolysis

Table 1: Faraday Constant Formulae

Quantity
Formula / Value
Faraday constant
\(F=\frac{E}{z}\)
Using charge and mass
\(F=\frac{Eq}{m}\)
If \(m=E\)
\(F=q\)
1 Faraday
\(96500\ C/gram\ equivalent\)
Relation with Avogadro number
\(F=N_Ae\)
Value
\(F=6.022\times10^{23}\times1.6\times10^{-19}=96500\ C\)
ECE relation
\(z=\frac{E}{F}=\frac{E}{96500}\)
❖ Examples:

Table 1: Charge Required for 1 Gram Equivalent

Substance
Mass liberated/deposited by 96500 C
Hydrogen
\(1.008\ g\)
Copper
\(31.5\ g\)
Silver
\(108\ g\)
❖ Important Point: 1 Faraday is the quantity of charge carried by one mole of electrons
▢ Electroplating:
❖ Definition: Process of depositing thin layer of one metal over another metal by electrolysis
❖ Purpose: Cheap metal articles are coated with precious metals like silver or gold to make them attractive and protective

Table 1: Electroplating Setup

Part
Made as
Article to be electroplated
Cathode
Metal to be deposited
Anode
Electrolyte
Soluble salt of depositing metal
❖ Thickness of Deposit:
◉ Formulae:
  • •
    \(\rho=\frac{m}{V}=\frac{m}{Ad}\)
  • •
    \(d=\frac{m}{\rho A}\)
  • •
    \(d=\frac{zIt}{\rho A}\)
◉ Symbols:
  • •
    \(\rho\) = density of deposited material
  • •
    \(A\) = area of deposition
  • •
    \(d\) = thickness of deposited layer
❖ Important Points:
  • •
    Weak current is preferred for electroplating
  • •
    Strong current makes deposit brittle
  • •
    AC cannot be used for electroplating or electrolysis due to change in polarity
▢ Electrochemical Cell:
❖ Definition: Arrangement converting chemical energy into electrical energy due to chemical action
❖ Energy Limitation: Total energy depends on amount of reactants
❖ Types:
  • •
    Primary cell
  • •
    Secondary cell
▢ Primary Cell:
❖ Definition: Cell producing electrical energy at cost of chemical energy
❖ Chemical Reaction: Irreversible
❖ Recharge: Cannot be recharged; chemicals must be replaced after long use
❖ Examples:
  • •
    Voltaic cell
  • •
    Daniel cell
  • •
    Leclanche cell
  • •
    Bunsen cell
  • •
    Fuel cell

Table 1: Primary Cells Comparison

Feature
Daniel cell
Leclanche cell
Dry cell
Positive electrode
Copper vessel
Carbon rod
Carbon rod with brass cap
Negative electrode
Zinc rod
Zinc rod
Zinc vessel
Electrolyte
Dilute \(H_2SO_4\)
\(NH_4Cl\) solution
Paste of \(NH_4Cl\) and saw dust
Depolarizer
\(CuSO_4\) solution
\(MnO_2\)
\(MnO_2\)
▢ Secondary Cell:
❖ Definition: Cell in which electrical energy is first stored as chemical energy during charging and later chemical energy converts to electrical energy during discharge
❖ Chemical Reaction: Reversible
❖ Also Called: Storage cell / accumulator
❖ Examples:
  • •
    Lead acid accumulator
  • •
    Alkali accumulator / Edison cell
❖ Important Point: Secondary cell has low internal resistance, so it gives stronger current than primary cell
▢ Lead Acid Accumulator:

Table 1: Lead Acid Accumulator Components

Part
Material
Electrolyte
20% \(H_2SO_4\)
Cathode / positive electrode
Perforated lead plates coated with \(PbO_2\)
Anode / negative electrode
Perforated lead plates coated with pure lead
Fully charged emf
\(2.2\ V\)
❖ Basic Reaction: \(PbO+H_2SO_4\rightarrow PbSO_4+H_2O\)
❖ During Charging:
◉ Meaning: Charging means storing electrical energy as chemical energy
◉ Electrolyte Change: \(H_2SO_4\) is regenerated
◉ Ionization: \(H_2SO_4\rightarrow2H^+ +SO_4^{2-}\)

Table 1: Charging Reactions

Electrode
Reaction
Positive electrode / cathode
\(PbSO_4+SO_4^{2-}+2H_2O-2e^-\rightarrow PbO_2+H_2SO_4\)
Negative electrode / anode
\(PbSO_4+2H^+ +2e^-\rightarrow Pb+H_2SO_4\)
❖ During Discharging:
◉ Meaning: When cell is connected to external circuit, current is drawn from cell
◉ Electrolyte Change: \(H_2SO_4\) is used
◉ Ionization: \(H_2SO_4\rightarrow2H^+ +SO_4^{2-}\)

Table 1: Discharging Reactions

Electrode
Reaction
Negative electrode / anode
\(Pb+SO_4^{2-}\rightarrow PbSO_4+2e^-\)
Positive electrode / cathode
\(PbO_2+4H^+ +SO_4^{2-}+2e^-\rightarrow PbSO_4+2H_2O\)
▢ Alkali Accumulator:
❖ Also Called: Ni-Fe cell / Edison cell

Table 1: Edison Cell Components

Part
Material
Positive electrode / cathode
Perforated steel plate packed with nickel hydroxide \([Ni(OH)_2]\)
Negative electrode / anode
Steel grid packed with iron hydroxide \([Fe(OH)_2]\) and trace of mercury oxide
Electrolyte
20% KOH in distilled water + 1% LiOH
Mercury oxide function
Lowers internal resistance
LiOH function
Makes electrolyte more conducting
❖ Important Points:
  • •
    Efficiency is lower than acid cell due to higher internal resistance
  • •
    During discharging, emf of Edison cell decreases
▢ Read and Digest:

Table 1: Important Points

Fact
Answer
Current in electrolyte
Due to flow of positive and negative ions
Voltameter
Used to determine electrochemical equivalent \((z)\)
Voltmeter
Used to determine potential difference
Faraday's laws
Followed by electrolytic conductors
Ohm's law
Followed by metallic and electrolytic conductors
Nature of electrolyte
Determines emf between two metals placed in electrolyte
Electroplating current
Weak current preferred
Strong current in electroplating
Deposit becomes brittle
AC in electrolysis/electroplating
Cannot be used due to change in polarity
Secondary cell internal resistance
Low
Single Daniel cell and water decomposition
Cannot decompose water
Reason
Daniel cell emf = 1.1 V, minimum p.d. required = 1.8 V
Fully charged lead acid cell emf
2.2 V
Alkali cell efficiency
Lower than acid cell due to higher internal resistance
During discharging of Edison cell
Emf decreases
1 Faraday
Charge carried by one mole of electrons
Faraday constant
\(F=N_Ae=96500\ C\)
ECE
\(z=\frac{E}{F}=\frac{E}{96500}\)
Charge for 1 gram equivalent
\(96500\ C\)
▢ High-Yield Recall:

Table 1: Chemical Effect of Current One-Liners

Fact
Answer
Voltameter
Electrolysis vessel
Cation
Positive ion moving to cathode
Anion
Negative ion moving to anode
Faraday first law
\(m\propto q\)
Mass deposited
\(m=zq=zIt\)
ECE
Mass deposited by 1 C charge
ECE SI unit
\(kgC^{-1}\)
Faraday second law
\(m\propto E\)
Chemical equivalent
\(E=\frac{Atomic\ weight}{Valency}\)
Second law ratio
\(\frac{m_1}{m_2}=\frac{E_1}{E_2}=\frac{z_1}{z_2}\)
Faraday constant
\(F=\frac{E}{z}\)
1 Faraday
\(96500\ C\)
Faraday constant relation
\(F=N_Ae\)
ECE relation
\(z=\frac{E}{96500}\)
Electroplating
Thin metal layer deposition by electrolysis
Electroplated article
Cathode
Depositing metal
Anode
Thickness of deposit
\(d=\frac{zIt}{\rho A}\)
Electrochemical cell
Chemical energy → electrical energy
Primary cell reaction
Irreversible
Secondary cell reaction
Reversible
Secondary cell
Storage cell / accumulator
Lead acid electrolyte
20% \(H_2SO_4\)
Lead acid cathode
\(PbO_2\)
Lead acid anode
Pb
Lead acid charged emf
2.2 V
Edison cell positive electrode
\(Ni(OH)_2\)
Edison cell negative electrode
\(Fe(OH)_2\)
Edison cell electrolyte
KOH + LiOH
Daniel cell emf
1.1 V
Water decomposition p.d.
1.8 V
AC electrolysis
Not used due to polarity reversal
Q1.
\(2\ \mathrm{g}\) of copper is deposited in a voltmeter in \(30\ \mathrm{minutes}\) at \(12\ \mathrm{volt}\). What mass is deposited at \(6\ \mathrm{volt}\) and for time of \(45\ \mathrm{minutes}\)? [IOM 2014]
📅IOM 2014
Q2.
A charge of \(4.8\times10^3\ \mathrm{C}\) passes through an electrolyte solution of copper sulphate. The number of \(\mathrm{Cu}^{2+}\) ions liberated from the electrolyte is:
Q3.
What is the amount of charge required to convert \(0.2\ \mathrm{moles}\) of \(\mathrm{Cu}^{2+}\) to \(\mathrm{Cu}\)?
Q4.
A Leclanché cell supplies a current of \(1\ \mathrm{A}\) for \(1\ \mathrm{hour}\). Atomic weight of Mn = \(55\), of oxygen = \(16\), of zinc = \(65\) and ECE of hydrogen = \(1.04\times10^{-8}\ \mathrm{g/C}\). Then the mass of hydrogen liberated is
Q5.
In the above question, the mass of zinc consumed is
Q6.
The ECE of silver is \(1.118\times10^{-6}\ \mathrm{kg/C}\). Its atomic weight is \(108\) and Avogadro’s number is \(6.02\times10^{23}\ \mathrm{mol}^{-1}\). The charge on one \(\mathrm{Ag}^{+}\) ion is
Q7.
A certain charge liberates \(0.8\ \mathrm{g}\) of oxygen. The mass of silver liberated by the equal charge is
Q8.
What is the volume of hydrogen liberated at NTP by the amount of charge which liberates \(0.3175\ \mathrm{g}\) of copper?
Q9.
When \(1\ \mathrm{kg}\) of hydrogen forms water, \(34\times10^7\ \mathrm{cal}\) of heat is liberated. If ECE of hydrogen is \(\frac{1}{96500000}\ \mathrm{kg/C}\), then the minimum voltage required for decomposition of water is
Q10.
The commercial aluminium (At wt. = \(27\)) is generally obtained by electrolysis. What is the total charge required to deposit \(9\ \mathrm{g}\) of aluminium?
Q11.
Silver and Zinc voltameters are connected in series and same current is passed through both of them for same time. If \(2x\ \mathrm{kg}\) of silver is liberated then the amount of zinc liberated will be very near to
Q12.
A current of \(10\ \mathrm{A}\), deposits \(10.8\ \mathrm{g}\) of silver in \(900\ \mathrm{s}\). The mass of the copper deposited by \(9\ \mathrm{A}\) of current in \(1200\ \mathrm{s}\) will be [\(E_{Cu}=31.5\) and \(E_{Ag}=108\)].
Q13.
Two electroplating cells, one of silver and another of aluminium are connected in series. The ratio of number of silver atoms to that of aluminium atoms deposited during time \(t\) will be
Q14.
It is required to have a deposition of copper on surface of a plate having a total area of \(250\ \mathrm{cm}^2\). Given that the density of copper is \(8.9\ \mathrm{g/cm}^3\) and electrochemical equivalent of copper is \(0.0003295\ \mathrm{g/C}\), the thickness of copper deposited when \(1\ \mathrm{A}\) of current is allowed to pass through for \(100\ \mathrm{minutes}\) is
Q15.
When a copper voltameter is connected with a battery of emf \(12\ \mathrm{V}\), \(2\ \mathrm{g}\) of copper is deposited in \(30\ \mathrm{minutes}\). If the same voltameter is connected across a \(6\ \mathrm{V}\) battery, the mass of copper deposited in \(45\ \mathrm{minutes}\) would be
Q16.
If Cu and Ag voltameter are connected in series and same amount of current is passed for same time when mass of Ag deposited is \(x\ \mathrm{mg}\) then mass of Cu deposited in mg will be [MOE 2008].
📅MOE 2008
Q17.
The faradays constant is \(9.65\times10^7\ \mathrm{A\,s/kg\,equivalent}\). When fused sodium chloride is electrolysed, the amount of sodium deposited in negative electrode when \(16\ \mathrm{A}\) current is passed in \(10\ \mathrm{minutes}\). [IOM 2017]
📅IOM 2017