Chemical effect of Current

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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

▢ 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
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:

•
•
\(A\) = 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
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
Negative electrode / anode

❖ 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

▢ Alkali Accumulator:

❖ Also Called: Ni-Fe cell / Edison cell

Table 1: Edison Cell Components

Part	Material
Positive electrode / cathode
Negative electrode / anode
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
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
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 grams of copper is deposited in a voltmeter in 30 minutes at 12 volt. What mass is deposited at 6 volt and for time of 45 minutes? [IOM 2014]

📅IOM 2014

Q2.

A charge of 4.8×10³ coul passes through an electrolyte solution of copper sulphate. The number of Cu²⁺ ions liberated from the electrolyte is:

Q3.

What is the amount of charge required to convert 0.2 moles of Cu²⁺ to Cu?

Q4.

A Leclanché cell supplies a current of 1 Amp for 1 hour. Atomic weight of Mn = 55, of oxygen = 16, of zinc = 65 and ECE of hydrogen = 1.04×10^-8 g/coul. Then the mass of hydrogen liberated is

Q5.

In the above question, the mass of zinc consumed is

Q6.

The ECE of silver is 1.118×10^-6 kg/coulomb. Its atomic weight is 108 and Avogadro's number is 6.02×10²³ per gm mole. The charge on one Ag⁺ ion is

Q7.

A certain charge liberates 0.8 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 g of copper?

Q9.

When 1 kg of hydrogen forms water, 34×10⁷ calories of heat is liberated. If ECE of hydrogen is 96500000 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 grams 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 kg of silver is liberated then the amount of zinc liberated will be very near to

Q12.

A current of 10A, deposits 10.8 gram of silver in 900s. The mass of the copper deposited by 9A of current in 1200s 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 cm². Given that the density of copper is 8.9 g/cc and electrochemical equivalent of copper is 0.0003295 g/C, the thickness of copper deposited when 1A of current is allowed to pass through for 100 minutes is

Q15.

When a copper voltameter is connected with a battery of emf 12V, 2g of copper is deposited in 30 minutes. If the same voltameter is connected across a 6V battery, the mass of copper deposited in 45 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 mg then mass of Cu deposited in mg will be [MOE 2008].

📅MOE 2008

Q17.

The faradays constant is 9.65×10⁷ amp/sec/kg. When fused sodium chloride is electrolysed, the amount of sodium deposited in negative electrode when 16 ampere current is passed in 10 minute. [IOM 2017]

📅IOM 2017

40Chemical effect of Current

Table 1: Electrolysis Terms

Table 1: First Law Formulae

Table 1: ECE Values

Table 1: Second Law Formulae

Table 1: Faraday Constant Formulae

Table 1: Charge Required for 1 Gram Equivalent

Table 1: Electroplating Setup

Table 1: Primary Cells Comparison

Table 1: Lead Acid Accumulator Components

Table 1: Charging Reactions

Table 1: Discharging Reactions

Table 1: Edison Cell Components

Table 1: Important Points

Table 1: Chemical Effect of Current One-Liners

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