Voltaic Cell Notes

A voltaic cell, left, does not produce current, or electron flow, until its electrodes are externally joined, right.

1. When the cell is not in use, the molecules of the acid in the electrolyte dissociate, or divide, into electrically charged portions called ions. This means the sulfuric acid electrolyte (H₂SO₄) dissociates into two positively charged hydrogen (2H⁺) ions and one negatively charged sulfate ion (SO₄⁼).

2. Once the copper electrode starts drawing electrons through an external connection, a chemical reaction helps to keep the current going. Every zinc atom that loses electrons to the copper electrode becomes a zinc ion (Zn⁺⁺) with a double positive charge. Sulfate ions promptly attract the zinc ions into the solution where they combine to form dissolved zinc sulfate (Zn⁺⁺ + SO₄ ⁼ ZnSO₄).

3. Once the hydrogen ions (2H⁺) are free, they seize electrons at the copper electrode, become normal hydrogen atoms (H), and form bubbles of gaseous hydrogen (H₂). This allows the copper electrode to draw more electrons, which keeps the current flowing

Dry cell (zinc carbon cell):

Positive electrode: centrally located carbon rod

Negative electrode: zinc can

The electrolyte: paste of ammonium chloride, zinc chloride, manganese dioxide and carbon dust

Ammonium chloride paste reacts with zinc to liberate hydrogen

___________Zn+2 NH4CL                           Zn CL2 + 2 NH3 + H2

The hydrogen reacts with manganese dioxide as

__________H2+2 MnO2                              Mn2O3 +H2O

Hydrogen is liberated at faster rate while reaction of MnO2 is slow. Due to this, hydrogen accumulates in the form of thin layer on carbon rod. This is called polarization. Due to this the cell e.m.f goes down if cell is operated for longer time.

Zinc-carbon batteries usually provide 1.4 to 1.7 volts of D.C. electric power that gradually declines to .9 volts during use

Lead acid battery:

Negative plate: Pure lead (Pb) which is grey in colour

Electrolyte: Aqueous solution of sulphuric acid (H2SO4)

Positive plate: lead peroxide PbO2 (dark brown in colour)

During discharging:

When the cell discharges that means it sends current through the external load then H2SO4 is dissociated into positive H2 and negative SO4 ions. As the current is flowing from cathode to anode, H2 ions move to anode and SO4 ions move to the cathode. 

At anode:

Pbo2 + H2 + H2so4_________Pbso4 + 2H2O

At cathode:

Pb+So4_________Pbso4

It will be noted that

1. Both anode and cathode become Pbso4

2. Voltage of the cell decreases

3. The cell gives out energy

During charging:

When the cell is recharged the H2 ions move to cathode and So4 ions go to anode and the following changes takes place

__________At cathode: Pbso4+H2                                     Pb+ H2so4

At anode: Pbso4+So4+2H2o      _________    Pbo2+2H2so4

1. The anode becomes dark brown in colour and cathode becomes grey metallic lead

2. There is a rise in voltage

3. Energy is absorbed by the cell

  

Nickel iron (or) Edison batteries:

Positive plate: - Ni (OH) 4 (nickel hydroxide)

Negative plate: Powdered iron (Fe)

Electrolyte: KOH (potassium hydrate)

During discharging:

Positive plate: Ni (OH) 4 +2K     __________ Ni (OH)2 + 2KOH

Negative plate: Fe + 2OH   _____________   Fe (OH)2

During charging:

Positive plate:  Ni (OH)2 + 2OH  _________ Ni (OH) 4

Negative plate: Fe (OH)2 + 2K   _________ Fe+ 2KOH

Nickel cadmium:

Positive plate: - Ni (OH) 4 (nickel hydroxide)

Negative plate: a mixture of cadmium (Cd)

Electrolyte: KOH (potassium hydrate)

During discharging:

Positive plate: Ni (OH) 4 +2K ___________ Ni (OH)2 + 2KOH

Negative plate: Cd + 2OH ___________ Cd (OH)2

During charging:

Positive plate:    Ni (OH)2 + 2OH__________ Ni (OH) 4

Negative plate:   Cd(OH)2 + 2K  __________ Cd+ 2KOH