Unit 6: Galvanic Cells in Practice

Exploring the real-world applications of electrochemical cells, from disposable batteries to car batteries and fuel cells.

6.18 Primary and Secondary Cells

Galvanic cells, commonly known as batteries, convert chemical energy into electrical energy. As the reactants are consumed, the cell's voltage drops until it is "flat." Based on their ability to be recharged, cells are classified into two types.

Primary Cells (Non-rechargeable)
  • In a primary cell, the electrochemical reaction is irreversible.
  • Once the reactants are used up, the cell cannot be recharged and must be discarded.
  • They are designed for single use and are common in devices like remote controls, torches, and watches.
  • Example: Zinc-carbon dry cells, alkaline batteries.
Secondary Cells (Rechargeable)
  • In a secondary cell, the electrochemical reaction is reversible.
  • The cell can be recharged by applying an external electrical current. This forces the electrons to flow in the opposite direction, reversing the spontaneous reaction and regenerating the original reactants.
  • They are designed for multiple uses and are found in mobile phones, laptops, and cars.
  • Example: Lead-acid batteries, lithium-ion batteries.
Solved Examples:
  1. What is the key difference between a primary and a secondary cell?
    Solution: The reversibility of the chemical reaction. Primary cells are irreversible, while secondary cells are reversible.
  2. Is a car battery a primary or secondary cell?
    Solution: Secondary, because it is recharged by the car's alternator.
  3. What does it mean to "recharge" a battery?
    Solution: It means applying an external voltage to reverse the cell's chemical reaction, restoring the reactants to their original state.
  4. Give an example of a device that uses a primary cell.
    Solution: A television remote control.
  5. Why can't a standard alkaline battery be recharged?
    Solution: The chemical reactions that produce the electricity are not easily reversible under normal conditions.
  6. What happens to the EMF of a battery as it is used?
    Solution: The EMF gradually decreases as the concentration of reactants decreases.
  7. What type of energy conversion occurs in a galvanic cell?
    Solution: Chemical energy is converted into electrical energy.
  8. What type of energy conversion occurs when recharging a secondary cell?
    Solution: Electrical energy is converted back into chemical energy.
  9. Are lithium-ion batteries (used in phones) primary or secondary cells?
    Solution: Secondary, as they are designed to be recharged hundreds of times.
  10. What is a "flat" battery?
    Solution: A battery in which the reactants have been consumed to the point where the cell can no longer produce a useful voltage.

6.19 Examples of Electrochemical Cells

The Daniell Cell

An early, classic example of a galvanic cell consisting of zinc and copper half-cells.

  • Anode (Oxidation): $Zn(s) \rightarrow Zn^{2+}(aq) + 2e^-$
  • Cathode (Reduction): $Cu^{2+}(aq) + 2e^- \rightarrow Cu(s)$
  • Overall: $Zn(s) + Cu^{2+}(aq) \rightarrow Zn^{2+}(aq) + Cu(s)$ (EMF ≈ 1.1 V)

The Lead-Acid Battery (Secondary Cell)

Used in cars, it consists of multiple cells connected in series. The electrolyte is sulphuric acid.

  • Anode (Oxidation): $Pb(s) + SO_4^{2-}(aq) \rightarrow PbSO_4(s) + 2e^-$
  • Cathode (Reduction): $PbO_2(s) + 4H^+(aq) + SO_4^{2-}(aq) + 2e^- \rightarrow PbSO_4(s) + 2H_2O(l)$
  • This reaction is reversed during recharging.

Dry Cells (Primary Cell)

Common disposable batteries. The electrolyte is a moist paste, not a liquid, preventing leakage. A common example is the zinc-carbon cell where a zinc casing acts as the anode and a central carbon rod is the cathode.

Fuel Cells

A special type of galvanic cell where the reactants (a fuel and an oxidant) are continuously supplied from an external source. They do not run down or need recharging as long as fuel is provided.
Example: The Hydrogen-Oxygen Fuel Cell (in alkaline conditions).

  • Anode (Oxidation): $2H_2(g) + 4OH^-(aq) \rightarrow 4H_2O(l) + 4e^-$
  • Cathode (Reduction): $O_2(g) + 2H_2O(l) + 4e^- \rightarrow 4OH^-(aq)$
  • Overall: $2H_2(g) + O_2(g) \rightarrow 2H_2O(l)$
The only product is water, making them a very clean source of energy.

Solved Examples:
  1. What are the two metals used in a Daniell cell?
    Solution: Zinc and copper.
  2. What is the main advantage of a dry cell?
    Solution: It can be used in any orientation without the risk of the electrolyte spilling.
  3. What is the overall product of a hydrogen-oxygen fuel cell?
    Solution: Water ($H_2O$).
  4. In a lead-acid battery, what compound is formed at both the anode and cathode during discharge?
    Solution: Lead(II) sulphate ($PbSO_4$).
  5. How is a fuel cell different from a regular battery?
    Solution: A fuel cell requires a continuous external supply of fuel and oxidant, whereas a regular battery has its reactants sealed inside.
  6. In the Daniell cell, which electrode increases in mass?
    Solution: The copper electrode (cathode), because copper ions are reduced to solid copper.
  7. What is the electrolyte in a car's lead-acid battery?
    Solution: Sulphuric acid ($H_2SO_4$).
  8. What is the fuel in a hydrogen-oxygen fuel cell?
    Solution: Hydrogen gas ($H_2$).
  9. What is the oxidant in a hydrogen-oxygen fuel cell?
    Solution: Oxygen gas ($O_2$).
  10. Why are fuel cells considered a "clean" energy source?
    Solution: Because their primary product is water, with no greenhouse gases or major pollutants produced.

Knowledge Check (20 Questions)

Answer: Primary.

Answer: The zinc electrode.

Answer: Water.

Answer: Secondary.

Answer: Sulphuric acid.

Answer: Its electrolyte is a paste, not a liquid.

Answer: Copper ions are reduced to copper metal ($Cu^{2+} + 2e^- \rightarrow Cu$).

Answer: A fuel (like hydrogen) and an oxidant (like oxygen).

Answer: Reversible.

Answer: It decreases, as it is consumed in the reaction.

Answer: 1.1 Volts.

Answer: The anode (the fuel electrode).

Answer: Primary.

Answer: $Pb(s) + PbO_2(s) + 2H_2SO_4(aq) \rightarrow 2PbSO_4(s) + 2H_2O(l)$.

Answer: Recharging.

Answer: The zinc casing.

Answer: Its only product is water, making it non-polluting.

Answer: It decreases as the zinc is oxidised to zinc ions.

Answer: Electrical energy to chemical energy.

Answer: False. It runs as long as fuel is supplied.
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