Electrochemical cells

1. Definition and Purpose

• Electrochemical cells convert between chemical energy and electrical energy.
• There are two types of electrochemical cells:
  1. Galvanic (Voltaic) Cells – produce electrical energy from spontaneous redox reactions.
  2. Electrolytic Cells – use electrical energy to drive non-spontaneous redox reactions.

2. Galvanic (Voltaic) Cells

• Converts chemical energy into electrical energy.
• Uses a salt bridge to allow ion flow and maintain charge balance.
• Spontaneous redox reaction occurs.
• Exothermic (releases energy).
• Electrode Reactions:
  • Anode (negative electrode): Oxidation occurs.
  • Cathode (positive electrode): Reduction occurs.
• Electromotive force (emf) is always positive.

3. Electrolytic Cells

• Converts electrical energy into chemical energy.
• No salt bridge is needed.
• Non-spontaneous redox reaction occurs.
• Endothermic (requires energy input).
• Electrode Reactions:
  • Anode (positive electrode): Oxidation occurs.
  • Cathode (negative electrode): Reduction occurs.
• Electromotive force (emf) is always negative.

4. Key Differences Between Galvanic and Electrolytic Cells

Feature	Galvanic Cell	Electrolytic Cell
Energy Conversion	Chemical → Electrical	Electrical → Chemical
Reaction Type	Spontaneous	Non-spontaneous
Energy Change	Exothermic	Endothermic
Salt Bridge	Present	Not needed
Anode Charge	Negative	Positive
Cathode Charge	Positive	Negative
Electrons Flow	From anode to cathode	From anode to cathode
Emf Sign	Positive	Negative

5. Key Concepts and Terminology

• Oxidation: Loss of electrons.
• Reduction: Gain of electrons.
• Redox Reaction: A reaction involving both oxidation and reduction.
• Electrode: A conductor where redox reactions occur.
• Anode: Where oxidation occurs.
• Cathode: Where reduction occurs.

6. Example: Electrolysis of Copper(II) Chloride $(CuCl_2)$

• Half-reactions:

  • Anode (Oxidation): $2Cl^-(aq) \rightarrow Cl_2(g) + 2e^-$

  • Cathode (Reduction): $Cu^{2+}(aq) + 2e^- \rightarrow Cu(s)$

• Overall Reaction: $CuCl_2(aq) \rightarrow Cu(s) + Cl_2(g)$

7. Key Takeaways

• Galvanic cells produce electricity, while electrolytic cells require electricity.
• Anode is negative in galvanic cells but positive in electrolytic cells.
• Cathode is positive in galvanic cells but negative in electrolytic cells.
• Electrons always flow from anode to cathode.