Solutions

Electrochemical Cells

Electrochemical cells are devices that convert chemical energy into electrical energy or vice versa. A classic example is the Daniell cell, which consists of zinc and copper electrodes dipped in their respective salt solutions. The spontaneous redox reaction occurring in the Daniell cell is Zn(s) + Cu²⁺(aq) → Zn²⁺(aq) + Cu(s). This reaction liberates chemical energy that is converted into electrical energy, producing an electrical potential of approximately 1.1 V when the ion concentrations are 1 mol dm⁻³. Such a device is called a galvanic or voltaic cell. When an external voltage opposing the cell potential is applied and gradually increased, the reaction continues until the external voltage equals the cell potential (1.1 V), at which point the reaction stops and no current flows. If the external voltage is increased further, the reaction reverses, and the cell functions as an electrolytic cell, where electrical energy drives a non-spontaneous chemical reaction. Thus, electrochemical cells can be classified into galvanic cells (spontaneous reactions producing electricity) and electrolytic cells (electricity driving non-spontaneous reactions). In the Daniell cell, electrons flow from the zinc electrode (anode) to the copper electrode (cathode) during spontaneous operation. Zinc dissolves at the anode, releasing Zn²⁺ ions, and copper deposits at the cathode. When the external voltage exceeds 1.1 V, the electron flow reverses, zinc deposits at the zinc electrode, and copper dissolves at the copper electrode. This fundamental understanding of electrochemical cells forms the basis for various applications including batteries, electroplating, and industrial electrolysis.

• Electrochemical cells convert chemical energy to electrical energy or vice versa.
• Daniell cell is a classic galvanic cell with zinc and copper electrodes.
• Spontaneous redox reaction in Daniell cell produces ~1.1 V emf at 1 M ion concentration.
• Applying external voltage equal to cell emf stops the reaction; higher voltage reverses it.
• Galvanic cells produce electricity spontaneously; electrolytic cells require external voltage.
• Electron flow direction reverses when external voltage exceeds cell emf.
• 📌 Electrochemical cell: device converting chemical energy to electrical energy or vice versa.
• 📌 Galvanic cell: electrochemical cell with spontaneous redox reaction producing electricity.
• 📌 Electrolytic cell: electrochemical cell where external voltage drives non-spontaneous reaction.

Galvanic Cells

A galvanic cell is an electrochemical cell that converts the chemical energy of a spontaneous redox reaction into electrical energy. The Daniell cell is a prime example where zinc metal is oxidized to Zn²⁺ ions at the anode, and Cu²⁺ ions are reduced to copper metal at the cathode. The overall cell reaction is Zn(s) + Cu²⁺(aq) → Zn²⁺(aq) + Cu(s). This overall reaction is composed of two half-reactions: (i) Reduction at cathode: Cu²⁺ + 2e⁻ → Cu(s) (ii) Oxidation at anode: Zn(s) → Zn²⁺ + 2e⁻ Each half-cell consists of a metal electrode dipped in an electrolyte containing its ions. The two half-cells are connected externally by a wire and internally by a salt bridge, which maintains electrical neutrality by allowing ion flow. At the metal-electrolyte interface, a potential difference called electrode potential develops due to the tendency of metal atoms to ionize and metal ions to deposit. The electrode potential depends on the concentration of ions and the nature of the metal. The standard electrode potential is the electrode potential measured under standard conditions (1 M concentration, 1 bar pressure, 25°C). In a galvanic cell, the electrode where oxidation occurs is called the anode and has a negative electrode potential, while the electrode where reduction occurs is called the cathode and has a positive electrode potential. The cell potential (emf) is the difference between the cathode and anode potentials and drives the flow of electrons through the external circuit.

• Galvanic cells convert spontaneous redox reactions into electrical energy.
• Daniell cell reaction splits into oxidation at zinc anode and reduction at copper cathode.
• Half-cells consist of metal electrodes in electrolyte solutions of their ions.
• Salt bridge maintains electrical neutrality by ion flow between half-cells.
• Electrode potential arises from charge separation at metal-electrolyte interface.
• Cell emf equals cathode potential minus anode potential.
• 📌 Half-cell: part of galvanic cell where either oxidation or reduction occurs.
• 📌 Salt bridge: ionic conductor connecting two half-cells to maintain charge balance.
• 📌 Electrode potential: potential difference between electrode and its electrolyte.