What Is a Galvanic Cell?
Redox reactions, in which one species loses electrons while another species gains them, can be divided into half-reactions. In practical application, the species involved can be physically divided into half-cells. These reactions can be demonstrated using a system called an electrochemical cell that either produces or uses electrical energy.
A galvanic cell, also called a voltaic cell, is an electrochemical cell formed when metal strips connected by a wire are immersed in different solutions and the solutions are connected by a salt bridge. Each metal strip in the galvanic cell is an electrode, an electrical conductor through which current enters or leaves an electrochemical cell. The anode is the electrode where the oxidation half-reaction takes place. The cathode is the electrode where the reduction half-reaction takes place. The solutions contain aqueous ions of the two electrodes. The salt bridge is an inert connection between the two half-cells, composed of either a glass tube filled with an inert salt solution or a strip of filter paper soaked in an inert salt solution, that transfers ions between the half-cells. This bridge does not take part in the reaction.As the redox reaction continues, electrons are transferred from one half-cell to the other. This creates a positive charge on one half-cell and a negative charge on the other half cell. The ions in the salt bridge move within the salt bridge to balance out this difference. The salt bridge allows the two half-cells to remain electrically neutral.
Galvanic cell notation is a shorthand method of describing the setup of a galvanic cell. For example, a galvanic cell containing a zinc (Zn) electrode immersed in zinc sulfate (ZnSO4) on the anode side and a copper (Cu) electrode immersed in copper sulfate (CuSO4) on the cathode side is written as in galvanic cell notation. The concentrations of the aqueous solutions affect the rate of the redox reaction and therefore are given in the notation. The vertical bar | in galvanic cell notation represents a phase boundary. It is used instead of the arrow that is commonly used in chemical reactions. The double vertical bars represent the salt bridge of the galvanic cell.
In this notation, the components that make up the cell are written in order, starting with the anode and then moving through each solution to the cathode, resulting in an oxidation half-reaction on the left and a reduction half-reaction on the right. Spectator ions that do not take part in the electrical reaction, such as the sulfate ions in each solution, are not included in this notation.If the species in a galvanic cell is a poor conductor of electricity, an inert electrode—that is, an electrode that does not take part in the chemical reaction but serves only to transfer electrons—may be used. Platinum, for example, is an element that does not take part in the chemical reaction and is commonly used as an inert electrode. Consider a galvanic cell in which a magnesium (Mg) electrode is immersed in a solution of magnesium chloride (MgCl2) and a platinum (Pt) electrode is immersed in hydrochloric acid (HCl). In this galvanic cell, platinum is an inert electrode. Hydrogen ions (H+) still get reduced in the hydrochloric acid (HCl) solution, releasing hydrogen gas (H2). The galvanic cell notation is . Platinum is included in the notation even though it is inert because it acts as a catalyst for the hydrogen reaction.
Galvanic Cell with Inert Electrode
Standard Reduction Potentials
Standard Hydrogen Electrode
Standard Reduction Potential Cell
The Nernst Equation
Cell potential is related to Gibbs free energy (G), which is an indicator of the spontaneity of a reaction. The equation relating cell potential and Gibbs free energy is where n is the number of electrons transferred in the balanced redox reaction; F is the Faraday constant, the charge of one mole of electrons (96,485 C/mol); and E is the cell potential. A positive indicates a nonspontaneous reaction, while a negative indicates a spontaneous one.Standard cell potential is taken under standard conditions, but in real life, conditions are not always standard. The Nernst equation, can be used to find the cell potential under conditions that are not standard.
The electrochemical potential, E, is defined as the decrease in Gibbs free energy per unit of charge transferred. In this equation, is the decrease in Gibbs free energy, F is the Faraday constant, and n is the number of transferred electrons.
The Nernst equation and the equation for Gibbs free energy can be used in calculations of the cell potential when the electrodes are different or when they are the same as in a concentration cell.
Galvanic Cell Applications
Another use of a galvanic cell is a fuel cell. A fuel cell is a device that generates electrical power through the ionization of hydrogen or another molecule. A fuel cell is different than a battery. In a fuel cell, fuel is being continually consumed to produce electricity, and the fuel cell functions as long as fuel is available. Batteries are limited by the mass of electrodes they contain. Fuel cells are used in many industrial applications and have begun to be introduced as power for vehicles.Hydrogen fuel cells are the most common type of fuel cell. In a hydrogen fuel cell, oxygen reacts at the cathode and hydrogen reacts at the anode. The electrons given off when hydrogen is ionized are used as the fuel. In some hydrogen fuel cells, hydrogen travels through the electrolyte to meet the oxygen at the cathode. In others, oxygen travels through the electrolyte to meet the hydrogen at the anode. In both types, when the oxygen and hydrogen meet, they form water, which is the only exhaust given off. Because of this, hydrogen fuel cells are considered clean energy. However, they are not yet in widespread use because they are very expensive and inefficient compared to other methods of electricity generation.