Name: galvanic cells and battery reactions
Uploaded: 2020-09-24T15:06:52.000Z
Duration: 2 min 47 s
Channel: JoVE

A spontaneous redox reaction generates electricity, which can power electrical devices. But how does it work?
Consider the redox reaction between copper and a silver nitrate solution. When placed in one vessel, the copper is oxidized to cuprous ions, while the silver ions are reduced to a silver precipitate. Here, the transfer of electrons is direct and does not generate an electric current.
However, when these half-reactions are physically separated and connected via an external circuit, an electric current is observed. This set-up is called an electrochemical cell, further defined as a galvanic or voltaic cell for a spontaneous redox reaction.
A galvanic cell consists of two half-cells containing the half-cell reaction mixtures of a solid metal electrode in its corresponding electrolyte solution. Here, it is copper in aqueous copper nitrate and silver in aqueous silver nitrate.
The separated reactants experience a cell potential, but without an external connection, no reaction or flow of electrons occurs. Once externally connected, the electrons are forced to flow indirectly over the external circuit, enabling the reactions to occur at the respective electrodes.
Per convention, oxidation occurs at the anode bearing a negative charge, and reduction at the cathode bearing a positive charge.
As the transfer of electrons occurs, positive ions are formed in the oxidation half-cell and negative ions in the reduction half-cell. This causes a positive and negative charge buildup preventing further electron flow and must be continuously neutralized for the reaction to progress.
Thus, the two half-cells are linked by a salt bridge: an inverted U-tube containing a paste or gel of an inert electrolyte like sodium nitrate. As the redox reaction progresses, the counter-ions flow into the respective half-cells to ensure charge neutrality without mixing of electrolytes.
Special notations, or cell schematics, are used to describe a galvanic cell.
The oxidation is represented on the left side and reduction on the right. A double vertical line indicates a salt bridge connecting the two half-cells, while single vertical lines indicate interfaces between the component phases.
If two or more components exist in the same phase, they are separated using commas.

voltaic/galvanic cells

Voltaic/Galvanic Cells

Spontaneous Chemical Reactions
Spontaneous redox reactions occur abundantly in nature. The chemical reaction occurring in a disposable AA battery powering our remote controls is one such example of a spontaneous redox reaction. Another example is the immersion of coiled copper wire into an aqueous silver nitrate solution. The reaction shows a gradual, visually impressive color change from colorless to bright blue and the formation of a grey precipitate on the copper wire. In this experiment, the copper undergoes oxidation to form cuprous ions, which renders the solution blue, while the silver ions are reduced to form a silver precipitate on the wire.
The reaction can be summarized as follows:
<img alt="Static equilibrium concept; &#931;Fx=0, &#931;Fy=0 diagram for educational use." src="/files/ftp_upload/11434/11434_Equation_1.png" style="text-align: center; height: 105px;" data-altupdatedat="1754904545000" data-alt="DNA replication diagram; enzyme functions, strand synthesis, helicase, polymerase, ligase actions." />
Here, there is a direct and spontaneous transfer of electrons from the copper wire to the aqueous silver ions.
Electrochemical Cells and Galvanic Cells
Suppose a device contains all the reactants and products of a redox system but prevents physical contact between the reactants. Therefore, the direct transfer of electrons is prevented; instead, it takes place indirectly through an external circuit that contacts the separated reactants. Devices of this type are generally referred to as electrochemical cells.
An electrochemical cell is an instrument in which an electric current is generated by a redox reaction without direct contact between the reacting atoms. The electrochemical cells in which a current is created due to a spontaneous redox reaction are known as galvanic or voltaic cells (named after Luigi Galvani and Alessandro Volta).
Components of a Galvanic Cell 
A galvanic cell based on the spontaneous reaction between copper and silver(I) is depicted in Figure 1. A typical galvanic cell consists of two half-cells, each containing a redox conjugate pair, or couple, of a single reactant. The half-cell on the left contains the Cu(0)/Cu(II) couple in the form of a solid copper foil and an aqueous solution of copper nitrate. The right half-cell contains the Ag(I)/Ag(0) couple as solid silver foil and an aqueous silver nitrate solution. An external circuit is connected to each half-cell at its solid foil, meaning the Cu and Ag foil each function as an electrode. The reaction occurs at the interface between each half-cell reaction mixture and its respective electrode. The anode is the electrode at which oxidation occurs and it carries a negative charge, while the cathode is the electrode at which reduction occurs, and it bears a positive charge.
The two half-cells are connected by a salt bridge, which is an inverted U-tube containing a gel or paste of an inert electrolyte like potassium chloride or ammonium nitrate. The salt bridge helps to keep the reaction mixtures separate while ensuring the reaction&#8217;s charge balance. The spontaneous reaction in this cell produces Cu2+ cations in the anode half-cell and consumes Ag+ ions in the cathode half-cell, resulting in a compensatory flow of inert ions from the salt bridge that maintains charge balance. The salt bridge provides a flow of inert ions, ensuring that the reaction continues by neutralizing the buildup of positive and negative ions at the anode and cathode. Hence, increasing concentrations of Cu2+ in the anode half-cell are balanced by an influx of NO3&#8722; from the salt bridge, while a flow of Na+ into the cathode half-cell compensates for the decreasing Ag+ concentration.
<img alt="Galvanic cell diagram, external circuit, Cu&#178;&#8314;/Ag&#8314; exchange, electrochemical setup, ion flow." src="/files/ftp_upload/11434/11434_Image1.png" style="text-align: center; height: 350px;" data-altupdatedat="1754904545000" data-alt="Electrochemical cell diagram, Cu2+/Ag+ ions, external circuit, NO3-, Na+ ion flow, voltaic process." />
Figure 1: A galvanic cell based on the spontaneous reaction between copper and silver(I) ions.
Representation of a Galvanic Cell using Cell Schematics or Cell Notations
The galvanic cell is represented symbolically using cell schematics or cell notations.
&#8226; In cell schematics, all interfaces between the component phases are represented by vertical parallel lines. 
&#8226; If two or more components exist in the same phase, they are separated in the representation using commas. 
&#8226; The anode is represented at the left of the schematic and the cathode to the right. 
&#8226; A double vertical line indicates a salt bridge or a porous partition between the two half-reactions. 
&#8226; By convention, the schematic begins with the anode and proceeds left-to-right, identifying phases and interfaces encountered within the cell, ending with the cathode.
For example, the above galvanic cell consists of a solid copper anode immersed in an aqueous copper(II) nitrate connected via a salt bridge to an aqueous silver(I) nitrate solution immersed in which is a solid silver cathode. Converting this statement to symbolism following the above guidelines results in the cell schematic:
<img alt="DNA translation process, biological protein synthesis; diagram with codons and amino acids sequence." src="/files/ftp_upload/11434/11434_Equation_2.png" style="text-align: center; height: 48px;" data-altupdatedat="1754904545000" data-alt="Kirchhoff's junction rule &#931;I=0 diagram illustrating current conservation at a circuit node." />
This text is adapted from <a href="https://openstax.org/books/chemistry-2e/pages/17-2-galvanic-cells">Openstax, Chemistry 2e</a><a href="https://openstax.org/books/chemistry-2e/pages/17-2-galvanic-cells">,</a><a href="https://openstax.org/books/chemistry-2e/pages/17-2-galvanic-cells">17.2 Galvanic Cells. </a>

Spontaneous Chemical Reactions
Spontaneous redox reactions occur abundantly in nature. The chemical reaction occurring in a disposable AA battery powering ...

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galvanic cells and battery reactions

Galvanic cells use spontaneous redox reactions to produce electric current. A common example is a disposable AA battery, which powers devices by turning chemical energy into electrical energy. Another example is a copper wire placed in silver nitrate solution, where the solution slowly turns bright blue and a grey solid forms on the wire.

In the copper and silver reaction, copper is oxidized and silver ions are reduced. Copper forms cuprous ions, which give the solution its blue color. At the same time, silver ions gain electrons and become silver metal on the copper wire. This is a direct, spontaneous transfer of electrons from copper to the aqueous silver ions.

An electrochemical cell keeps the reactants and products of a redox reaction in the same device but prevents the reacting atoms from touching. Because of this separation, electrons move indirectly through an external circuit instead of moving by direct contact. When the current comes from a spontaneous redox reaction, the cell is called a galvanic cell or a voltaic cell. These names come from Luigi Galvani and Alessandro Volta.

A galvanic cell has two half-cells, and each half-cell contains one redox couple. In the copper-silver cell, the left half-cell contains the Cu(0)/Cu(II) couple as solid copper foil in copper nitrate solution. The right half-cell contains the Ag(I)/Ag(0) couple as solid silver foil in silver nitrate solution. Each metal foil is an electrode because the external circuit connects to it.

The reaction happens at the boundary between each solution and its electrode. Oxidation occurs at the anode, which carries a negative charge. Reduction occurs at the cathode, which carries a positive charge. The two half-cells are joined by a salt bridge, an inverted U-tube with a gel or paste of an inert electrolyte such as potassium chloride or ammonium nitrate.

The salt bridge keeps the half-reactions separate and helps maintain charge balance. As copper ions build up in the anode half-cell, inert nitrate ions move in to balance the charge. At the cathode, silver ions are used up, so inert sodium ions can move in to compensate. This ion flow helps the cell keep running.

A galvanic cell can also be written as a cell notation, or cell schematic. Vertical lines show phase boundaries, and a double line shows the salt bridge or a porous partition. By convention, the anode is written on the left and the cathode on the right. The notation starts with the anode and moves left to right through each phase and interface until it ends at the cathode, such as in the copper-silver cell described here.

Galvanic Cells and Battery Reactions

balancing redox equations

Redox Reactions and Half-Reaction Balancing

Redox reactions involve electron transfer and changes in oxidation number. In electrochemistry, these reactions connect chemical changes with electrical energy. They also power technologies such as industrial metal processing and rechargeable batteries used in electric vehicles. Oxidation number is a way to track how an atom's electronic environment changes compared with the pure element, where the oxidation number is zero. For a simple ionic compound, the oxidation number matches the ion...

electromotive force

Cell Voltage and Electron Flow

Electric current is the flow of electric charge through a conducting medium or a solution. In wires, that charge is carried by electrons. In some solutions, ions can also move and carry charge. When electrons move through a wire, they produce an electric current. This idea connects to redox reactions, where electrons are lost and gained. In the spontaneous reaction between zinc and copper ions, zinc is oxidized to zinc ions. Copper ions are reduced to solid copper. Even so, this direct...

Galvanic cells use spontaneous redox reactions to produce electric current. A common example is a disposable AA battery, which powers devices by turning chemical energy into electrical energy. Another example is a copper wire placed in silver nitrate solution, where the solution slowly turns bright blue and a grey solid forms on the wire. In the copper and silver reaction, copper is oxidized and silver ions are reduced. Copper forms cuprous ions, which give the solution its blue color. At the...

standard electrode potentials

Using Electrode Potentials to Predict Reactions

Electrode potentials help predict whether a redox reaction will happen on its own. A redox reaction involves electron transfer. In electrochemistry, the measured value is called cell potential or cell voltage. It shows the energy linked to charge transfer and is measured in volts, where 1 volt equals 1 joule per coulomb. A single half-cell cannot be measured by itself because electron transfer needs both a donor and a receiver. For that reason, scientists measure the difference between two...

cell potential and free energy

Redox Cell Potential, ΔG°, and K

Redox cell potential connects chemical energy, free energy change, and the equilibrium constant in an electrochemical cell. In these reactions, chemical energy is converted into electrical energy. That link helps predict whether a reaction is spontaneous and how far it may proceed. Thermodynamics is the study of how heat and other forms of energy are related. In an electrochemical cell, cell potential can also reflect oxidizing strength or reducing strength. A similar idea appears in acid-base...

the nernst equation

Cell Potential Under Changing Conditions

Cell potential changes when a redox reaction does not run at standard conditions. Standard cell potentials connect with standard free energy change, ΔG°, and the equilibrium constant K, but those relationships only hold at 1 M concentration, 298 K, and 1 atmosphere. A zinc(II) and tin(II) redox cell with E°cell = +0.291 V and ΔG° = −56.2 kJ is spontaneous under those standard conditions. As a cell discharges, the reactant concentration changes and the cell potential steadily drops. Many redox...

concentration cells

Cell Potential from Concentration Differences

Concentration cells are voltaic cells that use two almost identical half-cells. The half-cells use the same electrode and the same half-reaction. They differ only in the concentration of one redox species, so the cell potential depends on that concentration difference. In a concentration cell, the Nernst equation can be used to find the cell potential. For the zinc example shown, the calculated cell potential is positive. A positive cell potential means the overall reaction is spontaneous. <br...

batteries and fuel cells

Primary, Rechargeable Batteries and Fuel Cells

Batteries and fuel cells are galvanic cells that supply electrical power for many applications. They range from tiny button batteries in wristwatches to large systems that can provide backup energy to municipal power grids. Some are primary cells, which are single-use and cannot be recharged. Others are secondary cells, which use reversible reactions and can be charged again by an external power source. A common primary battery is the dry cell. It uses zinc as both the container and the anode,...

corrosion

Rusting and Metal Protection

Corrosion is the natural breakdown of metals through electrochemical reactions. It can appear as rust on iron, tarnish on silver, or a blue-green patina on copper. In each case, the metal is oxidized. Some corrosion products can protect the surface, while others keep damaging the metal. Rusting is the harmful type of corrosion seen in iron. It happens when iron is exposed to oxygen and water. A galvanic cell forms on the iron surface, and this drives oxidation at the anodic region and...

electrolysis

Electrolysis in Batteries and Metal Production

Electrolysis uses electrical energy to drive a nonspontaneous redox reaction. In a galvanic cell, a spontaneous redox reaction does electrical work on the surroundings by sending electrons through an external circuit. In electrolysis, an external circuit does work on the redox system by applying a voltage large enough to force the reaction in the opposite direction. This process is used in several real-world settings. Recharging a battery is one example, because an outside power source drives...

key terms and concepts

Explore how galvanic cells turn spontaneous redox reactions into electric current, using battery chemistry, half-cells, electrodes, salt bridges, and cell notation.

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Galvanic Cells and Battery Reactions

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