Name: cell voltage and electron flow
Uploaded: 2020-09-24T15:06:52.000Z
Duration: 2 min 36 s
Channel: JoVE

When electrons flow through a wire, or ions flow through a solution, they generate electricity. This flow of electrons, or, more specifically, electric charge, is defined as an electric current. But what causes the electrons to flow?
Consider a redox reaction between copper and zinc. When a zinc electrode is placed into a copper ion solution, electrons are transferred from one substance to another. 
Zinc, having a greater tendency to lose electrons, is oxidized to zinc ions, while copper ions are reduced to solid copper. In this reaction, electrons flow from zinc to copper, but this reaction does not generate electricity. 
Now, consider that the reactants, zinc and copper, are physically separated and connected via an external conductor such as a wire. The reactants&#8217; tendency to gain or lose electrons still persists, driving the electrons to flow through the wire, which connects the two half-reactions. 
This electron flow constitutes an electrical current and can power electrical appliances such as a lightbulb.
Electrical current is measured in amperes. One ampere equals the flow of one coulomb of electrical charge per second, corresponding to 6.24 &#215; 1018 electrons per second.
The flow of electrical current is similar to water flowing down a waterfall. The water is driven by the difference in gravitational potential energy, while the flow of electrons is driven by the difference of the electrical potential energy between the reactants. 
This difference in electrical potential energy is described by the terms potential difference, electromotive force, or cell potential. The cell potential is a measure of the driving force between two reactants and the tendency for electron transfer.
The cell potential of two isolated reactants is measured with a voltmeter, which is read in cell voltage. One volt correlates to one joule of potential energy per one coulomb of electrical charge.
A high cell potential indicates a large driving force and greater ease of electron transfer. Lastly, the cell potential, or electromotive force, depends on the reactants&#8217; nature, reaction temperature, and the concentration of ions present in the reaction.

electromotive force

Electricity is generated by either electrons or ions flowing through a solution or a conducting medium. This flow of electrons or specifically electrical charge is defined as an electric current. When electrons move through a wire, they generate an electric current. It can be recalled that in a redox reaction, electrons are lost and gained. In the spontaneous redox reaction of zinc with copper, when zinc is immersed in a copper ion solution, a transfer of electrons from one substance to another occurs.
<img alt="Force-body diagram of static equilibrium with &#931;Fx=0 equations, vector arrows, angles, tension analysis." src="/files/ftp_upload/11433/11433_Equation_1.png" style="text-align: center; height: 50px;" data-altupdatedat="1761830439000" data-alt="Eq1" />
Zinc, having a greater tendency to lose electrons, is oxidized to zinc ions, while copper ions are reduced to solid copper. However, this reaction does not generate electricity.
Electrical Current and How Electrons Flow
Electron transfer occurs directly from a reducing agent to an oxidizing agent in a solution. Even if the components of half-reactions are physically isolated in separate vessels and connected via an external conductor such as a wire, the tendency to lose and gain electrons by the reactants still persists. However, now, electrons are forced to flow through the wire connecting the two half-reactions. This electron flow through the wire constitutes an electric current and can power electronic appliances, such as a light-bulb. Electric current is measured in amperes. One ampere is equal to the flow of one coulomb of electrical charge per second and is equal to 6.24 &#215; 10&#8722;18 electrons per second.
<img alt="Ideal gas law formula PV=nRT equation diagram for thermodynamic studies and calculations." src="/files/ftp_upload/11433/11433_Equation_2.png" style="text-align: center; height: 72px;" data-altupdatedat="1761830439000" data-alt="Eq2" />
Since an electron has a charge of 1.602 &#215; 10&#8722;19 C, 1 ampere correlates to the flow of 6.242 &#215; 1018 electrons per second.
<img alt="Michaelis-Menten kinetics equation; enzyme-catalyzed reaction rate; mathematical formula." src="/files/ftp_upload/11433/11433_Equation_3.png" style="text-align: center; height: 50px;" data-altupdatedat="1761830439000" data-alt="Eq3" />
Driving Force for Electrical Current, Potential Difference, and Emf
The flow of electrical current is similar to water flowing down a waterfall. The water is driven by the difference in gravitational potential energy, while the flow of electrons is driven by the difference of the electrical potential energy between the reactants. This difference in electrical potential energy is described either by the terms potential difference, electromotive force (emf), or cell potential. The emf is a measure of the driving force between two reactants and the tendency for electron transfer.
Some redox reactions are spontaneous, while others are not. For example, a copper wire undergoes spontaneous oxidation by silver(I) ions, but fails to yield any reaction when immersed in a solution of lead(II) ions. This is due to the difference in the redox activity of the two species, Ag+ (aq) and Pb2+ (aq), towards copper: the silver ion spontaneously oxidizes copper, but the lead ion does not. This difference in redox reactivity in electrochemistry can be quantified using the term &#8216;cell potential&#8217;; also commonly known as &#8216;voltage&#8217;.
The cell potential of two isolated reactants is measured with a voltmeter, which is read in cell voltage. One volt correlates to one joule of potential energy per one coulomb of electrical charge.
<img alt="Optical-emission diagram of a helium-neon laser showing energy levels and transitions." src="/files/ftp_upload/11433/11433_Equation_4.png" style="text-align: center; height: 72px;" data-altupdatedat="1761830439000" data-alt="Eq4" />
A high cell potential indicates a large driving force and greater ease of electron transfer. Lastly, the electromotive force, or cell potential, depends on the reactants&#8217; nature, the reaction temperature, and the concentration of ions present in the reaction.
This text is adapted from <a href="https://openstax.org/books/chemistry-2e/pages/17-3-electrode-and-cell-potentials">OpenStax, Chemistry 2e, Section 17.3: Electrode and Cell Potentials</a>.

Electromotive Force

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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 reaction does not generate useful electricity.

Electricity is produced when the half-reactions are separated into different vessels and linked by an external wire. The tendency for electrons to move from the reducing agent to the oxidizing agent still remains. But now the electrons must travel through the wire instead of moving directly in solution. That forced movement is an electric current, and it can power devices such as a light bulb.

Electric current is measured in amperes. One ampere means one coulomb of charge moves each second. That is equal to about 6.24 × 10^18 electrons per second. Since one electron has a charge of 1.602 × 10^-19 C, the ampere gives a direct measure of electron flow.

The flow of current depends on a driving force, much like water flowing down a waterfall. Water moves because of a difference in gravitational potential energy. Electrons move because of a difference in electrical potential energy between the reactants. This difference is called potential difference, electromotive force, or cell potential.

Electromotive force, or emf, describes the driving force between two reactants and their tendency to transfer electrons. Some redox reactions are spontaneous, but others are not. For example, copper wire is oxidized spontaneously by silver(I) ions, but it does not react with lead(II) ions. That difference shows that silver ions and lead ions do not have the same redox reactivity toward copper.

Cell potential is another name for this redox driving force, and voltage is a common word for it. A voltmeter is used to measure the cell potential of two isolated reactants. The reading is given in volts. One volt equals one joule of potential energy per coulomb of charge. A high cell potential means a strong driving force and easier electron transfer. The electromotive force also depends on the nature of the reactants, the temperature, and the concentration of ions in the reaction.

Cell Voltage and Electron Flow

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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...

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...

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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...

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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...

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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...

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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 redox reactions create electric current, what cell potential means, and how voltage and emf measure the driving force for electron flow.

Cell Voltage and Electron Flow | JoVE K12

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Cell Voltage and Electron Flow

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