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

当电子流过导线，或离子流过溶液时，就会产生电流。这种电子流动，或者更具体地说，电荷的流动，被定义为电流。但是，是什么导致了电子的流动呢？请考虑铜和锌之间的氧化还原反应。当将锌电极放入铜离子溶液时，电子从一种物质转移到另一种物质。锌更容易失去电子，被氧化为锌离子，而铜离子 被还原为固态铜。在这个反应中，电子从锌流向铜，但是这个反应不会产生电。现在，请考虑反应物锌和铜，它们在物理上是分开的，并通过 外部导体连接，例如导线。反应物获得或失去电子的趋势 仍然存在，从而驱动电子 流过连接两个半反应的 导线。这种电子流构成了电流，并且可以 给电器供电，例如电灯泡。电流的度量以安培为单位。1 安培等于每秒 1 库仑 电荷的流量，相当于 每秒 6.24 10¹⁸ 个电子。电流的流动 类似于流下瀑布的水。水由 重力势能的差驱动，而电子流则由 反应物之间的电势能差 驱动。电势能的这种差异 可以由术语 电势差、电动势或电池电势来描述。电池电势是两种反应物之间的驱动力 以及电子转移趋势的量度。用电压表测量两个孤立反应物的电池电势，它的读数为电池电压。一伏特对应于每一库仑电荷的势能 是一焦耳。高电池电势表明驱动力大，电子转移更容易。最后，电池电势，或电动势，取决于反应物的性质、反应温度 和反应中存在离子的浓度。

电动势

电是由流经溶液或导电介质的电子或离子产生的。 这种电子流或特定的电荷被定义为电流。 当电子通过电线移动时，它们会产生电流。 可以回顾 ，在氧化还原反应中，电子会丢失和获得。 在锌 与铜的自发氧化还原反应中，当锌浸入铜离子溶液中时，电子从一种物质转移到另一种物质。 
 <img alt="Eq1" src="/files/ftp_upload/11433/11433_Equation_1.png" style="text-align: center; height: 50px;" />
 锌具有更大的失去电子的倾向，被氧化成锌离子，而铜离子则被还原为固体铜。 但是，这种反应不会产生电。 
 电流和电子的流动方式 
 电子转移直接从溶液中的还原剂转移到氧化剂。 即使半反应的组成部分在单独的容器中被物理隔离并通过外部导体 (如电线) 连接，反应物丢失和获取电子的趋势仍然存在。 但是，现在，电子被迫流过连接两个半反应的导线。 这种通过电线的电子流构成了电流，可以为电子设备 (如灯泡) 供电。 电流以安培为单位测量。 一安培等于每 秒一个库伦电荷的流量，等于每秒 6.24 × 10−18 个电子。 
 <img alt="Eq2" src="/files/ftp_upload/11433/11433_Equation_2.png" style="text-align: center; height: 72px;" />
 由于电子的电荷为 1.602 × 10−19 C ， 1 安培与每秒 6.242 × 1018 个电子的流量相关。 
 <img alt="Eq3" src="/files/ftp_upload/11433/11433_Equation_3.png" style="text-align: center; height: 50px;" />
 用于电流，电势差和电动势的驱动力 
 电流的流动类似于流过瀑布的水。 引力势能的差异驱动着水，而电子的流动则由反应物之间的电气势能的差异驱动。 “电势差” (Potential Difference) ，“电动势 (EMF) ” (电势差) 或“电池电势” (电势差) 一词描述了电气势能的这一差异。 EMF 是两个反应物和电子传输趋势之间的驱动力的度量单位。 
 有些氧化还原反应是自发的，而另一些反应则不是自发的。 例如，铜线会被银㈠离子自发氧化，但浸入铅㈡离子的溶液时，未能产率任何反应。 这是由于 AA+ (AQ) 和 Pb2+ (AQ) 这两个物种对铜的氧化还原活动不同：银离子会自动氧化铜，但铅离子不会氧化。 电化学中氧化还原反应的这一差异可以使用 ‘电池电势’ (电压) 一词来量化，通常也称为 ‘Voltage ’ (电压)。 
 两个隔离反应物的电池电势由一个电压表测量，电压表以单元电压为单位。 一伏电压与每一个库伦的电荷一焦耳势能相关。 
 <img alt="Eq4" src="/files/ftp_upload/11433/11433_Equation_4.png" style="text-align: center; height: 72px;" />
 高电池电势表示驱动力大且电子传输更容易。 最后，电动势或电池电势取决于反应物的性质，反应温度和反应中存在的离子的浓度。 
 本文改编自<a href="https://openstax.org/books/chemistry-2e/pages/17-3-electrode-and-cell-potentials"> OpenStax, 化学 2e, 第 17.3 节：电极和电池电势</a>。

电力是由流经溶液或导电介质的电子或离子产生的。这种电子流或特别是电流......

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KEY TERMS AND CONCEPTS

关键术语与概念

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 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

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

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...

voltaic/galvanic cells

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...

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 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

chemistry

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Cell Voltage and Electron Flow

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