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

電子がワイヤーを流れるとき、またはイオンが溶液を流れるとき、電子は電気を生成します。この電子の流れ、より具体的には電荷は、電流として定義されます。しかし、電子が流れる原因は何ですか?
銅と亜鉛の間の酸化還元反応を考えてみましょう。銅イオン溶液に亜鉛電極を入れると、ある物質から別の物質に電子が移動します。 
電子を失う傾向が強い亜鉛は亜鉛イオンに酸化され、銅イオンは固体銅に還元されます。この反応では、亜鉛から銅に電子が流れますが、この反応では電気は発生しません。 
ここで、反応物である亜鉛と銅が物理的に分離され、ワイヤーなどの外部導体を介して接続されていると考えてください。反応物が電子を獲得または失う傾向は依然として持続し、電子がワイヤーを通って流れるように駆動し、2つの半反応を接続します。 
この電子の流れは電流を構成し、電球などの電化製品に電力を供給することができます。
電流はアンペアで測定されます。1アンペアは、1秒あたり1クーロンの電荷の流れに相当し、1秒あたり6.24×1018電子に相当します。
電流の流れは、滝を流れ落ちる水に似ています。水は重力ポテンシャルエネルギーの差によって駆動され、電子の流れは反応物間の電位エネルギーの差によって駆動されます。 
この電位エネルギーの差は、電位差、起電力、またはセル電位という用語で表されます。セル電位は、2つの反応物間の駆動力と電子移動の傾向の尺度です。
2つの単離された反応物のセル電位は、セル電圧で読み取られる電圧計で測定されます。1ボルトは、電荷1クーロンあたり1ジュールの位置エネルギーに相関します。
セル電位が高いということは、駆動力が大きく、電子移動が容易であることを示しています。最後に、セル電位、または起電力は、反応物の性質、反応温度、および反応中に存在するイオンの濃度に依存します。

起電力

電気は、電子やイオンが溶液や導電性媒体を流れることで生じます。この電子の流れ、具体的には電荷の流れを「電流」と定義します。電線の中を電子が移動すると、電流が発生します。酸化還元反応では、反応の過程で電子の授受を伴う。亜鉛と銅の自発的な酸化還元反応では、亜鉛を銅イオン溶液に浸すと、片方からもう一方への電子移動が起こります。
 <img alt="Eq1" src="/files/ftp_upload/11433/11433_Equation_1.png" style="text-align: center; height: 50px;" />
 電子を失いやすい亜鉛は酸化されて亜鉛イオンとなり、銅イオンは還元されて固体の銅となります。ただし、この反応では電気は発生しません。.
 電流と電子の流れ方
 溶液中では、還元剤から酸化剤へと直接電子が移動します。片方の反応物が物理的に別の容器へと隔離され、電線などの外部導体を介して接続された場合も、反応物による電子の放出や受容の傾向は変わりません。ここで異なるのは、2つの半反応をつなぐ導線の中を電子が流れるという点です。この電子の流れが電流となり、電球などの電子機器を動かしています。電流の単位はアンペアです。1アンペアは、1秒間に1 クーロンの電荷が流れることに相当し、1秒間に6.24×10−18個の電子が流れることに相当します。
 <img alt="Eq2" src="/files/ftp_upload/11433/11433_Equation_2.png" style="text-align: center; height: 72px;" />
 電子は1.602 × 10−19 Cの電荷を持っているため、1アンペアは1秒間に6.242 × 1018 個の電子が流れることに相当します。
 <img alt="Eq3" src="/files/ftp_upload/11433/11433_Equation_3.png" style="text-align: center; height: 50px;" />
 電流・電位差・起電力の駆動力
 電流の流れは、滝を下る水の流れに似ています。水の流れは重力の位置エネルギーの差によって駆動されるが、電子の流れは反応物間の電気的なポテンシャル差によって駆動されます。この電位の差は、電位差、起電力、電池電位のいずれかの用語で表現されます。起電力は、2つの反応体間の駆動力と電子移動の方向を示す指標です。
 酸化還元反応には、自発的に起こるものとそうでないものがあります。例えば、銅線は銀(I)イオンによって自発的に酸化されるが、鉛(II)イオンの溶液に浸しても何の反応も起こりません。これは、Ag+ (aq)とPb2+ (aq)という2つの化学種の銅に対する酸化還元活性の違いによるもので、銀イオンは銅を自発的に酸化しますが、鉛イオンは酸化しません。このような電気化学における酸化還元反応性の違いは、「電極電位」（一般的には「電圧」とも呼ばれる）という言葉で定量化することができます。
 孤立した2つの反応物の電極電位は、電圧計で測定され、電池の電圧として読み取られます。1ボルトは、1クーロンの電荷あたり1ジュールのエネルギーに相当します。
 <img alt="Eq4" src="/files/ftp_upload/11433/11433_Equation_4.png" style="text-align: center; height: 72px;" />
 電極電位が高いほど起電力が大きく、電子の移動が容易であることを示します。このような起電力（電極電位）は、反応物の性質、反応温度、反応に含まれるイオンの濃度などによって変化します。
 上記の文章は以下から引用しました。 <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>.

電気は、溶液または導電性媒体を流れる電子またはイオンのいずれかによって生成されます。この電子の流れ、または特に電気の流れ...

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JoVE Core

Chemistry

化学

Electrochemistry

電気化学

KEY TERMS AND CONCEPTS

主要用語と概念

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

JoVE Core 化学は、簡潔で理解しやすいアニメーションビデオレッスンを使用して、一般的な化学の概念を生き生きとしたものにします。さらに、活動する科学者のビデオでは、現在世界中の実験室で行われている古典的そして独創的な研究・実験をお見せします。

Cell Voltage and Electron Flow

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