Name: using electrode potentials to predict reactions
Uploaded: 2020-09-24T15:06:52.000Z
Duration: 3 min 2 s
Channel: JoVE

请考虑两个有不同液位的容器。连接时，液体从较高的位置 流向较低的位置。通过测量各个液位，可以预测流向。相比之下，在原电池中，了解各个电极电势 有助于预测氧化剂、还原剂和 电子流。这里，每个半电池都有其相应的 电极电势。然而，不能直接测量单个电极的电势，只能测量两个半电池之间的 电势差。那么，是如何计算的呢？为了确定单个电极的电势，指定一个电极的电势为零，另一个电极的电势是 相对于它来测量的。电势为零的电极 是标准状态下的氢电极，也称为标准氢电极或 SHE。SHE 在 25℃下运行，由部分浸没在 1 摩尔盐酸中的惰性铂电极组成，并暴露于 1 个大气压的 氢气流中。当将锌电极连接到 SHE 时，其质量降低，表明其被氧化为 Zn2+离子。同时增加的氢气产量 意味着氢离子的减少。因此，锌是阳极，而 SHE 是阴极。锌的标准电极电位为 0.76 V，表明它的氧化电位高于 SHE。然而，当铜电极连接到 SHE 时，随着 Cu2+还原成铜，它的质量增加。因此，铜是阴极，SHE 是阳极。铜的标准还原电位为 0.34 V，表明它的还原电位高于 SHE。标准电极电位提供了氧化 或还原发生的量度-在标准条件下，正电位越高，还原的趋势越大。氧化还原反应的方向和自发性 是通过检查单个标准电极电位 来确定的。电极电位的总和为正 表示自发反应。因此，对于以锌为阳极、铜为阴极、电池电位为 1.10 V 的 铜-锌原电池，两种半电池反应都是正向 自发发生的。最后，标准电极电位 是一种固有属性，不受半反应化学计量的 任何变化的影响。

标准电极电势

通过比较银和铅的活性，可以观察到两种离子体 AFP+ (AQ) 和 Pb2+ (AQ) 在对铜的氧化还原反应方面有差异：银离子会自动减少，而铅离子不会自动减少。 这种相对氧化还原活动可以通过称为电池电势的属性在电化学细胞中轻松量化。 该属性在电化学中通常称为电池电压，它是电荷转移随附的能量的测量值。 电位是使用 SI 单位电压测量的，该单位电压定义为每充电一个库伦一焦耳能量。 因此， 
 <img alt="Eq1" src="/files/ftp_upload/11435/11435_Equation_1.png" style="text-align: center; height: 70px;" />
 标准电极电势 
 当出于电化学目的进行测量时，电池电势是特定类型电荷转移过程 (即反应物之间的电子转移) 的驱动力度量单位。 无法测量单个电极或单个半电池的电位，因为电子传输分别需要施药者和植入者，或还原剂和氧化剂。 相反，半电池电位只能相对于另一个半电池进行测量。 因此，只能测量两个半电池 (Ecell) 之间的电位差，这两个 Ecell 被定义为 
 <img alt="Eq2" src="/files/ftp_upload/11435/11435_Equation_2.png" style="text-align: center; height: 50px;" />
 其中，阴极和阳极分别代表两种不同的半电池分别用作阴极和阳极的电位。 标准电池电势 (E°) 单元是在两个半电池的标准状态条件下测得的电池电势 (即， 1 M 浓度， 1 bar 压力， 298 K) 
 <img alt="Eq3" src="/files/ftp_upload/11435/11435_Equation_3.png" style="text-align: center; height: 50px;" />
 为了轻松计算半反应电位，科学界指定了一个特定的半电池作为所有电池电势测量的通用参考，电位为 0 V。 该半电池被称为标准氢电极 (SHE) ，它基于以下半反应： 
 <img alt="Eq4" src="/files/ftp_upload/11435/11435_Equation_4.png" style="text-align: center; height: 50px;" />
 通常， SHE 由一个惰性铂电极组成，该电极浸入 1 M 水性 H+ 中，并在 1 bar 压力下产生气泡 H2 气体， 因此，半电池 X 的电极电位 (EX) 被定义为作为阴极的细胞 X 的电位测量值，而 SHE 则作为阳极。 
 <img alt="Eq5" src="/files/ftp_upload/11435/11435_Equation_5.png" style="text-align: center; height: 70px;" />
 因此， 
 <img alt="Eq6" src="/files/ftp_upload/11435/11435_Equation_6.png" style="text-align: center; height: 50px;" />
 在标准状态条件下，半电池 X 的电位等于标准电极电势 E°X 由于电池电势的定义要求半电池作为阴极运行，因此这些电位也称为标准降低电位。 
 预测自发性和氧化还原反应的方向 
 细胞和电极电位决定氧化还原反应的自发性。 据观察，自发反应显示电池电势呈阳性，而非自发过程显示电池电势呈阴性。 如果电极电位的总和为正，则反应被认为是自发的。 正向发生半电池电极电位正极反应，而数值小于氢电极的反应通常以相反顺序发生。 
 一种更强的氧化剂显示出更大的标准电极电势， E°。 随着电极电位测量减量， E° 的增加相当于物种减少和氧化能力提高后驱动力的增加。 因此，当 E°阴极 > E°阳极时， E°单元为正极。 
 考虑到这一点，它解释了为什么铜被银氧化而不是铅氧化： 
 <img alt="Eq7" src="/files/ftp_upload/11435/11435_Equation_7v2.png" style="text-align: center; height: 125px;" />
 预测矿物酸中的金属溶解 
 半电池电位的一个重要应用是了解特定金属在矿物酸中是否会发生溶解。 大多数酸 (如盐酸溶解金属) 通过将质子还原为氢气并将金属氧化至各自的离子。 如果锌与盐酸反应，则反应是自发的，因为锌的标准电极电势低于氢的。 但是，铜由于其电极电位较高，因此不会与盐酸发生反应。 
 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>

在比较银和铅的反应性时，观察到两种离子物质 Ag+ （aq） 和 Pb2+ （aq） 的氧化还原显示出差异......

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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 half-cells. The cell potential, Ecell, is defined from the cathode and anode potentials, and the standard cell potential, E°cell, is taken under standard conditions of 1 M concentration, 1 bar pressure, and 298 K.

To compare half-cells, chemistry uses the standard hydrogen electrode, or SHE, as a zero-volt reference. The SHE uses an inert platinum electrode in 1 M aqueous H+ with hydrogen gas bubbling at 1 bar and 298 K. A half-cell X is measured against the SHE when X acts as the cathode. Under standard conditions, this gives the standard electrode potential, E°X. These values are also called standard reduction potentials because the half-cell is written as a reduction.

The sign of the potential helps show the direction of a redox reaction. A spontaneous reaction has a positive cell potential, while a nonspontaneous one has a negative cell potential. When the sum of the electrode potentials is positive, the reaction is spontaneous. Half-cell reactions with positive electrode potential tend to go forward, and reactions with values below the hydrogen electrode usually go in reverse. A stronger oxidant has a larger standard electrode potential because it has a greater drive to gain electrons.

This idea explains why silver ions and lead ions behave differently toward copper. Silver ion, Ag+ (aq), is reduced spontaneously, but lead ion, Pb2+ (aq), is not. It also helps explain metal dissolution in mineral acids such as hydrochloric acid. These acids dissolve metals by reducing protons to hydrogen gas and oxidizing the metal to its ions. Zinc reacts spontaneously with hydrochloric acid because its standard electrode potential is lower than hydrogen’s, while copper does not react because its electrode potential is higher.

Using Electrode Potentials to Predict Reactions

using-electrode-potentials-to-predict-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...

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

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

Learn how electrode potentials and the standard hydrogen electrode help predict spontaneous redox reactions, metal reactivity, and whether metals dissolve in acid.

Using Electrode Potentials to Predict Reactions | JoVE K12

chemistry

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Using Electrode Potentials to Predict Reactions

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