Name: redox reactions and half-reaction balancing
Uploaded: 2020-09-24T15:06:52.000Z
Duration: 2 min 58 s
Channel: JoVE

K12 Standards

north-carolina-k-12-science-standard-course-of-study

NC K-12 Science Standard Course of Study

9-12

Physical Science

Matter and its Interactions

PS.PSc.2.3

Balancing Redox Equations

バッテリーは、ポータブルデバイスを動作させるための電力をどのように供給しますか?カットしたアボカドが茶色に変わったり、水中で金属が錆びたりするのはなぜですか? 
これらのプロセスは、ある原子から別の原子への電子移動を含む特定の種類の化学反応によって駆動されます。電子を失った原子は酸化され、電子を獲得した原子は減少します。これらの反応は酸化還元反応または酸化還元反応と呼ばれ、1つ以上の反応物の酸化状態の変化によって特徴付けられます。 
マグネシウムと濃塩酸との相互作用は、酸化還元反応の一例です。ここでは、マグネシウムがマグネシウム2+イオンに酸化され、プロトンが水素ガスに還元されます。
単純な化学式で簡単にバランスをとることができます。ただし、ほとんどの酸化還元方程式は複雑であるため、失われる電子と得られる電子の数を正当化する必要があります。
酸化還元方程式のバランスをとるときは、質量保存の法則を守ることが不可欠です。各元素の量と電子の利得または損失は、反応の両側でバランスが取れている必要があります。したがって、1つの反応物が還元されると、別の反応物は酸化されなければなりません。
半反応法は、酸化還元方程式のバランスをうまく取るために使用されます。
過マンガン酸塩とシュウ酸塩との反応を考えてみましょう。酸性水溶液の場合は、酸化状態を割り当て、反応をその成分半反応に分割します。
次に、水素と酸素を省略して各半反応のバランスを取ります。ここでは、マンガンはすでにバランスが取れていますが、シュウ酸塩には2の係数が必要です。第二に、水分子を追加して酸素原子のバランスを取ります。そして第三に、必要に応じて陽子を追加することで水素原子のバランスを取ります。
電子を追加して電荷のバランスを取ります。マンガンが還元されるため、反応物側に電子が5個追加されます。逆に、シュウ酸塩は酸化されます。したがって、製品側に2つの電子が追加されます。半反応に整数を掛けて、追加する電子の数が等しくなります。
最後に、バランスの取れた酸化還元反応を得るために、両側の種を打ち消すことにより、バランスの取れた半反応を追加して単純化します。
塩基性水溶液の場合、手順は似ていますが、1つの追加ステップが含まれます。ここでは、反応の両側に同数のヒドロキシルイオンを添加してプロトンを中和し、バランスの取れた半反応を合計します。

酸化還元反応の平衡

電気化学とは、電気反応と化学反応の相互変換に関する科学です。このような反応は、「還元-酸化」または「酸化還元反応」と呼ばれます。これらの反応は、1つまたは複数の反応要素の酸化状態の変化によって定義され、反応種間の電子の移動を伴う半反応(の部分集合)も含まれます。電気化学は、酸化還元化学の基本原理や、工業規模の冶金プロセスから電気自動車用の堅牢な二次電池まで、さまざまな技術について十分な知見が得られる分野として発展してきた。電子の移動を伴う反応は電気化学の根幹であり、酸化還元化学を理解するためのいくつかの要素を以下で述べます。
 酸化数
 酸化還元反応とは、1つまたは複数の反応物の酸化状態が変化する反応として定義されます。元素の酸化数とは、純粋な元素の原子と比較して、その原子の電子環境がどのように異なるかを評価したものです。この定義によれば、元素中の原子の酸化数は0です。原子の酸化数は、その化合物がイオンである場合の原子の電荷に等しいです。したがって、分子内のすべての原子の酸化数の合計は、その分子の電荷と同じになります。
 イオン性化合物
 単純なイオン性化合物は、元素がイオン電荷と等しい酸化数を持つため、形式酸化数を考える上で最も単純な例です。塩化ナトリウム（NaCl）は、Na+の陽イオンとCl−の陰イオンで構成され、ナトリウムと塩素の酸化数はそれぞれ+1と −1です。フッ化カルシウム（CaF2）は、Ca2+陽イオンとF−陰イオンからなり、カルシウムとフッ素の酸化数は、それぞれ+2と −1です。
 共有結合性化合物
 共有結合を持つ化合物は、形式酸化数を考える上でより困難です。水は、2つの水素原子と1つの酸素原子が極性共有結合で結合した共有結合化合物です。O−H結合を作っている共有電子は、より電気陰性のO原子に強く引き付けられます。そのため、酸素は単体の酸素に比べて部分的に負の電荷を帯びています。その結果、水分子中の水素原子は、水素ガス中の水素原子に比べて部分的にプラスの電荷を持つことになります。水分子の部分的な負電荷と正電荷の合計はゼロとなり、水分子は中性となります。
 もし、水のO−H結合内における共有電子の分極が非常に極端であると仮定すると、HからOへと電子が完全に移動し、水はO2−陰イオンとH+陽イオンからなるイオン化合物となります。そして、水の中の酸素と水素の酸化数は、それぞれ −2 と +1 です。これと同じ理屈を四塩化炭素（CCl4）に当てはめると、炭素の酸化数は+4、塩素の酸化数は −1となります。硝酸イオン（NO3−）では、窒素の酸化数は+5、酸素の酸化数は−2で、合計すると分子の電荷は−1になります。
 <img alt="Eq1" src="/files/ftp_upload/11432/11432_Equation_1.png" style="text-align: center; height: 70px;" />
 酸化還元反応における係数の決定
 以下の原子数が釣り合わない式は、塩化ナトリウムの分解を表しています。
 <img alt="Eq2" src="/files/ftp_upload/11432/11432_Equation_2.png" style="text-align: center; height: 48px;" />
 この反応は、Naの酸化数が+1から0に減少し（還元が進行）、Clの酸化数が −1から0に増加する（酸化が進行）ことから、酸化還元反応の定義に合致しています。この方程式の場合は、NaClとNaの化学量論的係数2を加えることで簡単に原子数のつり合いが取れます。
 <img alt="Eq3" src="/files/ftp_upload/11432/11432_Equation_3.png" style="text-align: center; height: 48px;" />
 水溶液中で起こる酸化還元反応は、電気化学の分野ではよく見られる現象であり、多くは水やそのイオンであるH+ (aq)やOH−(aq)が反応物や生成物として関与しています。
 このような場合、酸化還元反応を表す方程式を見ただけで両辺の原子数を釣り合わせることが非常に困難であり、半反応法と呼ばれる体系的なアプローチが有用です。この方法は次のようなステップで進めます。
 <ol>
 <li>この式を、構成している酸化および還元の半反応にそれぞれ分割します。</li>
 <li>OとH以外のすべての元素について、各半反応の原子数を釣り合わせます。</li>
 <li>式の中で必要に応じて水分子を加え、O原子が含む各半反応の原子数を釣り合わせます。</li>
 <li>式に必要な水素イオンを加えて、H原子を含む各半反応の原子数を釣り合わせます。</li>
 <li>最後に、必要に応じて電子を追加して元素の電荷を釣り合わせます。</li>
 <li>酸化の半反応で失われる電子の数と還元の半反応で得られる電子の数を等しくするために、半反応に適切な数を掛けます。</li>
 <li>両方の半反応を加え、式の両辺にある共通の化学種を消去することで式を単純化します。</li>
 <li>塩基性溶液で反応が起こる場合、手順7で得られた式にOH−イオンを加えて水素イオンを中和し（式の両辺に同数ずつ加える）、簡略化します。</li>
 <li>式を確認し、原子の電荷が釣り合っていることを確認します。</li>
 </ol>
 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/17-1-review-of-redox-chemistry">Openstax, Chemistry 2e, Chapter 17: Introduction,</a> and <a href="https://openstax.org/books/chemistry-2e/pages/17-1-review-of-redox-chemistry">Openstax, Chemistry 2e, Section17.2: Review of Redox Chemistry</a>.

電気化学は、電気反応と化学反応の相互変換に関与する科学です。このような反応は還元酸化、または...

Education

JoVE Core

Chemistry

化学

Electrochemistry

電気化学

KEY TERMS AND CONCEPTS

主要用語と概念

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 charge. Sodium chloride has Na at +1 and Cl at -1, and calcium fluoride has Ca at +2 and F at -1.

Covalent compounds need a closer look because electrons are shared. In water, the O-H bonds are polar, so oxygen pulls shared electrons more strongly than hydrogen does. That gives oxygen an oxidation number of -2 and hydrogen +1. The same idea gives carbon +4 and chlorine -1 in carbon tetrachloride, and nitrogen +5 with oxygen -2 in the nitrate ion, which adds up to the ion's -1 charge.

A simple redox example is the decomposition of sodium chloride. Sodium changes from +1 to 0, so it is reduced. Chlorine changes from -1 to 0, so it is oxidized. The equation is balanced by placing a coefficient of 2 in front of NaCl and Na.

Redox reactions in aqueous solution are often harder to balance because water, H+ ions, or OH- ions can appear as reactants or products. A systematic half-reaction method helps. First, split the equation into oxidation and reduction half-reactions. Then balance all elements except O and H, add water for oxygen, add protons for hydrogen, and add electrons to balance charge. After that, multiply the half-reactions so the electrons lost and gained are equal, add them together, and cancel species that appear on both sides.

If the reaction occurs in an alkaline medium, add OH- ions to neutralize the protons and simplify the equation. Finish by checking that both atoms and charge are balanced. This method gives a clear path for balancing redox equations in electrochemistry.

Redox Reactions and Half-Reaction Balancing

redox-reactions-and-half-reaction-balancing

balancing redox equations

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

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

Learn how oxidation numbers, redox changes, and the half-reaction method help balance redox equations in aqueous and alkaline solutions.

Redox Reactions and Half-Reaction Balancing | 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。

Texas Essential Knowledge and Skills

High School

Chemistry and Physics

IPC.8

IPC.8.B

Tennessee Academic Standards

Matter and Its Interactions

CHEM1.PS1.2

Virginia  Standards of Learning

HIgh School

PS.3

PS.3.d

CHEM.9

CHEM.9.A

Alabama Course of Study Science

Matter and Its Interactions: Energy

9-12.C.3.3.a

9-12.PS.3.6.d

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HS-PS1-2

HS-PS1-7

Florida Department of Education

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Georgia Standards of Excellence

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balancing-chemical-equations-and-reaction-outcomes

Balancing Chemical Equations and Reaction Outcomes

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phase-transitions-sublimation-and-deposition

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PS.PSc.1.2

physical-and-chemical-properties-of-matter

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Physical Properties of Matter

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comparing-intermolecular-forces-melting-point-boiling-point

Comparing Intermolecular Forces: Melting Point, Boiling Point, and Miscibility

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PS.PSc.1.3

The Atomic Theory of Matter

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Subatomic Particles

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The de Broglie Wavelength

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Use models to interpret the data presented in Bohr diagrams and electron dot diagrams for neutral atoms of elements 1 through 18.

PS.PSc.1.4

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Use models to compare representations of atoms, ions, and isotopes.

PS.PSc.1.5

The Periodic Table

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Periodic Classification of the Elements

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atomic-radii-and-effective-nuclear-charge

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Ionization Energy

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Alkali Metals

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Halogens

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Noble Gases


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Use the Periodic Table as a model to predict the relative properties (metallic/nonmetallic character, ionic charge, and reactivity) and arrangement of elements based on the pattern of valence electrons in the outermost energy levels of atoms.

PS.PSc.1.6

Types of Chemical Bonds

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Ionic Bonding and Electron Transfer

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Covalent Bonding and Lewis Structures

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Bonding in Metals

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Construct an explanation to classify the type of chemical bond that occurs (covalent, ionic, or metallic) in a given substance.

PS.PSc.2.1

Nomenclature of Alkanes

In the late 19th-century, the number of new chemical compounds discovered increased tremendously. Hence, the necessity arose to develop a naming system ...

Nomenclature of Alkenes

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Nomenclature of Alkynes

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structure-and-nomenclature-of-alcohols-and-phenols

Structure and Nomenclature of Alcohols and Phenols

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Structure and Nomenclature of Ethers

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IUPAC Nomenclature of Aldehydes

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IUPAC Nomenclature of Ketones

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IUPAC Nomenclature of Carboxylic Acids

IUPAC names of carboxylic acids are systematically derived following a few rules discussed below.
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Use models to apply International Union of Pure and Applied Chemistry (IUPAC) conventions to name and write formulas for simple compounds.

PS.PSc.2.2

Understand types, properties, and structure of matter.

Redox Reactions and Half-Reaction Balancing

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