Redox

Redox, short for reduction–oxidation, is a special kind of chemical reaction. In these reactions, some atoms lose tiny parts called electrons, while others gain them. This swapping of electrons is important in many processes around us.

One type of redox reaction happens when a single electron moves from one atom to another. Another type happens when whole atoms move from one place to another. For example, when iron rusts, iron atoms change and oxygen atoms also change at the same time. Redox reactions happen in many other processes too, like when hydrogen atoms help change other atoms.

Terminology

"Redox" is a mix of the words "reduction" and "oxidation". It was first used in 1928 by two scientists.

In chemistry, oxidation means a substance loses tiny parts called electrons. Reduction means a substance gains those electrons. These two processes always happen together in a reaction. One substance gives electrons, and another takes them.

Some substances can take electrons from others; these are called oxidizing agents. Others can give electrons; these are called reducing agents. When these reactions happen, they form pairs called redox pairs. For example, iron can change its form in such pairs. These reactions are important in many processes, like in batteries.

Rates, mechanisms, and energies

Redox reactions can happen at different speeds. For example, rust forms slowly, while burning fuel happens very quickly. When electrons move between atoms, the reaction is usually fast and happens as soon as the materials are mixed.

The way atoms move in these reactions can be complicated because many types of atoms can be involved. We can study the energy needed to break bonds and remove electrons from water to understand how much energy is involved in redox reactions.

Standard electrode potentials (reduction potentials)

Each half-reaction has a standard electrode potential (E^o
_cell), which is the voltage at balance under special standard conditions in an electrochemical cell. In this cell, the cathode reaction is the half-reaction we are looking at, and the anode is a special standard hydrogen electrode where hydrogen loses an electron.

The electrode potential for each half-reaction is also called its reduction potential (E^o
_red). This tells us how likely something is to gain electrons. For the hydrogen reaction, this value is set to zero. Some reactions have positive values, meaning they are more likely to gain electrons than hydrogen. Others have negative values, meaning they are less likely to gain electrons than hydrogen.

For a full redox reaction in a cell, the total voltage can be found by subtracting the anode's potential from the cathode's potential. Sometimes, we talk about oxidation potential instead, which is just the opposite of the reduction potential. Using this, we can add the cathode's reduction potential to the anode's oxidation potential to find the cell's voltage.

Examples of redox reactions

When hydrogen and fluorine react, hydrogen loses electrons and fluorine gains electrons. This reaction makes a strong bond and releases a lot of energy. We can see this reaction in two steps: one step where hydrogen loses electrons, and one step where fluorine gains electrons. These steps come together to form hydrogen fluoride.

In another type of reaction, one metal can replace another metal in a solution. For example, when zinc is placed in a solution of copper(II) sulfate, zinc replaces copper. This reaction also releases energy. Zinc loses electrons, and copper gains electrons.

Other examples include turning nitrate into nitrogen with acid, burning hydrocarbons to make water and carbon dioxide, and the step-by-step change of hydrocarbons by oxygen, which makes different substances like alcohol, aldehyde, ketone, carboxylic acid, and peroxide.

Corrosion is when metals slowly change by reacting with substances like oxygen. Rusting is a common example, where iron turns into iron oxides. Another example is when iron changes with hydrogen peroxide and acid.

A disproportionation reaction is special because one substance both loses and gains electrons. For example, thiosulfate can change into sulfur and sulfur dioxide, where one part loses electrons and another part gains electrons.

Redox reactions in industry

Cathodic protection helps stop metal from rusting by connecting it to another metal that rusts more easily. This keeps important structures safe.

Oxidation helps make cleaning supplies. Redox reactions are used in batteries to make electricity. They are also used to turn raw materials into pure metals. These reactions help put thin layers of materials on objects, like chrome on car parts or silver on cutlery.

Redox reactions in biology

Many important processes in living things involve redox reactions. For example, some processes need iron from the environment to start.

Aerobic cellular respiration is a process where substances like glucose are changed, and oxygen is changed into water. This process also depends on the change of NAD⁺ to NADH and back again. Photosynthesis is another important process. It is different from respiration, but they work together. In photosynthesis, carbon dioxide is changed into sugars, and water is changed into oxygen.

Living cells store and use energy through redox reactions. In plant cells, mitochondria help manage these energy processes.

Redox reactions in geology

Minerals form when metals change and combine with oxygen. For example, iron is found in rocks like magnetite (Fe₃O₄) and hematite (Fe₂O₃). To get the metal back, we change these minerals by adding substances like carbon or carbon monoxide. This happens in special factories called blast furnaces. In these factories, the minerals and carbon are heated together. One important reaction looks like this:

Fe₂O₃ + 3 CO → 2 Fe + 3 CO₂

Redox reactions in soils

Chemical reactions in soils often involve tiny particles called electrons being shared between different substances. This sharing is important for many processes in the soil, such as helping plants grow and changing the soil's properties. Scientists measure how likely a soil is to share these particles using a value called redox potential. This helps them understand how chemicals behave and how living things interact with the soil. This idea started with studies on soils that stay underwater, like those used for growing rice, and has since been used to study many other soil processes.

Mnemonics

Main article: List of chemistry mnemonics

Learning about redox reactions can be hard because the names sound alike. A substance that loses electrons is called the reducing agent, and one that gains electrons is called the oxidizing agent. Here are some fun phrases students use to remember these ideas:

• "OIL RIG" — oxidation is loss of electrons, reduction is gain of electrons
• "LEO the lion says GER [grr]" — loss of electrons is oxidation, gain of electrons is reduction
• "LEORA says GEROA" — the loss of electrons is called oxidation (reducing agent); the gain of electrons is called reduction (oxidizing agent).
• "RED CAT" and "AN OX", or "AnOx RedCat" ("an ox-red cat") — reduction occurs at the cathode and the anode is for oxidation
• "RED CAT gains what AN OX loses" – reduction at the cathode gains (electrons) what anode oxidation loses (electrons)
• "PANIC" – Positive Anode and Negative is Cathode. This applies to electrolytic cells which release stored electricity, and can be recharged with electricity. PANIC does not apply to cells that can be recharged with redox materials. These galvanic or voltaic cells, such as fuel cells, produce electricity from internal redox reactions. Here, the positive electrode is the cathode and the negative is the anode.