Defining redox reactions
To understand how the cool stuff works, let’s look at the atomic level and see how these reactions work. “Redox” is a shortened name for “reduction-oxidation.” Redox reactions are explained in different ways, but one of the easiest is seeing them in terms of atoms (or compounds) gaining or losing electrons. A simple mnemonic is used to differentiate the two:
OIL : RIG
Oxidation is loss : Reduction is gain
Oxidation requires that an atom or compound loses electrons, whereas reduction requires that an atom or compound gains electrons. If a compound loses electrons in a reaction, it makes sense for another one to gain them.
Less common definitions
Other, less common definitions of redox reactions include thinking in terms of oxygen transfer and hydrogen transfer.
Oxygen: Oxidation is gain, reduction is loss (follows intuition).
Hydrogen: Oxidation is loss, reduction is gain (follows OIL RIG mnemonic).
Oxidizing and reducing agents
This is where OIL RIG comes in handy. As one would expect, oxidizing agents oxidize something, and reducing agents reduce something. I.e., oxidizing agents cause something to lose electrons, reducing agents cause something to gain electrons. Oxidizing agents gain the lost electrons, reducing agents lose the electrons. For more information on the basics of redox reactions, click here.
Below is an example of a common redox reaction: rusting of a surface.
Fe + 3 OH ⇌ Fe(OH)3
Fe3+ + 3 OH– ⇌ Fe(OH)3
Both formulas are the same thing (I have omitted using solid, gas, aqueous identifiers for comprehensive purposes). The first simply shows a combination reaction. The second shows each atom and compound with their oxidation numbers (i.e., their charge).
Question: What is being reduced and what is being oxidized?
Answer: Respectively, iron and hydroxide. Iron goes from a positive charge to neutral, so it must be gaining electrons. Iron is reduced. Hydroxide goes from a negative charge to neutral, so it must be losing electrons. Hydroxide is oxidized.
Question: What are the reducing and oxidizing agents? (Remember: reducing agents cause something to gain electrons, oxidizing agents cause a loss).
Answer: Respectively, hydroxide and iron. Hydroxide causes iron to gain electrons, so hydroxide is the reducing agent. Iron causes hydroxide to lose electrons, so iron is the oxidizing agent.
Question: Why is rust so destructive if it only affects the surface?
Answer: The rust flakes off, revealing another layer to be oxidized. Iron rust flakes off because iron has multiple oxidation states which form different oxides. These oxides have different sizes and bump against each other. This leads to rust breaking off of the surface and flaking off.
Note that this formula results in an aqueous solution of hydroxide and iron (III). Once the solution dries, it will become rust, or iron (III) oxide (Fe2O3).
Batteries work by corroding. As is shown on the diagram, zinc (Zn) is reducing and cerium (Ce) is oxidizing. The electrolytes allow electrons to flow between the nodes, allowing the zinc and cerium to reduce and oxidize, respectively, thus providing charge.
Metabolism and Photosynthesis
Electrons are transferred from glucose to dioxide, which oxidizes the carbon molecules and reduces the dioxide. The dioxide is reduced to water, and the leftovers of this reaction gives our bodies energy. Plants photosynthesize the same way humans metabolize, only backwards. Switch the arrow in the formula the other way, and you have photosynthesis. (The energy comes from sunlight).
In short, combustion occurs when a hydrocarbon is heated and reacts with oxygen to yield carbon dioxide and water. Take a look at the formula below:
This is a generic formula for a combustion redox reaction. Below is a specific one of methane burning:
Let’s analyze each molecule. The carbon in the first compound, methane, has a -4 charge to balance out the total charge from the hydrogen atoms (4 atoms with +1 charge each). On the other side, it is bonded with 2 oxygen atoms. To balance this out, carbon needs to have a +4 charge (to balance out the -4 charge from the oxygen atoms). Since it lost electrons, the carbon was oxidized. The hydrogen has the same +1 charge on both sides. The oxygen in the dioxide molecule has a 0 charge, but reduces to a -2 charge in carbon dioxide and water on the other side. Not only do we see how fire starts, we now know why oxygen is necessary.
Our bodies work thanks to redox reactions. Surprisingly, they also fail because of them. Here is a quote from Emory University’s Department of Medicine discussing the redox theory of aging :
The glutathione redox system becomes oxidized with age; the immune system loses response, the brain accumulates protein aggregates, the lungs and kidneys decline in function, blood vessels lose flexibility and the heart begins to fail.
Ever wonder why antioxidants are so good for you? Oxidation damages cell membranes, lipids, and DNA. Free radicals are formed when our bodies metabolize oxygen, leading to heart disease and certain cancers, says Better Health Channel.
Two common types of rocket fuels are hypergolic and solid propellant. Hypergolic fuels consist of the fuel and an oxidizer to react with it. These two ignite upon contact, providing thrust, and are very easy to control. Solid propellants are the simplest, consisting of a casing with solid fuels and oxidizers inside of it, with a guided flame to ignite all of it. For more information on different types of rocket fuels, learn from the experts at NASA.