Chemical reaction

Rusting iron

A bonfire is an example for redox

A chemical reaction happens when one or more chemicals are changed into one or more other chemicals. Examples:

iron and oxygen combining to make rust
vinegar and baking soda combining to make sodium acetate, carbon dioxide and water
things burning or exploding
many reactions that happen inside living things, such as photosynthesis
electrochemical reactions when discharging or recharging batteries

Some reactions are fast, and others are slow. Some happen at different speeds, depending on temperature or other things. For example, wood does not react with air when it is cold, but if it is made hot enough, it will start to burn. Some reactions give out energy. These are exothermic reactions. In other reactions, energy is taken in. These are endothermic reactions.

Nuclear reactions are not chemical reactions. Chemical reactions involve only the electrons of atoms; nuclear reactions involve the protons and neutrons in the atomic nuclei.

Four basic types

The four basic chemical reactions types: synthesis, decomposition, single replacement and double replacement

Synthesis

In a synthesis reaction, two or more simple substances combine to form a more complex substance.

[math]\displaystyle{ A + B \longrightarrow AB }[/math]

"Two or more reactants giving one product" is another way to identify a synthesis reaction. One example of a synthesis reaction is the combination of iron and sulfur to form iron(II) sulfide:

[math]\displaystyle{ 8Fe + S_8 \longrightarrow 8FeS }[/math]

Another example is simple hydrogen gas combined with simple oxygen gas to produce a more complex substance, such as water.^[1]

Decomposition

A decomposition reaction is when a more complex substance breaks down into its more simple parts. It is thus the opposite of a synthesis reaction, and can be written as:^[1]^[2]

[math]\displaystyle{ AB \longrightarrow A + B }[/math]

One example of a decomposition reaction is the electrolysis of water to make oxygen and hydrogen gas:

[math]\displaystyle{ 2H_2O \longrightarrow 2H_2 + O_2 }[/math]

Another example of a decomposition reaction is calcium carbonate breaking down into calcium oxide and carbon dioxide under high temperatures:

CaCO₃ —> CaO + CO₂

Single replacement

In a single replacement reaction, a single uncombined element replaces another in a compound; in other words, one element trades places with another element in a compound^[1] These reactions come in the general form of:

[math]\displaystyle{ A + BC \longrightarrow AC + B }[/math]

One example of a single displacement reaction is when magnesium replaces hydrogen in water to make magnesium hydroxide and hydrogen gas:

[math]\displaystyle{ Mg + 2H_2O \longrightarrow Mg(OH)_2 + H_2 }[/math]

Double replacement

In a double replacement reaction, the anions and cations of two compounds switch places and form two entirely different compounds.^[1] These reactions are in the general form:^[2]

[math]\displaystyle{ AB + CD \longrightarrow AD + CB }[/math]

For example, when barium chloride (BaCl₂) and magnesium sulfate (MgSO₄) react, the SO₄²⁻ anion switches places with the 2Cl⁻ anion, giving the compounds BaSO₄ and MgCl₂.

Another example of a double displacement reaction is the reaction of lead(II) nitrate with potassium iodide to form lead(II) iodide and potassium nitrate:

[math]\displaystyle{ Pb(NO_3)_2 + 2 KI \longrightarrow PbI_2 + 2 KNO_3 }[/math]

Equations

A chemical reaction is being displayed by an equation:

[math]\displaystyle{ \mathrm{ A + B \longrightarrow C + D} }[/math]

Here, A and B react to C and D in a chemical reaction.

This is an example of a combustion reaction.

[math]\displaystyle{ \mathrm{C + O_2 \longrightarrow CO_2} }[/math]

carbon + oxygen → carbon dioxide

Chemical Reaction Media

A thermite reaction using iron(III) oxide. The sparks flying outwards are globules of molten iron trailing smoke in their wake.
Antoine Lavoisier developed the theory of combustion as a chemical reaction with oxygen.
As seen from the equation CH
4 + 2O
2 → CO
2 + 2 H
2O, a coefficient of 2 must be placed before the oxygen gas on the reactants side and before the water on the products side in order for, as per the law of conservation of mass, the quantity of each element does not change during the reaction.
An example of organic reaction: oxidation of ketones to esters with a peroxycarboxylic acid
Isomerization of azobenzene, induced by light (hν) or heat (Δ)
Illustration of a redox reaction
Sodium chloride is formed through the redox reaction of sodium metal and chlorine gas
Ferrocene – an iron atom sandwiched between two C5H5 ligands
Diagram showing the differences between a compound, a precipitate, a supernate, and a suspension.