HiSET Science: Reaction Types

Types of Reactions
One way to organize chemical reactions is to sort them into two categories: oxidation/reduction reactions (also called redox reactions) and metathesis reactions (which include acid/base reactions).
Oxidation/reduction reactions can involve the transfer of one or more electrons, or they can occur as a result of the transfer of oxygen, hydrogen, or halogen atoms. The species that loses electrons is oxidized and is referred to as the reducing agent. The species that gains electrons is reduced and is referred to as the oxidizing agent. The element undergoing oxidation experiences an increase in its oxidation number, while the element undergoing reduction experiences a decrease in its oxidation number. Single replacement reactions are types of oxidation/reduction reactions. In a single replacement reaction, electrons are transferred from one chemical species to another. The transfer of electrons results in changes in the nature and charge of the species.

Single Substitution, Displacement, and Replacement Reactions
Single substitution, displacement, or replacement reactions are when one reactant is displaced by another to form the final product (A + BC → AB + C). Single substitution reactions can be cationic or anionic. When a piece of copper (Cu) is placed into a solution of silver nitrate (AgNO3), the solution turns blue. The copper appears to be replaced with a silvery-white material.

The equation is 2AgNO3 + Cu → Cu (NO3)2 + 2Ag. When this reaction takes place, the copper dissolves and the silver in the silver nitrate solution precipitates (becomes a solid), resulting in copper nitrate and silver. Copper and silver have switched places in the nitrate.

Double displacement, double replacement, substitution, metathesis, or ion exchange reactions are when ions or bonds are exchanged by two compounds to form different compounds (AC + BD → AD + BC). An example of this is that silver nitrate and sodium chloride form two different products (silver chloride and sodium nitrate) when they react. The formula for this reaction is AgNO3 + NaCl → AgCl + NaNO3.

Combination and Decomposition Reactions
Combination, or synthesis, reactions:
In a combination reaction, two or more reactants combine to form a single product (A + B → C). These reactions are also called synthesis or addition reactions. An example is burning hydrogen in air to produce water. The equation is 2H2 (g) + O2 (g) → 2H2O (l).

Another example is when water and sulfur trioxide react to form sulfuric acid.
The equation is H2O + SO3 → H2SO4.
Decomposition (or desynthesis, decombination, or deconstruction) reactions: In a decomposition reaction, a reactant is broken down into two or more products (A → B + C).

These reactions are also called analysis reactions. Thermal decomposition is caused by heat. Electrolytic decomposition is due to electricity. An example of this type of reaction is the decomposition of water into hydrogen and oxygen gas. The equation is 2H2O → 2H2 + O2.


Acid/Base Reactions
In acid/base reactions, an acid is a compound that can donate a proton, while a base is a compound that can accept a proton. In these types of reactions, the acid and base react to form a salt and water. When the proton is donated, the base becomes water and the remaining ions form a salt. One method of determining whether a reaction is an oxidation/reduction or a metathesis reaction is that the oxidation number of atoms does not change during a metathesis reaction.

Isomerization and Neutralization Reactions
Isomerization, or rearrangement, is the process of forming a compound's isomer. Within a compound, bonds are reformed. The reactant and product have the same molecular formula, but different structural formulas and different properties (A → B or A → A'). For example, butane (C4H10) is a hydrocarbon consisting of four carbon atoms in a straight chain. Heating it to 100° C or higher in the presence of a catalyst forms isobutane (methylpropane), which has a branched-chain structure. Boiling and freezing points are greatly different for butane and isobutane. A rearrangement reaction occurs within the molecule.

A neutralization, acid-base, or proton transfer reaction is when one compound acquires H+ from another.
These types of reactions are also usually double displacement reactions. The acid has an H+ that is transferred to the base and neutralized to form a salt.