There are five basic types of chemical reactions namely synthesis, decomposition, displacement, double displacement, and combustion reactions. Understanding these types of reactions will be useful in predicting the products of chemical reactions when given only the reactants along with conditions.
Some other common types of chemical reactions including neutralization, redox, and precipitation reaction are also discussed here.
In a synthesis reaction, two or more substances combine to form one new substance. This type of reaction is also known as a combination reaction.
In general form it can be written as:
A + B → AB
Synthesis reactions release energy meaning that they are exothermic.
One typical example is the formation of sodium chloride from its constituent ions.
Na+ + Cl– → NaCl
Another example of synthesis reactions is the combination of metals (I and II group metals) with oxygen forming their respective oxides.
2Mg(s) + O2 (g) → 2MgO(s)
O2 + 2Ca → 2CaO
2. Decomposition reactions
The reactions in which a compound breaks down into two or more simpler substances are called decomposition reactions.
The general form of decomposition reactions is:
AB → A + B
Most decomposition reactions are endothermic. Unlike synthesis reactions, they require energy to break the bonds present in the reactant.
An example of this type of chemical reaction is the decomposition of hydrogen peroxide into water and hydrogen gas.
2H2O2 → 2H2O + O2
Mercuric oxide decomposes into mercury and oxygen upon heating.
2HgO(s) → 2Hg (l) + O2 (g)
Other metal oxides also give decomposition reactions upon heating. The decomposition, however, occurs at specific temperatures for different metal oxides.
Metal carbonates break down forming metal oxides and carbon dioxide. For example, limestone (calcium carbonate) forms calcium oxide and carbon dioxide when decomposed.
CaCO3 (s) → CaO(s) + CO2 (g)
Electrolytic decomposition of water is also an example of decomposition reactions.
H2O(l) → H2 (g) + O2 (g)
3. Single Displacement reactions
Single displacement reactions are also known as single replacement reactions. Here one element replaces a similar element in a compound. The general representation of this type of chemical reaction is:
A + BC → AC + B
In this reaction A replaces B. Keep in mind that metals can replace metals and nonmetals will replace nonmetals in a compound.
So if A and B are both metals, A can replace B and thus, form the product AC.
Similarly, if both A and B are nonmetals, A can displace B to give product AC.
The displacement cannot happen if:
- A is metal and B is a nonmetal.
- A is nonmetal and B is a metal.
An example of single displacement reaction is the reaction of tin chloride and zinc. Zinc being a more reactive metal (as of electrochemical series) replaces tin leading to the formation of zinc chloride.
SnCl2 + Zn → ZnCl2 + Sn
Similarly, magnesium is more reactive than copper.
Mg(s) + Cu(NO3)2 (aq) → Mg(NO3)2 (aq) + Cu(s)
4. Double displacement reactions
In double displacement reactions, two compounds exchange ions to form completely different compounds. The exchange during this type of reaction is either of cations or anions. It’s never both at the same time.
The general form is:
AB + CD → AC + BD
Double displacement reactions are also called metathesis. They usually occur in aqueous solutions.
An example of double displacement reaction is the reaction between potassium iodide and lead nitrate where lead cation and potassium ion switch places.
Pb(NO3)2 (aq) + 2 KI (aq) → 2KNO3 (aq) + PbI2 (s)
One of the most common double replacement reactions is the reaction between sodium chloride and silver nitrate resulting in sodium nitrate and silver chloride (ppt).
NaCl(aq) + AgNO3 (aq) → NaNO3 (aq) + AgCl(s)
5. Combustion reactions
Combustion reaction is often included in the basic types of chemical reactions. It involves burning of compounds and the release of energy in the form of heat and light. One of the reactants in combustion reactions is necessarily oxygen.
An example is the burning of naphthalene.
C10H8 (g) + 12O2 (g) → 10CO2 (g) + 4H2O (g)
Remember that hydrocarbons are usually burned as fuels. Their combustion always results in the formation of carbon dioxide and water. There is also a release of a huge amount of energy which is used for domestic as well as commercial purposes as fuel output.
Methane, also called natural gas, is the most used hydrocarbon. Upon burning a mole of methane (16 g), 810 KJ of energy is released.
O2 (g) + CH4 (g) → H2O (g) + CO2 (g)
Other common types of reactions
These are the 7 common types of chemical reactions:
An acid-base reaction that results in the formation of water and salt is known as a neutralization reaction. It is a double displacement reaction in which hydrogen ions from an acid react with hydroxyl ions from a base to form salt and water. It is the usual mechanism of an acid-base neutralization reaction but the products may change in some cases.
The general form of neutralization is:
HA + BOH → H2O + BA
A common example is the reaction of HCl and NaOH.
HCl + NaOH → NaCl + H2O
The hydrogen from acid leaves the chloride ion to attack the sodium metal making salt and water, the neutralization products.
A redox reaction is an oxidation-reduction reaction in which one molecule, atom, or ion changes its state by gaining electron(s) and another one changes it by losing electrons. The atom that gains electron(s) is said to be reduced and the one that loses electron(s) is oxidized. In other words, the oxidation number of elements gets changed during this type of reaction.
Note that a redox reaction can be a synthesis, decomposition, single displacement, double displacement, and combustion reaction.
An example of a redox reaction is a reaction between thiosulfate ion and iodine where I2 is reduced to I– and S2O2- is oxidized to S4O62-
2 S2O32−(aq) + I2 (aq) → S4O62−(aq) + 2 I−(aq)
Precipitation reaction involves the formation of precipitates, the insoluble solid products that separate out.
Like the neutralization reaction, it also qualifies as a double displacement reaction.
An example is the precipitation reaction is:
NaCl(aq) + AgNO3 (aq) → AgCl(s)↓ + NaNO3 (aq)
In the above reaction, AgCl precipitates out and settles at the bottom of the vessel.
Polymerization is also known as a chain reaction. It is the formation of a chain-like network called polymer as a result of a chemical combination of relatively small units called monomers.
For example, the polymerization of ethylene (C2H4) produces polyethylene -(-C2H4-)-n.
C2H4 + C2H4 + C2H4 → (C2H4)n
Hydro means water and lysis means to break down. As the name suggests it is the breakdown of molecules with the help of water.
The general formula of a hydrolysis reaction is.
AB + H2O → AH + BOH
For example, dissolving sulfuric acid (H2SO4) in water (H2O) yields bisulfate ions (HSO3–) and hydronium ions (H3O+).
H2SO4 + H2O → HSO3– + H3O+
Dehydration is the opposite of hydrolysis where two molecules combine to form a new molecule with the elimination of water.
The term dehydration is used for losing water and the word synthesis suggests the formation of a new substance.
For example, dehydration of ethanol at 170 ⁰C gives ethene.
CH3–CH2–OH → CH2=CH2 + H2O
Photochemical reaction is the type of chemical reaction triggered by the absorption of light by molecules within a substance.
A common example is the photosynthesis reaction.
Similarly, silver chloride (AgCl) decomposes into silver (Ag) and chlorine (Cl2) gas upon exposure to radiation (light).
2 AgCl + hν → 2 Ag + Cl2
What are the 5 primary types of chemical reactions?
- Synthesis reactions
- Decomposition reactions
- Single displacement reactions
- Double displacement reactions
- Combustion reactions
What type of chemical reaction is 2NO → N2 + O2?
It is a decomposition reaction.
Why are chemical reactions considered to be important?
Chemical reactions help us understand the properties of matter. In a way, simple chemical reactions enable us to understand the vast and complicated processes going on around the universe and inside living organisms.
What is the basis for balancing a chemical equation?
A chemical equation is balanced based on the law of conservation of mass. It states that “Matter can neither be created nor be destroyed”.
Are all combination reactions exothermic?
Most of the combination reactions are exothermic but not all. Some endothermic combustion reactions also exist like the combustion of nitrogen gas.