Chemical reactions are an essential part of technology, contributing to various human activities that are a part of our daily lives. Examples of chemical reactions that we encounter everyday include the burning of fuels and the making of wine and beer. Chemical reactions are also widely present in nature, from the chemical weathering of rocks, photosynthesis in plants and the respiration process in animals.
In the broader aspect, there are three types of reactions: physical, chemical and nuclear. Chemical reactions can be further divided into many categories. Six common types of chemical reactions are: synthesis, decomposition, single-displacement, double-displacement, combustion and acid-base reactions. Scientists classify them based on what happens when going from reactants to products. This is helpful in predicting the reactivity of reagents and the products formed from the reactions.
Types of Reactions
A chemical reaction is a process in which one or more substances, the reactants, undergo chemical transformation to form one or more different substances, the products. It is a process that involves rearrangement of the constituent atoms of the reactants to form products, without changing the nuclei of the atoms.
For instance, in a process used to manufacture soda and seltzer, carbon dioxide is bubbled into water under pressurized conditions and forms a new compound known as carbonic acid (H2CO3). By this equation, you know a chemical reaction has occurred.
CO2(g) + H2O(l) —> H2CO3(aq)
A physical reaction is different from a chemical reaction. Physical changes only involve the change of state, for example, the freezing of water to ice and the sublimation of dry ice to carbon dioxide. In both scenarios, the chemical identity of reactants, H2O and CO2, did not change. The products are still made up of the same compounds as the reactants.
H2O(l) —> H2O(s)
CO2(s) —> CO2(g)
A nuclear reaction is also distinguished from a chemical reaction. It involves the collision of two nuclei to form one or more nuclides that are different from parent nuclei. For example, Ernest Rutherford performed the first artificial transmutation by exposing nitrogen gas to alpha particles, forming the isotope 17O and ejecting a proton in this process. The element in the reactant changed, thus a reaction had taken place.
14N + α —> 17O + p
Types of Chemical Reactions
The most common types of chemical reactions are synthesis, decomposition, single displacement, double displacement, combustion and acid-base. However, such categorization is not exclusive. For example, an acid-base reaction can also be classified as a double displacement reaction.
A synthesis reaction is one in which two or more substances are combined to form a more complex one. The chemical equation for a general form of synthesis reaction is as follows:
A + B —> AB
One example of a synthesis reaction is the combination of iron (Fe) and sulfur (S) to form iron sulfide.
Fe(s) + S(s) —> FeS(s)
Another example is when sodium and chlorine gas are combined to produce a more complex molecule, the sodium chloride.
2Na(s) + Cl2(g) —> 2NaCl(s)
A decomposition reaction works quite the opposite to a synthesis reaction. It is a reaction where a more complex substance breaks apart into simpler ones. A general form of a decomposition reaction can be written as:
AB —> A + B
An example of a decomposition reaction is the electrolysis of water to form hydrogen and oxygen gas.
H2O(l) —> H2(g) + O2(g)
Decomposition can also be thermal, such as the conversion of carbonic acid to water and carbon dioxide under heating conditions. It is commonly seen in carbonated beverages.
H2CO3(aq) —> H2O(l) + CO2(g)
Single Displacement Reaction
Also known as the single replacement reaction, the single displacement reaction is when a pure element switches places with another element in a compound. It is in the general form:
A + BC —> AC + B
Many metals can react with a strong acid. For example, magnesium reacts with hydrochloric acid to form hydrogen gas and magnesium chloride. In this reaction, magnesium switches places with the hydrogen in hydrochloric acid.
Mg(s) + 2HCl(aq) —> H2(g) + MgCl2(aq)
Magnesium can also react with water to generate magnesium hydroxide and hydrogen gas.
Mg(s) + 2H2O(l) —> H2(g) + Mg(OH)2(aq)
Another type of chemical reactions is double displacement, in which the cations of the two reactants switch places to form two completely different products. A general form of this reaction is:
AB + CD —> AD + CB
One example of a double displacement reaction is when barium chloride reacts with magnesium sulfate to form barium sulfate and magnesium chloride. In this reaction, barium and magnesium cations in the reactants switch places to new barium and magnesium compounds.
BaCl2 + MgSO4 —> BaSO4 + MgCl2
Another example is the reaction of lead nitrate with potassium iodide to form lead iodide and potassium nitrate.
Pb(NO3)2 + 2KI —> PbI2 + 2KNO3
In both cases, the reaction generates a precipitate (BaSO4 and PbI2) from two soluble reactants, so they are also grouped under precipitation reactions.
A combustion reaction is an exothermic redox chemical reaction where a fuel reacts with oxygen to produce gaseous products. Although it is usually initiated by a form of energy, such as using a lit match to light a fire, the released heat provide energy for sustaining the reaction.
A complete combustion reaction occurs when excess oxygen is present and yields primarily common oxides such as carbon dioxide and sulfur dioxide. In order to ensure full combustion, the oxygen present has to be twice or three times the theoretical amount calculated by stoichiometry. A complete combustion of a hydrocarbon can be expressed in the form:
4CxHy + (4x+y)O2 —> 4xCO2 + 2yH2O + heat
Combustion of methane, which is a saturated hydrocarbon, releases substantial heat (891 kJ/mol) and can be summarized by the equation as follows:
CH4 + 2O2 —> CO2 + 2H2O + heat
Naphthalene is another example of hydrocarbon and its complete combustion also generates carbon dioxide, water and heat.
C10H8 + 12O2 —> 10CO2 + 4H2O + heat
Alcohols can also serve as a source of fuel for combustion, such as methanol.
CH3OH+ O2 —> CO2 + 2H2O + heat
An incomplete combustion occurs when there is not enough oxygen to fully react with the fuel to produce carbon dioxide and water. Such an example is when methane is burnt in a limited supply of oxygen to produce a combination of carbon monoxide, carbon dioxide, carbon ash and water. It can be expressed by the equations below, arranged by the amount of oxygen present.
CH4 + O2 —> C + 2H2O
2CH4 + 3O2 —> 2CO + 4H2O
More but not enough oxygen:
4CH4 + 7O2 —> 2CO + 2CO2 + 8H2O
Too much carbon monoxide can result in air poisoning because it combines with hemoglobin to form carboxyhemoglobin and reduces its capacity for delivering oxygen. Therefore it is important to ensure complete combustion of fuel for household and industrial uses.
The acid-base reaction is a reaction between an acid and a base, and water is one of the products. It is a special type of double displacement reaction (A and B switch places) and these chemical reaction examples are written as:
HA + BOH —> BA + H2O
A simple example of an acid-base reaction is when an antacid (calcium hydroxide) neutralizes stomach acid (hydrochloric acid).
Ca(OH)2 + 2HCl —> CaCl2 + 2H2O
Another example is the reaction of vinegar (acetic acid) with baking soda (sodium bicarbonate). In this process, water and carbon dioxide are formed but no heat is released, so it is not a combustion reaction.
CH3COOH + NaHCO3 —> CH3COONa + H2O + CO2
- Combustion processes are rarely perfect. Realistically, you would see secondary combustion reactions. Secondary reactions often give out products such as carbon monoxide (CO). Carbon monoxide produced by this process indicates incomplete combustion. Though less prominent than the primary CO2-generating reaction, incomplete combustion matters. Running a car engine in a closed garage can be fatal—the small percentage of gas burned “incompletely” into CO adds up to toxic levels.
About the Author
Lan Luo has a PhD in Organic Chemistry from University of Chicago and a BS in Chemistry from Worcester Polytechnic Institute. She has years of research experience in asymmetric catalysis, natural product synthesis, drug discovery and drug delivery. She has served as a contributor for Synfacts and a reviewer for journal articles.