Studying chemical reactions and the conditions that affect their progression is the chief concern of the entire discipline of chemistry. A chemical reaction is the creation of one or more new substances from one or more existing ones; the former are called products and the latter reactants.
Often, the aim of chemists is to, well, make things. Formally, this process is known as synthesis, and it occurs with no human input in the body in the form of biosynthesis. The reactions associated with creating a new compound from multiple existing substances are called chemical synthesis reactions, and examples of these abound in everyday life, from industrial settings to academic chemistry laboratories.
Types of Chemical Reactions
The five basic types of chemical reactions are briefly detailed below. Some reactions can be of more than one type, as you'll soon see. The basic principles of any chemical reaction dictate that it must be balanced in elements and charge, meaning that these elements and charge are conserved before and after the reaction. These reactions might take place in a variety of situations. Some might happen spontaneously while others might require a catalyst to initiate the reaction.
- Decomposition reaction: This kind of reaction has the general form: AB → A + B. An example is the decomposition of hydrogen peroxide into water and oxygen: 2H2O2→ 2H2O + O2
- Combustion reaction: In these reactions, one reactant, called a fuel, reacts with oxygen gas, O2 (one oxygen atom bonded to another). Often the result is visible as fire
There are two types of displacement reactions. They are also sometimes called single or double replacement reactions.
- Single displacement (or single replacement) reaction: These reactions involve one reactant "poaching" from another. The general form is A + BC → AB + C
- Double displacement (or double replacement) reaction: These are "trades" of the form AB + CD → AD + BC
- Synthesis reaction (also called a composition or combination reaction): A reaction of the general form A + B → AB. In the general example, it creates a single product from multiple inputs. An example is the synthesis of calcium carbonate from calcium oxide and carbon dioxide: CaO + CO2 → CaCO3
The process of rearranging elements and quantities to get a balanced chemical equation is called stoichiometry. This makes sure the the chemical compounds on either side of a reaction make up the same amount of reactants and products.
Synthesis Reaction: General Equation
As you saw, the synthesis reaction "word equation" (using letters to represent unspecified atoms or molecules) is A + B → AB. A and B can consist of single elements, either alone as ions (such as Na+) or as diatomic molecules (such as Cl2). One or both can also consist of molecular groups, e.g., NO<3-).
Seven elements are naturally found as diatomic molecules: H2, N2, O2, F2, Cl2, Br2, and I2>.
Occasionally, composition reactions will include more than two elements and assume the form A + B + C → ABC.
Synthesis Reactions in Everyday Life
If someone asked for a list of chemical reactions in everyday life, or even, say, just a list of single-replacement reaction examples in everyday life, an experienced chemist could go on for a very long time! But limiting the discussion to synthesis reaction examples alone hardly spoils the fun. A few common such reactions are described below.
- Two of the substances you have already seen combine to form ordinary table salt, sodium chloride: 2Na + Cl2> → 2NaCl
- The reaction of calcium oxide and carbon dioxide above is an example of a class of synthesis reactions that involve metal oxides combining with CO2 to produce the corresponding metallic carbonate
- Magnesium metal and oxygen gas combine to form magnesium oxide: 2Mg + O2 → 2MgO. This is also a combustion reaction and applies to various other metals
- The synthesis of zinc oxide (handy for preventing nose sunburns) is a synthesis reaction that also involves oxidation, or the loss of electrons, often as a result of interacting with oxygen. The balanced reaction for this synthesis is 2Zn + O2 → 2ZnO
- Potassium chloride reacts with ordinary oxygen gas to produce potassium chlorate. This is also an example of a general process – the combination of oxygen and a binary chloride to produce a chlorate of the metal: KCl + O2 → KClO3.
When chemical reactions involve carbon, they are generally considered to fall under the umbrella of organic chemistry. When multiple organic compounds and reactions are combined together to create a new element this is an organic example of a synthesis reaction.
Photosynthesis is likely the most famous example of this type of reaction; simple inputs of carbon-dioxide, water, and energy (sunlight) are used to create complex molecules and complex products. The reaction mechanisms involved include many types of reactions, but the predominant mechanism of photosynthesis creates natural products from natural reagents – organic synthesis.
The chemical bonds and more complex particles store this input energy in the form of glucose and other organic molecules.
About the Author
Kevin Beck holds a bachelor's degree in physics with minors in math and chemistry from the University of Vermont. Formerly with ScienceBlogs.com and the editor of "Run Strong," he has written for Runner's World, Men's Fitness, Competitor, and a variety of other publications. More about Kevin and links to his professional work can be found at www.kemibe.com.