essay on chemical reactions

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Introduction & Top Questions

Historical overview

The conservation of matter
Energy considerations
Kinetic considerations
Gas-forming reactions
Precipitation reactions
Oxidation-reduction reactions
The Arrhenius theory
The Brønsted-Lowry theory
The Lewis theory
Decomposition reactions
Substitution, elimination, and addition reactions
Polymerization reactions
Solvolysis and hydrolysis
Chain reactions
Photolysis reactions

What are the basics of chemical reactions?

What happens to chemical bonds when a chemical reaction takes place, how are chemical reactions classified.

What are acids and bases?
How are acids and bases measured?

Leonardo da Vinci (1452-1519), Italian Renaissance painter from Florence. Engraving by Cosomo Colombini (d. 1812) after a Leonardo self portrait. Ca. 1500.

chemical reaction

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Table Of Contents

A chemical reaction is a process in which one or more substances, also called reactants, are converted to one or more different substances, known as products. Substances are either chemical elements or compounds .
A chemical reaction rearranges the constituent atoms of the reactants to create different substances as products. The properties of the products are different from those of the reactants.
Chemical reactions differ from physical changes, which include changes of state, such as ice melting to water and water evaporating to vapor. If a physical change occurs, the physical properties of a substance will change, but its chemical identity will remain the same.

According to the modern view of chemical reactions, bonds between atoms in the reactants must be broken, and the atoms or pieces of molecules are reassembled into products by forming new bonds. Energy is absorbed to break bonds, and energy is evolved as bonds are made. In some reactions the energy required to break bonds is larger than the energy evolved in making new bonds, and the net result is the absorption of energy. Hence, different types of bonds may be formed in a reaction. A Lewis acid-base reaction , for example, involves the formation of a covalent bond between a Lewis base, a species that supplies an electron pair, and a Lewis acid, a species that can accept an electron pair. Ammonia is an example of a Lewis base. A pair of electrons located on a nitrogen atom may be used to form a chemical bond to a Lewis acid.

Chemists classify chemical reactions in a number of ways: by type of product, by types of reactants, by reaction outcome, and by reaction mechanism. Often a given reaction can be placed in two or even three categories, including gas -forming and precipitation reactions. Many reactions produce a gas such as carbon dioxide , hydrogen sulfide , ammonia , or sulfur dioxide . Cake batter rising is caused by a gas-forming reaction between an acid and baking soda (sodium hydrogen carbonate). Classification by types of reactants include acid-base reactions and oxidation-reduction reactions , which involve the transfer of one or more electrons from a reducing agent to an oxidizing agent. Examples of classification by reaction outcome include decomposition, polymerization , substitution , and elimination and addition reactions. Chain reactions and photolysis reactions are examples of classification by reaction mechanism, which provides details on how atoms are shuffled and reassembled in the formation of products.

chemical reaction , a process in which one or more substances, the reactants , are converted to one or more different substances, the products. Substances are either chemical elements or compounds . A chemical reaction rearranges the constituent atoms of the reactants to create different substances as products.

Chemical reactions are an integral part of technology, of culture , and indeed of life itself. Burning fuels, smelting iron , making glass and pottery , brewing beer , and making wine and cheese are among many examples of activities incorporating chemical reactions that have been known and used for thousands of years. Chemical reactions abound in the geology of Earth , in the atmosphere and oceans , and in a vast array of complicated processes that occur in all living systems.

Chemical reactions must be distinguished from physical changes. Physical changes include changes of state, such as ice melting to water and water evaporating to vapour. If a physical change occurs, the physical properties of a substance will change, but its chemical identity will remain the same. No matter what its physical state, water (H 2 O) is the same compound , with each molecule composed of two atoms of hydrogen and one atom of oxygen . However, if water, as ice, liquid, or vapour, encounters sodium metal (Na), the atoms will be redistributed to give the new substances molecular hydrogen (H 2 ) and sodium hydroxide (NaOH). By this, we know that a chemical change or reaction has occurred.

The concept of a chemical reaction dates back about 250 years. It had its origins in early experiments that classified substances as elements and compounds and in theories that explained these processes. Development of the concept of a chemical reaction had a primary role in defining the science of chemistry as it is known today.

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The first substantive studies in this area were on gases . The identification of oxygen in the 18th century by Swedish chemist Carl Wilhelm Scheele and English clergyman Joseph Priestley had particular significance. The influence of French chemist Antoine-Laurent Lavoisier was especially notable, in that his insights confirmed the importance of quantitative measurements of chemical processes. In his book Traité élémentaire de chimie (1789; Elementary Treatise on Chemistry ), Lavoisier identified 33 “elements”—substances not broken down into simpler entities. Among his many discoveries, Lavoisier accurately measured the weight gained when elements were oxidized, and he ascribed the result to the combining of the element with oxygen . The concept of chemical reactions involving the combination of elements clearly emerged from his writing, and his approach led others to pursue experimental chemistry as a quantitative science.

The other occurrence of historical significance concerning chemical reactions was the development of atomic theory . For this, much credit goes to English chemist John Dalton , who postulated his atomic theory early in the 19th century. Dalton maintained that matter is composed of small, indivisible particles, that the particles, or atoms , of each element were unique, and that chemical reactions were involved in rearranging atoms to form new substances. This view of chemical reactions accurately defines the current subject. Dalton’s theory provided a basis for understanding the results of earlier experimentalists, including the law of conservation of matter (matter is neither created nor destroyed) and the law of constant composition (all samples of a substance have identical elemental compositions).

Thus, experiment and theory, the two cornerstones of chemical science in the modern world, together defined the concept of chemical reactions. Today experimental chemistry provides innumerable examples, and theoretical chemistry allows an understanding of their meaning.

Basic concepts of chemical reactions

When making a new substance from other substances, chemists say either that they carry out a synthesis or that they synthesize the new material. Reactants are converted to products, and the process is symbolized by a chemical equation . For example, iron (Fe) and sulfur (S) combine to form iron sulfide (FeS). Fe(s) + S(s) → FeS(s) The plus sign indicates that iron reacts with sulfur. The arrow signifies that the reaction “forms” or “yields” iron sulfide, the product. The state of matter of reactants and products is designated with the symbols (s) for solids , (l) for liquids , and (g) for gases .

Types of Chemical Reactions

List of Common Reactions and Examples

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A chemical reaction is a process generally characterized by a chemical change in which the starting materials (reactants) are different from the products. Chemical reactions tend to involve the motion of electrons , leading to the formation and breaking of chemical bonds. There are several different types of chemical reactions and more than one way of classifying them. Here are some common reaction types:

Oxidation-Reduction or Redox Reaction

In a redox reaction, the oxidation numbers of atoms are changed. Redox reactions may involve the transfer of electrons between chemical species. The reaction that occurs when In which I 2 is reduced to I - and S 2 O 3 2- (thiosulfate anion) is oxidized to S 4 O 6 2- provides an example of a redox reaction : 2 S 2 O 3 2− (aq) + I 2 (aq) → S 4 O 6 2− (aq) + 2 I − (aq)

Direct Combination or Synthesis Reaction

In a synthesis reaction , two or more chemical species combine to form a more complex product. A + B → AB The combination of iron and sulfur to form iron (II) sulfide is an example of a synthesis reaction: 8 Fe + S 8 → 8 FeS

Chemical Decomposition or Analysis Reaction

In a decomposition reaction , a compound is broken into smaller chemical species. AB → A + B The electrolysis of water into oxygen and hydrogen gas is an example of a decomposition reaction: 2 H 2 O → 2 H 2 + O 2

Single Displacement or Substitution Reaction

A substitution or single displacement reaction is characterized by one element being displaced from a compound by another element. A + BC → AC + B An example of a substitution reaction occurs when zinc combines with hydrochloric acid. The zinc replaces the hydrogen: Zn + 2 HCl → ZnCl 2 + H 2

Metathesis or Double Displacement Reaction

In a double displacement or metathesis reaction two compounds exchange bonds or ions in order to form different compounds . AB + CD → AD + CB An example of a double displacement reaction occurs between sodium chloride and silver nitrate to form sodium nitrate and silver chloride. NaCl(aq) + AgNO 3 (aq) → NaNO 3 (aq) + AgCl(s)

Acid-Base Reaction

An acid-base reaction is a type of double displacement reaction that occurs between an acid and a base. The H + ion in the acid reacts with the OH - ion in the base to form water and an ionic salt: HA + BOH → H 2 O + BA The reaction between hydrobromic acid (HBr) and sodium hydroxide is an example of an acid-base reaction: HBr + NaOH → NaBr + H 2 O

A combustion reaction is a type of redox reaction in which a combustible material combines with an oxidizer to form oxidized products and generate heat ( exothermic reaction ). Usually, in a combustion reaction oxygen combines with another compound to form carbon dioxide and water. An example of a combustion reaction is the burning of naphthalene: C 10 H 8 + 12 O 2 → 10 CO 2 + 4 H 2 O

Isomerization

In an isomerization reaction, the structural arrangement of a compound is changed but its net atomic composition remains the same.

Hydrolysis Reaction

A hydrolysis reaction involves water. The general form for a hydrolysis reaction is: X - (aq) + H 2 O(l) ↔ HX(aq) + OH - (aq)

The Main Types of Chemical Reactions

There are hundreds or even thousands of types of chemical reactions ! If you are asked to name the main 4, 5 or 6 types of chemical reactions , here is how they are categorized . The main four types of reactions are direct combination, analysis reaction, single displacement, and double displacement. If you're asked the five main types of reactions, it is these four and then either acid-base or redox (depending who you ask). Keep in mind, a specific chemical reaction may fall into more than one category.

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Chemical Properties of Matter
Examples of Physical Changes
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Acidic Solution Definition
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