What is a Chemical Reaction?

17th June 2020


A chemical reaction is distinct from a physical or phase change (e.g. evaporation, which is a change from liquid to gas) because the reaction involves a change in atomic combinations. For instance, the chemical reaction of water with sodium is an exothermic violent reaction that releases one hydrogen atom per molecule of water to form sodium hydroxide.

A chemical reaction is also different from atomic or nuclear reactions, in which the nuclei of atoms are either split or fused together, releasing tremendous amounts of energy. In a chemical reaction, the electrons in the outer shells are the ones that interact. The nuclei remain intact.

Bonds and chemical reactions

This all means that a chemical reaction is basically about the breaking of chemical bonds of molecules and the forming of new ones. There are four basic bonds:

  • Ionic bonds, which involves electron transfer
  • Covalent bonds, where electrons are shared
  • Hydrogen bonds, where two adjacent water molecules link together
  • Polar bonds, a midway between ionic and covalent bonding, where one end of the molecule is negatively charged and the other is positively charged

In many types of chemical reactions, covalent bonds are broken between atoms to form new bonds. Energy is released or absorbed in the process.

A chemical reaction can either be slow, like rusting iron, or fast, like a bomb exploding. It can either be simple, like the replacement of a hydrogen atom by another hydrogen atom (H + H2→H2 + H), or it can be as complex as protein synthesis, which involves several systems and chains of chemical reactions.

An explosion is an example of a fast chemical reaction

An explosion is an example of a fast chemical reaction

Most of the chemical reactions that occur in biological organisms are complex. Our own metabolic processes are complex chains of chemical reactions.

What happens in a chemical reaction?

In a chemical reaction, there’s a change in the atomic bonds of the elements comprising a molecule. Chemical bonds tie elements together in specific ways to form compound molecules. 

However, a molecule can simply be composed of the same element, such as in the case of diatomic molecules like nitrogen and oxygen. Even in molecules composed of the same element, a chemical reaction is said to have occurred when the bonds are broken.

For example, the breaking of bonds in a molecule of ozone (O3) is considered as a chemical reaction. In fact, it is a potentially corrosive reaction.

Generally speaking, four things can occur during a chemical reaction:

  1. Breaking of bonds – before new bonds can form, old bonds must first be broken. You can think of it like exchanging partners in a dance. Chemical reactions between reactants are predictable based on the electronegativity of the elements.
  2. Formation of new bonds – similar to breaking of bonds, this mainly involves the interaction of the electrons in the outer shells of the atoms. They may either form covalent bonds or ionic bonds.
  3. Release of energy – many chemical reactions are exothermic, or involve the release of heat. The most common example of this is combustion, which is sometimes explosive, such as the reaction of hydrogen with oxygen.
  4. Absorption of energy – endothermic chemical reactions are less common. This involves the absorption of heat from the environment, such as in the reaction of thionyl chloride (SOCl2) with cobalt(II) sulfate heptahydrate.

How do you classify chemical reactions?

More than 66 million inorganic compounds have been catalogued and about 12,000 new ones are added daily. Most are naturally occurring while others are man made or synthetic. There are also more than nine million known organic compounds.

The possible number of chemical reactions are simply astronomical. However, there are only five basic categories of chemical reactions. These categories are helpful in terms of predicting the outcome. These reactions can be generalised into formulas. However, some reactions can be grouped into more than one category. These categories are:

  1. Oxidation–Reduction: oxidant + reductant → reduced oxidant + oxidized reductant
  2. Acid–Base: acid + base → salt
  3. Exchange: Single Replacement: AB + C → AC + B or Double Replacement: AB + CD → AD + CB
  4. Combination (synthesis): A + B → AB
  5. Decomposition: AB → A + B

Different types of chemical reactions

Where do chemical reactions take place?

Chemical reactions happen almost everywhere. They occur in biological organisms, in rocks, in the air, in the oceans, underneath the earth, and in outer space. In fact, chemical reactions occur in the entire universe. If conditions such as temperature and presence of chemicals are right, reactions can occur.

Most chemical reactions are naturally occurring. Some happen randomly, while other reactions happen regularly. Some can be destructive like tooth decay, which is the result of bacteria producing acid that reacts with the calcium minerals of the teeth.

There are also synthetic chemical reactions which are designed for specific purposes. For example, raw materials like crude oil can be converted into plastic through a series of chemical reactions that trigger polymerisation.

Tooth decay is the result of a chemical reaction

Tooth decay is the result of a chemical reaction

How can you tell when a chemical reaction has happened?

Chemical reactions are either very subtle or conspicuously dramatic. Some chemical reactions, such as weathering, are barely noticeable because they are either very slow or not very obvious. On the other hand, some chemical reactions are immediately noticeable, such as in the case of lighting a match.

Here are some of the most common indications of chemical reactions. Although these are not conclusive indicators, they can help you assess a situation:

  • Gas bubbles appear – the gas element of one of the reactants is released. One example would be the bubbles formed on electrodes during electrolysis. Depending on whether an electrode is positive or negative, the bubbles could either be hydrogen or oxygen.
  • Formation of a precipitate – precipitates are heavier by-products in a chemical reaction, which are not soluble in the solvent.
  • Colour change – a copper that is dissolved in a sulphuric acid solution can turn the solution blue-green. This change in colour is an indication of chemical change. However, not all colour changes are indicators of a chemical reaction.
  • Temperature change – a chemical reaction almost always involves a change in temperature. It may either be endothermic or exothermic.
  • Production of light – combustions and explosions are some examples of chemical reactions that involve the production of photons. Bioluminescence, on the other hand, is a more complicated biochemical reaction that involves luciferin and oxygen. 
  • Change in volume – chemical compounds have a specific density under certain temperatures and pressures. If a chemical reaction occurs, that density could change. Since density is inversely proportional to volume, an increase in density would mean a decrease in volume and vice versa.
  • Change in smell or taste – any change in the chemical composition of a substance also has corresponding change in its odour and taste. For instance, in the process of making cheese, fermentation is involved. This makes milk taste sour and smell bad.

What factors influence the rate of a chemical reaction?

The rate or speed of chemical reaction is determined by different factors. The rate may either go faster or slower, depending on the factors involved. For example, a higher temperature typically accelerates chemical reactions because it agitates the molecules to move faster.

For every pair or group of reactants, there are ideal factors that optimise the rate of reaction. These factors have to be adjusted to get the perfect condition. In wine making, for example, grape extracts should be fermented at a temperature range of between 20 and 30 degrees Celsius.

Here are some of the factors that affect the rate of chemical reactions:

  • Concentration of the reactants
  • Presence of catalysts
  • Surface area of a solid reactant
  • Physical phase of the reactant
  • Temperature
  • Pressure


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