What is a chemical reaction?
A chemical reaction is a process that rearranges given chemical substances into other chemical substances.
Changes due to chemical reactions involve the formation and breaking of chemical bonds between atoms due to changes in the arrangement of outer electrons, without affecting the nucleons in the nucleus. A chemical reaction can split a compound but cannot split an atom, hence the reason why we define an atom as being chemical indivisible.
A chemical reaction can be represented by a chemical equation which shows, among other things the:
- reactants or reagents of the reaction
- products of the reaction
What are reagents (reactants) and products in a chemical reaction?
- Reactants (or reagents) are chemical substances that are initially involved in a chemical reaction.
- Products are the chemical substances produced by the chemical reaction.
Chemical reactions involve a chemical change, and produce one or more products, which usually have properties different from the reactants. For example, when hydrogen burns in oxygen to form water according to the following equation:
hydrogen + oxygen → water
- hydrogen and oxygen are the reactants
- water is the product
- the arrow (→) shows the direction the chemical reaction will proceed.
Water (the product) has completely different chemical properties from the reactants, hydrogen and oxygen.
Types of chemical reactions
There are basically three types of chemical reactions:
- Substitution /replacement /displacement reactions
- Decomposition reactions
- Synthesis /addition reactions
The are two types of substitution reactions:
- Single replacement reactions
- Double replacement reactions
Single replacement reactions
A single replacement reaction is a reaction in which a single uncombined element replaces another element in a compound.
A single replacement reaction can be represented as follows:
A + BC → AB + C
which means A displaces C from the compound BC to form compound AB.
Usually for such reactions to proceed forward, A has to be more reactive than C. That is because a more reactive element displaces a less reactive element from its compound. Some examples of single replacement reactions are when:
- zinc displaces hydrogen from sulphuric acid to form zinc sulphate as follows:
- Zn + H2SO4 → ZnSO4 + H2
- zinc displaces hydrogen from water to zinc oxide as follows:
- Zn + H2O → ZnO + H2
- aluminium displaces iron from iron (ii) oxide to form aluminum oxide as follows:
- 2Al + Fe2O3 → Al2O3 + 2Fe
Double replacement reactions
A double replacement reaction is a reaction in which the anions and cations of two compounds switch places and form two entirely different compounds.
A double replacement reaction can be represented as follows:
AB + CD → AC + BD
One example of a double displacement reactions is the reaction between barium chloride (BaCl2) and magnesium sulphate (MgSO4). In this reaction, the SO42− anion trades places with the 2Cl− anion, to produce the products BaSO4 and MgCl2.
BaCl2 + MgSO4 → BaSO4 + MgCl2
A decomposition reaction is a reaction in which a larger chemical substance breaks down into more simpler substances.
A decomposition reaction can be represented as follows:
AB → A + B
Examples of decomposition reactions are:
- the decomposition of calcium carbonate by the action of heat to produce calcium oxide and carbon dioxide gas.
- CaCO3 → CaO + CO2
- the electrolysis of water to yield hydrogen gas and oxygen gas.
- 2H2O → 2H2 + O2
A synthesis reaction is a reaction in which two or more simple chemical substances combine to form a larger substance.
Synthesis reactions are also know as addition reactions. A synthesis reaction can be represented as follows:
A + B → AB
Examples of synthesis reactions are:
- the reaction between calcium oxide and carbon dioxide to form calcium carbonate.
- CaO + CO2 → CaCO3
- the combustion of hydrogen gas in oxygen gas to yield water.
- 2H2 + O2 → 2H2O
You can read more on chemical reactions here