Such processes in which the original substances lose their nature and identity and form new chemical substances with different properties are called chemical changes. The process involving a chemical change is called a chemical reaction.
Characteristics of chemical reactions
When a chemical reaction takes place, some changes are observed. The easily observable changes that take place in a chemical reaction are called the characteristics of the chemical reaction. These changes help us to check that a chemical reaction has taken place. Some important characteristics of chemical reactions are:
(1) Formation of a precipitate: Some chemical reactions are accompanied by the formation of a precipitate. A precipitate is a solid substance formed on mixing two solutions.
(2) Evolution of a gas: Some chemical reactions are accompanied by the evolution of a gas.
(3) Change in colour: Some chemical reactions are accompanied by change in colour.
(4) Change in state: Some chemical reactions are accompanied by change in state.
(5) Change in temperature: Some chemical reactions are accompanied by change in temperature, i.e., rise or fall of temperature.
Reactions which are accompanied by rise in temperature are those in which heat is evolved. Such reactions are called exothermic reactions.
Reactions which are accompanied by fall in temperature are those in which heat is absorbed. Such reactions are called endothermic reactions.
Balanced and Unbalanced (skeletal) chemical equations
Such an equation in which the number of atoms of each element on the two sides of the equation are equal is called abalanced chemical equation.
Such an equation in which the number of atoms of different elements (one or more) is not equal on the two sides of the equation is calledunbalanced or skeletal equation.
Types of chemical reactions
Depending upon the nature of reaction, i.e., the type of chemical change taking place, the various chemical reactions have been broadly classified into the following types:
(1) Combination reactions
(2) Decomposition reactions
(3) Displacement reactions
(4) Double displacement reactions
(5) Precipitation reactions
(6) Neutralization reactions
(7) Reduction-oxidation (redox) reactions
Those reactions in which two or more elements or compounds combine together to form a single compound are called combination reactions.
There are three types combination reactions possible depending upon:
(a) Combination between two elements.
(b) Combination between two compounds.
(c)Combination between an element and a compound.
Burning of a magnesium ribbon
2Mg (s) + O2(g) −−−→ 2MgO (s)
Magnesium Oxygen Magnesium oxide
Those reactions in which a single compound breaks down to give two or more simpler substances are called decomposition reactions.
Thus, these reactions are just opposite of composition reactions. The decomposition reaction takes place only when the energy in the form of heat, electricity or light is supplied. Thus, there are three types of decomposition reactions possible. These are:
(a) Decomposition reactions which takes place by absorption of heat. These are called thermal decomposition reactions.
(b) Decomposition reactions which take place when electric current is passed through the compound in the molten state or in aqueous solution. These are called electrolytic decomposition reactions or simply electrolysis.
(c) Decomposition reactions which take place on absorption of light. These are called photo-decomposition reactions or simply photolysis.
Those reactions in which a more active element displaces a less active element from its compound are called displacement reactions.
Double Displacement Reactions
Those reactions in which two different atoms or groups of atoms are exchanged are called double displacement reactions.
Reaction between silver nitrate solution and sodium chloride solution
AgNo3(aq) + NaCl (aq) −−→ AgCl (s) ↓ + NaNO3(aq)
Silver nitrate Sodium Chloride Silver chloride Sodium Nitrate
Those reactions in which aqueous solutions of two compounds on mixing react to form an insoluble compound which separates out as a solid (called precipitate) or such an insoluble compound is formed when a gas is passed through the solution of a compound are called precipitation reactions.
A reaction in which an acid reacts with a base to form salt and water is called neutralization reaction, i.e.,
Acid + Base → Salt + Water
NaOH + HCl → NaCl + H2O
Sodium hydroxide Hydrochloric acid Sodium Chloride Water
Oxidation is defined as a process which involves
(i) Gain of oxygen or (ii) loss of hydrogen
Reduction is defined as a process which involves
(i) Gain of hydrogen or (ii) loss of oxygen
Q1. What is a Chemical Reaction?
Q2. What is a precipitate?
Q3. What is neutralization reaction?