↪ A chemical change is a process that involves formation of one or more than one new substance having properties entirely different from the original substance.
↪ Substance that reacts is called the reactant and that forms after chemical reaction is called product.
The following are the general characters of a chemical change:
↪ A chemical change is a permanent change.
↪ A chemical change is accompanied either by energy evolution or absorption.
↪ A chemical change is always conserved un mass.
⁕ Differences between Physical and Chemical Change
|Physical Change||Chemical Change|
|1. It is temporary change and process can be easily reversed.||1. It is permanent and irreversible change.|
|2. No new compound is formed, only the physical state and color may be changed.||2. New compound is formed with entirely different properties.|
|3. No change of mass.||3. Mass is always conserved.|
|4. Energy change is very small.|
4. Energy change is large.
⁕ Chemical Equation
↪ A symbolic representation of a chemical change is called chemical equation.
↪ Reactants and products are denoted by their formulae.
↪ A chemical change is always conserved in mass, total number of atoms of reactants and products must be balanced in the given chemical equation.
↪ A balanced chemical equation is also called as stoichiometric equation.
Suppose a chemical change is given with word equation as:
and the chemical equation of the reaction is:
⁕ Significance of a Chemical Equation
Consider a chemical equation of photosynthesis reaction:
↪ The given equation tells that carbon dioxide gas and water vapor undergoes photosynthesis reaction to give glucose and oxygen gas.
↪ Hence one can identify reactant and products with their physical states.
Consider the equation:
The equation tells that:
6 molecules of CO2 + 6 molecules of H2O gives 1 molecule of C6H12O6 + 6 molecules of O2
or, 6 mol of CO2 + 6 mol of H2O gives 1 mol of C6H12O6 + 6 mol of O2
↪ Molecular mass of a substance expressed in gram is one mole or g. mole
6x44 g of CO2 + 6x18 g of H2O give 180 g of C6H12O6 + 192 g of O2
↪ Hence a chemical equation has its quantitative significance.
⁕ Limitations of Chemical Equation
↪ A chemical equation does not give the concentration of reactants at the start and concentration of product and the end.
↪ It does not give rate of reaction and the time required for the completion of the reaction.
↪ It does not gives conditions required to carry out chemical reaction.
↪ A chemical equation does not give whether the heat is absorbed or evolved in the reaction.
⁕ Removal of defects
In order to make chemical equation more informative the following corrections can be done:
↪ Physical states of components involved in the reaction are denoted by the symbols s, l, g and aq for solid, liquid, gas and aqueous respectively in the equation as:
↪ Reversibility of reaction is denoted by the notation ⇌ in the chemical equation.
↪ Exothermicity and endothermicity are mentioned by notation ∆H = -ve for exothermic reaction and ∆H = +ve for endothermic reaction at product side.
↪ External conditions can be given above and below the arrow indicating chemical equation.