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Introduction to Chemical Reactions!



  1. Chemical reactions are used in many ways in our daily
  2. Chemical changes take place by chemical reactions.
  3. Changes in energy in energy are due to the chemical
  4. Some chemical reactions release and absorb energy in the form of heat, light and sound such as a burning of wood in campfire and fireworks are an example of chemical

Chemical reactions: A Chemical Reaction or a chemical change processes in which atoms of one or more substances they react or rearrange together to form products or in other words chemical reactions form new substances by breaking and making new bonds.

  Important  terminologies: Reactants: Reactants are substances which are present at the initial stage or at the beginning of the chemical reactions. Products: Products are the substances which are formed at the final stage or at the end of the chemical reaction. For example: CH4 + 2O2 → CO2 + 2H2O Here CH4 and O2 are the reactants and CO2 and H2O are the products and the arrow represent that it is a forward reaction Forward Reaction: A chemical reaction which goes in the forward direction or the chemical reaction in which reactants react together to yield the products is called a forward reaction. For example:   N2 + 3H2 → 2NH3 Backward Reaction: A Chemical reaction which goes in the backward direction or the chemical reaction in which products yield back the reactants is called the backward reaction. For example: N2 + 3H2 <– 2NH3 Exothermic reactions: In general “exo” means “exit or goes out” and “thermic” means “heat or energy” hence collectively exothermic reactions are the reaction in which heat energy is released and the temperature goes up or a chemical reaction in which heat is given out is termed as an Exothermic Reaction.


For example:

  • All Combustion reactions are exothermic in
  • An example of the exothermic reaction is light due to
  • Respiration
  • Combustion of fuels

Above graphical representation for exothermic reaction shows that energy of reactants is greater than that of products. Endothermic reaction: In general “endo” means “enter or comes in” and “thermic” means “heat or energy” hence collectively endothermic means reactions in which heat energy is absorbed and the temperature goes down are termed as Endothermic Reactions.


For example:

  • Photosynthesis is an Endothermic reaction
  • Reaction of acetic acid and sodium bicarbonate

Above graph for Endothermic reaction shows that energy of products is greater than that of reactants. Evidence of chemical reactions:

  • Color Change
  • Precipitate formation
  • Formation of gas
  • Exothermic and Endothermic Reaction
  • Change in odor
  • Release of energy as heat and light

Difference between Physical and Chemical Changes: Physical changes:

  • A change in the state
  • Solid to liquid
  • Liquid to gas
  • Gas to liquid
  • Atoms/molecules of the substances do not change

  Chemical changes:

  • Atoms and molecules broke down or combine together to form new
  • Properties of new substances formed are different from that of the original

Types of chemical reactions:

  • Combination Reaction (Synthesis Reaction )
  • Decomposition Reactions
  • Single Replacement Reactions
  • Double Replacement Reactions
  • Neutralization Reactions (Acid / Base )
  • Combustion Reactions

A detailed study of the above-mentioned types of chemical reactions. Combination Reaction (Synthesis Reaction): A reaction in which two or more substances (elements or compounds) combine together to form a new substance is called a combination or synthesis reaction. A combination reaction is also known as Formation reaction. In general:   A + B → C For example: H2(g) + Cl2(g)  → 2HCl(g) Mg(s) + Cl2(g) → MgCl2(s)    d Decomposition Reaction: A reaction in which substance is broken down into two or more simpler substances is known as decomposition reaction. A decomposition reaction takes place only when some energy in the form of heat, light and electricity is supplied to the reactants. In general: AB → A + B For example: 2NaHCO3 → 2CO2 + H2O + Na2O ZnCO3(g)  → ZnO(s) + CO(g) e Single Replacement Reaction: In single replacement reaction, one element replaces another element in the compound. The general expression for single replacement reaction is as follows: A + BC → B + AC For example: Fe  + CuSO4 → Cu + FeSO4 2NaBr  + Cl2 → 2NaCl + Br2 f Double Replacement Reactions: In double replacement reactions, a compound reacts with another compound to form two new compound in the product. The general expression for the double replacement reaction is as follows: AB + CD → AD + BC  For example: HCl + NaOH → NaCl + H2O KOH + H2SO4 →  K2SO4 + H2O g   Neutralization Reaction: A neutralization reaction is also known as an acid-base reaction. A chemical reaction between an acid and an alkali (base) is called the neutralization reaction. In general: Acid + base → salt + water For example: NaOH +  HCl → NaCl  + H2O (base)      (acid)    (salt)   (water) 2NaOH +  H2SO4  → Na2SO4 + 2H2SO4   h   Combustion Reaction: The combustion reaction is also called “Burning Reaction”. A combustion reaction is a chemical reaction in which an organic compound (hydrocarbon) reacts with oxygen. In general: CxHx + O2 → CO2 + H2O For example: CH4 + 2O2 → CO2 + 2H2O 2C4H10 + 13O2 → 8CO2 + 10 H2O i Corrosion: It is a process in which the metals are attacked by moisture, acids etc. Examples of corrosion in our daily life are the black coating which is present on the silver articles and copper articles used. Rusting is an example of corrosion in our day to day life. There are a few ways in which metals can be prevented from corroding. They are listed below:

  • Painting the metal articles
  • Powder coating
  • Oiling metals
  • Galvanization


  • Rancidity is the common phenomenon observed in our day to day In general, rancidity is defined as the spoiling of food items when it becomes unsuitable for consumption.
  • Food turns rancid when the fats and oils within them are oxidized and the taste and smell of the food changes. Oxidation of food generates highly reactive molecules in the rancid food which is responsible for the unpleasant odor and flavour.

Rancidity can be prevented using the following methods:

  • Addition of antioxidants to
  • Airtight containers being used for storage purpose of food.
  • Keeping food in the Freezer which basically slows down the rancidity
  • Keeping chips in airtight packets which contain nitrogen