Theory - To study Exothermic and Endothermic Reaction, Chemistry, Science - Notes | Study Science Class 9 - Class 9

Objective: 

To study exothermic and endothermic reaction.

Theory:

1. Exothermic reaction : An exothermic reaction is a chemical reaction that releases energy in the form of light or heat.It gives net energy to its surroundings. That is, the energy needed to initiate the reaction is less than the energy that is subsequently released.

reactants → products + energy

Examples of exothermic reactions:

• Calcium oxide reacts vigorously with water to produce slaked lime(Calcium hydroxide) used in white washing releasing large amount of heat.
• Burning of natural gas: when methane reacts with oxygen it releases vast amount of heat and cabon oxide and water.
                                                         CH₄+2O₂------->CO₂+H₂O 
• Respiration: During digestion, food is broken down into simpler substances. For example, rice, potatoes and bread contain carbohydrates. These carbohydrates are broken down to form glucose. This glucose combines with oxygen in the cells of our body and provides energy. The special name of this reaction is respiration.

1. Endothermic reaction : An endothermic reaction is any chemical reaction that absorbs heat from its environment.

reactants + energy → products

Example of endothermic reactions :

• Melting of ice: Ice takes heat from the surroundings and turns in to liquid water. 
• Decomposition of Calcium Carbonate: When limestone is heated at above 900 ºC it decomposes into quick lime and releases carbondioxide.
                                           CaCO₃(limestone)-------------> CaO(quick lime) + CO₂
• SilverChloride turns grey when exposed to sunlight.Due to decomposition of silver chloride into silver and chlorine
                                                          2AgCl ---------------->2Ag+Cl₂