Electrochemical reactions

        ELECTROCHEMICAL REACTIONS 

Chemical reactions in which electrical energy is consumed or produced are known as electrochemical reactions.

Electrolysis

The process in which a substance undergoes decomposition by the absorption of electrical energy is known as electrolysis.

Electrochemical reactions 

Observe the figure below.

Arrangements in which electricity is produced as a result of a chemical reaction between the acid and the metals kept immersed in it. Such arrangements which produce electricity as a result of chemical reaction are known as Electrochemical cells.

Photochemical reactions

         PHOTOCHEMICAL REACTIONS

Chemical reactions which liberate or absorb light energy are known as photochemical reactions.

Photosynthesis 

Photosynthesis is a chemical reaction that is responsible for sustaining life on earth. In this reaction, plants produce glucose by absorbing light. Note the chemical equation of this reaction.

Water + Carbon dioxide + Light —>Glucose + Oxygen

          6H2O + 6CO2 + Light —-> C6H12O6 + 6O2

The glucose thus formed is stored by plants in the form

of starch.

Colour change of silver bromide

Take some silver bromide in two dry watch glasses. Wrap one of them with a black paper. Keep both of them in sunlight for some time.

Observations:

Colour of silver bromide faded in watch glass which is placed in sunlight. This was due to the deposition of silver produced as a result of the decomposition of silver bromide by the absorption of light.

Thermochemical reactions

         THERMOCHEMICAL REACTIONS

Chemical reactions which emit or absorb heat energy is called thermochemical reactions.

Exothermic reactions

Chemical reactions which liberate heat are called exothermic reactions.

Example, add 5 mL dilute hydrochloric acid to a piece of magnesium taken in a test tube.

Observations:

As a result of reaction, Hydrogen gas liberated and the bottom of test tube got warm.

Magnesium + Hydrochloric acid o Magnesium chloride + Hydrogen + heat

                Mg+2HCl—->MgCl2 +H2 +heat

Endothermic reactions  

Chemical reactions which absorb heat are called endothermic reactions.

Example, take a few crystals of potassium permanganate in a dry test tube. Heat the test tube. Bring a glowing incense stick to the mouth of the test tube. 

Observations:

Colour of potassium permanganate faded and the incense stick got lit.