Question

Differentiate between the following energy changes in a chemical change with a suitable example :

photochemical and electrochemical reaction.

Answer 1

Photochemical reaction- Those reactions which take place by absorption of light energy is called photochemical reactions.

• Generally, it takes place by the absorption of ultraviolet light, visible light, or infrared radiation.

Electrochemical reaction- An electrochemical reaction is a process in which electrons flow between a solid electrode and a substance, such as an electrolyte.

• This flow triggers an electric current through the electrodes, causing the reaction to liberate or absorb heat.

Photochemical reaction	Electrochemical reaction
The chemical reaction is initiated with the absorption of light energy.	The chemical reaction is accompanied by the passage of an electric current.
The energy source is sunlight.	The energy source is the electric current.
An example is photosynthesis.	An example is a reaction occurring in an electric cell.

Example of photochemical reaction-

In black and white photography Silver bromide(AgBr) gets decomposed due to light and forms Silver(Ag) and bromine gas(Br).

$\underset{\mathrm{Silver}\mathrm{bromide}}{2\mathrm{AgBr}\left(\mathrm{s}\right)}\stackrel{\mathrm{sunlight}}{\to }\underset{\mathrm{Silver}}{2\mathrm{Ag}\left(\mathrm{s}\right)}+\underset{\mathrm{Bromine}}{{\mathrm{Br}}_2\left(\mathrm{g}\right)}$

Example of electrochemical reaction-

When current is passed through Zinc and water we get Zinc cation on the cathode and hydrogen gas is liberated.

$\underset{\mathrm{Zinc}}{\mathrm{Zn}\left(\mathrm{s}\right)}+\underset{\mathrm{Hydrogen}\mathrm{ion}}{2{\mathrm{H}}^{+}\left(\mathrm{aq}\right)}⟶\underset{\mathrm{Zinc}\mathrm{ion}}{{\mathrm{Zn}}^{+2}\left(\mathrm{aq}\right)}+\underset{\mathrm{Hydrogen}}{{\mathrm{H}}_2\left(\mathrm{g}\right)}$