Question

Hydrogen and chlorine combine together to form hydrochloric acid when exposed to sunlight. This type of reactions are called :

• A. photochemical reactions
• B. electrochemical reactions
• C. physical changes
• D. periodic changes

Answer 1

The correct option is A photochemical reactions

Exposure to sunlight initiates the reaction between chlorine and Hydrogen giving rise to hydrochloric acid. Thus, it is called photochemical reaction.
overall reaction: $H_2+C{l}_2\stackrel{hv}{\to }2HCl$
Initiation: $C{l}_2+hv\to 2Cl$
Propagation: $Cl+H_2\to HCl+H$
$H+C{l}_2\to HCl+Cl$
Termination: $2H\to H_2$
$2Cl\to C{l}_2$
$H+Cl\to HCl$
The enthalpy for the overall reaction is -184.6KJ/ 2mols of HCl. This reaction is catalyzed by the light(photochemical reaction) towards the violet end of the visible spectrum.