Question

Hydrogen and chloride 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

Studying all the above phenomena we can understand and provide the answer for the question asked. There are several steps included to form hydrochloric acid from using hydrogen and chlorine. Photochemical reactions are said to be the reaction which gets processed in the presence of sunlight or any source of light which provides the reactants with photons.

Complete step by step answer:
We will first understand how hydrochloric acid $(HCl)$ is formed using hydrogen and chlorine;
There are several steps involved in forming the above mentioned acid:
Initiation reaction: chlorine gas in the presence of sunlight forms two molecules of nascent chlorine.
$C{l_2}\xrightarrow{{hv}}2C{l^{}}$
Propagation reaction: now in this step nascent chlorine reacts with hydrogen gas to form one molecule of hydrochloric acid and one hydrogen atom which is highly unstable.
$Cl + {H_2} \to HCl + H$
Now the unstable hydrogen atom reacts with another molecule of chlorine gas to form one molecule of hydrochloric acid and nascent chlorine atom.
$H + C{l_2} \to HCl + Cl$
Termination reaction: the propagation reaction yields two different atoms which are highly unstable, that is hydrogen atom and nascent chlorine atom.
Hydrogen and chlorine reacts with each other to form hydrochloric acid;
Hydrogen atom reacts with chlorine gas to form one molecule of hydrochloric acid and nascent chlorine atom;
nascent chlorine reacts with hydrogen gas to form one molecule of hydrochloric acid and one hydrogen atom.
And these steps repeat themselves until the reactants get exhausted.
Overall reaction:
${H_2} + C{l_2}\xrightarrow{{hv}}2HCl$
it is an exothermic reaction having enthalpy of $- 184.6KJ/2mol$.
This whole process occurs in the presence of sunlight and hence it is called photochemical reaction.

Note:
Electrochemical reactions are those reactions which happen due to the presence of electricity, that is electricity is used to proceed the reaction from the reactants. Several ore and minerals are formed by this process. Polishing and plating of utensil are done by this process