Sulfur dioxide ( SO2) reacts with chlorine gas (Cl2) in the presence of moisture to give HCl and H2SO4.
Sulfuric acid (H2SO4) and hydrochloric acid (HCl) are strong acids in aqueous solutions.
SO2 + Cl2 + 2H2O = H2SO4 + 2HCl
Now we are going to study, how to balance this equation step by step.
In SO2, sulfur is at +4 oxidation state. In the right side of the reaction, sulfur of H2SO4 is at +6 oxidation number. So sulfur atom is oxidized from +4 to +6 in the reaction.
In Cl2 molecule, chlorine atom is at 0 oxidation state and in the right side of the reaction, chlorine of HCl molecule is at -1 oxidation state. So chlorine atom is reduced from 0 to -1 in the reaction.
In the earlier section, we knew, sulfur atoms are oxidized while chlorine atoms are reduced. Next thing is, balance number of atoms of oxidizing and reducing.
In the unbalanced equation, there are one sulfur atom in both sides of the reaction and does not need to equal the number of atoms of sulfur.
But, there are two chlorine atoms in the left side of the reaction and only one in the right side. So make two chlorine atoms by adding two HCl.
Total oxidation number change of sulfur is 2 (from +4 to +6).
Oxidation number change of chlorine is 2 (from 0*2 to -1*2).
Both oxidation number changes are 2. Therefore they are cancel out. Now we can write the equation as below.
SO2 + Cl2 + H2O = H2SO4 + 2HCl
Now try to balance hydrogen and oxygen atoms. In the right side, there are four hydrogen atoms and only two in left. So make two water molecules.
Then check balance of oxygen atoms. You can see, oxygen atoms are already balance.
Both are gases at room temperature and highly toxic gases. So you have to do this reaction very safely otherwise it may causes deathly injuries to people.
Both gases have unpleasant smell. So you can notice any kind of leakage if there is unpleasant smell.
SO2 dissolves in water to produce H2SO3 acid. Cl2 dissolves in water to produce HCl and HOCl acids. So both SO2 and Cl2 are acidic compounds.