Q.How is chlorine prepared in the laboratory? How does it react with the following?

Chlorine is made in the lab from MnO2 + conc. HCl on heating; it reacts with iron to give FeCl3, with hot conc. NaOH to give a chlorate, and with sodium thiosulphate depending on the amount of Cl2 used.

Laboratory preparation of chlorine:

Chlorine gas is prepared in the laboratory by heating manganese dioxide (MnO2) with concentrated hydrochloric acid (HCl):

MnO2 + 4HCl (conc.) --heat--> MnCl2 + Cl2(g) + 2H2O

Here MnO2 acts as an oxidising agent, oxidising Cl- of HCl to Cl2 gas, while Mn is reduced from +4 to +2.

(Chlorine can also be prepared using KMnO4 + conc. HCl at room temperature, without heating.)

Reactions of chlorine:

1. With iron: Chlorine reacts with iron on heating (dry Cl2 passed over heated iron) to form anhydrous ferric chloride: 2Fe + 3Cl2 --heat--> 2FeCl3
2. With hot, concentrated NaOH: Chlorine undergoes disproportionation with hot and concentrated NaOH, giving chloride and chlorate ions (unlike with cold dilute NaOH, which gives chloride and hypochlorite): 3Cl2 + 6NaOH (hot, conc.) -> 5NaCl + NaClO3 + 3H2O Here chlorine (oxidation state 0) disproportionates into Cl- (-1, in NaCl) and ClO3- (+5, in NaClO3).
3. With sodium thiosulphate (Na2S2O3): The reaction depends on the relative amount of chlorine used:

• With excess/sufficient chlorine (and water), thiosulphate is oxidised all the way to sulphate, with sulfur precipitating and HCl formed: Na2S2O3 + 4Cl2 + 5H2O -> 2NaHSO4 + 8HCl …