Physical and chemical characteristic of chlorine and its acidic compounds

Chlo­rine (Cl) is a chem­i­cal el­e­ment with the atom­ic num­ber 17 and an atom­ic mass of 35.453. Chlo­rine in free form is a heavy gas of a yel­lowy-green col­or with a suf­fo­cat­ing, harsh smell.

Ex­is­tence in na­ture and the his­to­ry of its dis­cov­ery

Chlo­rine is con­tained in the Earth’s crust, con­sti­tut­ing 0.013% of its to­tal mass. Chlo­rine is present in sea wa­ter in a sig­nif­i­cant con­cen­tra­tion, in the form of the Cl ion – ap­prox­i­mate­ly 18.8g/l. Chem­i­cal chlo­rine has high ac­tiv­i­ty, and is not en­coun­tered in na­ture in free form. Chlo­rine is present in the fol­low­ing min­er­als: rock salt (NaCl), car­nal­lite (KCl·Mg­Cl₂·6H₂O) and bischof­ite (Mg­Cl₂·6H₂O). It is also found in soil and oth­er rocks.

Chlo­rine was first ob­tained and de­scribed in 1774 by the Swedish chemist Karl Scheele. The sci­en­tist heat­ed hy­drochlo­ric acid with min­er­al py­ro­lusite, and ob­served the re­lease of a yel­low-green gas with an un­pleas­ant smell. Ini­tial­ly Scheele took the new gas to be an ox­ide of hy­drochlo­ric acid and called it “de­phlo­gis­ti­cat­ed hy­drochlo­ric acid”. An­oth­er sci­en­tist, Lavoisi­er, also be­lieved that chlo­rine was an ox­ide of “muria” (hy­drochlo­ric acid). Only the Eng­lish sci­en­tist Humphrey Davy, who tried many times to break down “muria ox­ide” into sim­ple sub­stances, es­tab­lished in 1811 that the poi­sonous green gas was a sim­ple sub­stance that cor­re­spond­ed to a chem­i­cal el­e­ment. Here you’ll find an easy ex­per­i­ment on ob­tain­ing chlo­rine at home.

The chem­i­cal and phys­i­cal prop­er­ties of chlo­rine

In or­di­nary con­di­tions, chlo­rine is a yel­low-green gas, with a den­si­ty ex­ceed­ing the den­si­ty of air by 2.5 times. The melt­ing tem­per­a­ture of chlo­rine in a sol­id state is -100.98 de­grees Cel­sius, and its boil­ing point is -33.97 de­grees Cel­sius.

In a free state, chlo­rine is present in the form of di­atom­ic mol­e­cules Cl₂. Chlo­rine is poor­ly sol­u­ble in wa­ter and high­ly sol­u­ble in many non-po­lar liq­uids.

Chlo­rine en­ters into an in­ter­ac­tion with the ma­jor­i­ty of met­als and many non-met­als. For ex­am­ple, with­out heat­ing chlo­rine en­ters into a re­ac­tion with al­ka­line and al­ka­line earth met­als, and with an­tin­o­my. The equa­tion of the re­ac­tion is:

2Sb + 3Cl₂ = 2S­b­Cl₃

On heat­ing, chlo­rine re­acts with alu­minum:

3Сl₂ + 2Аl = 2А1Сl₃,

and iron:

2Fe + 3Cl₂ = 2Fe­Cl₃.

With hy­dro­gen, chlo­rine in­ter­acts ei­ther on ig­ni­tion, or when a mix­ture of chlo­rine and hy­dro­gen is ex­posed to ul­tra­vi­o­let light. Hy­dro­gen chlo­ride forms as a re­sult:

Н₂ + Сl₂ = 2НСl

A so­lu­tion of hy­dro­gen chlo­ride in wa­ter is called hy­drochlo­ric acid. At room tem­per­a­ture, chlo­rine in­ter­acts with sul­fur and flu­o­rine. On heat­ing, chlo­rine in­ter­acts with phos­pho­rous, ar­senic, boron and oth­er non-met­als. Chlo­rine does not re­act di­rect­ly with car­bon and in­ert gas­es, oxy­gen and ni­tro­gen. With oth­er halo­gens, chlo­rine forms in­ter­halo­gen com­pounds: for ex­am­ple, strong ox­i­diz­ers — the flu­o­rides ClF, ClF₃ and ClF₅.

The re­ac­tion of chlo­rine with oxy­gen — oxy­gen com­pounds of chlo­rine

Chlo­rine does not di­rect­ly re­act with oxy­gen. But a num­ber of com­pounds with this el­e­ment can be ob­tained in­di­rect­ly:

Hypochlor­ous acid HClO

It is a very weak monoba­sic acid, in which chlo­rine has the de­gree of ox­i­da­tion of +1. It ex­ists ex­clu­sive­ly in so­lu­tions. In aque­ous so­lu­tions, hypochlor­ous acid par­tial­ly breaks down into pro­tons and hypochlo­rite an­ions. It is un­sta­ble and grad­u­al­ly breaks down even in di­lut­ed aque­ous so­lu­tions. Hypochlor­ous acid and its salts are hypochlo­rites, strong ox­i­diz­ers. It re­acts with hy­drochlo­ric acid, HCl, form­ing molec­u­lar chlo­rine. Hypochlor­ous acid forms in the dis­so­lu­tion of chlo­rine or chlo­rine ox­ide (I) in wa­ter (a dis­pro­por­tion­ate re­ac­tion). In in­dus­try, hypochlo­rites of cal­ci­um, sodi­um, potas­si­um and lithi­um are man­u­fac­tured by the chlo­ri­na­tion of lime milk and cor­re­spond­ing al­ka­lis. Hypochlor­ous acid and hypochlo­rites break down eas­i­ly with the re­lease of monatom­ic oxy­gen, and so they are wide­ly used for whiten­ing cel­lu­lose and fab­rics, and also for medic­i­nal pur­pos­es.

Chlo­ric acid HClO₃

It is a strong monoba­sic acid, in which chlo­rine has the de­gree of ox­i­da­tion of +5. It does not ex­ist in free form, and in aque­ous so­lu­tions in a con­cen­tra­tion be­low 30% it is quite sta­ble at cold tem­per­a­tures, but in so­lu­tions of high­er con­cen­tra­tion it breaks down. No an­hy­dride of chlo­ric acid is known. When con­cen­trat­ed sul­fu­ric acid re­acts with KClO₃, a yel­low-brown gas with a char­ac­ter­is­tic smell is re­leased — chlo­rine diox­ide ClO₂. This is a very un­sta­ble com­pound, which on heat­ing, im­pact or con­tact with com­bustible sub­stances eas­i­ly breaks down with an ex­plo­sion into chlo­rine and oxy­gen. Chlo­ric acid re­sem­bles HNO₃ by its prop­er­ties, and in a mix­ture with hy­drochlo­ric acid it is a strong ox­i­diz­er (of the same type as aqua re­gia). Chlo­rates, the salts of chlo­ric acid (HCl₃), are strong ox­i­diz­ers. The most im­por­tant chlo­rates are Berthol­let’s salt (KClO₃) and sodi­um chlo­rate (Na­ClO₃), used in the man­u­fac­ture of dyes and match­es, in medicine and fire­works, and in ex­plo­sives and her­bi­cides. The main con­sumers of Na­ClO₃ are the pulp and pa­per and tex­tile in­dus­tries, where the salt is used in the man­u­fac­ture of chlo­rine diox­ide – an ef­fec­tive whiten­ing agent.

Per­chlo­ric acid HClO₄

It is a heavy, thick liq­uid, a monoba­sic acid. It con­tains chlo­rine atoms (Cl) at the high­est de­gree of ox­i­da­tion, is a very strong ox­i­diz­er, and is ex­plo­sive. It is a volatile col­or­less liq­uid, which smokes strong­ly in air, and is monomer­ic in va­por. It is un­sta­ble and re­ac­tive. For per­chlo­ric acid, auto-de­hy­dra­tion is char­ac­ter­is­tic. It mix­es well with wa­ter in any ra­tios. It forms sev­er­al hy­drates. Con­cen­trat­ed so­lu­tions of per­chlo­ric acid have a slight­ly oily con­sis­ten­cy. Aque­ous so­lu­tions of per­chlo­ric acid are sta­ble, and have a low ox­i­da­tion abil­i­ty. Per­chlo­ric acid forms a azeotrop­ic mix­ture with wa­ter, boil­ing at a tem­per­a­ture of 203 de­grees Cel­sius and con­tain­ing 72% HClO₄.