Hydrogen sulfide

Hy­dro­gen sul­fide (H₂S) is a col­or­less gas with the smell of rot­ten eggs. It is heav­ier than hy­dro­gen by its den­si­ty. Hy­dro­gen sul­fide is a dead­ly poi­son for hu­mans and an­i­mals. Even a small amount in the air caus­es headaches and nau­sea, but the worst thing is that it is in­haled for a lengthy pe­ri­od, you cease to de­tect the smell.

How­ev­er, as in the case with oth­er poi­sons, there is a sim­ple an­ti­dote: in cas­es of hy­dro­gen sul­fide poi­son­ing you should sniff a hand­ker­chief soaked in acetic acid, with a piece of lime chlo­ride wrapped in it.

Hy­dro­gen sul­fide is ob­tained by the in­ter­ac­tion of sul­fur with hy­dro­gen at a tem­per­a­ture of 350 °С:

H₂ + S → H₂S↑

This is an ox­i­da­tion-re­duc­tion re­ac­tion, as the de­grees of ox­i­da­tion of the el­e­ments change.

In the lab­o­ra­to­ry, hy­dro­gen sul­fide is ob­tained by re­act­ing iron sul­fide with sul­fu­ric or hy­drochlo­ric acid:

FeS + 2HCl → Fe­Cl₂ + H₂S

This is an ex­change re­ac­tion, as the in­ter­act­ing sub­stances ex­change ions. This process is usu­al­ly car­ried out with Kipp’s ap­pa­ra­tus.

Prop­er­ties of hy­dro­gen sul­fide

In the com­bus­tion of hy­dro­gen sul­fide, sul­fur ox­ide 4 and wa­ter va­por form:

2H₂S + 3О₂ → 2Н₂О + 2SO₂

H₂S burns with a bluish flame, and if you hold an up­turned beaker over this flame, trans­par­ent drops of con­dens­ing wa­ter from the wa­ter va­por will form.

How­ev­er, if the tem­per­a­ture is re­duced slight­ly, this re­ac­tion takes place some­what dif­fer­ent­ly – and a yel­low coat­ing of free sul­fur ap­pears on the walls of the cooled beaker:

2H₂S + О₂ → 2Н₂О + 2S

The in­dus­tri­al method of ob­tain­ing sul­fur is based on this re­ac­tion. When heat­ing a pre­pared gaseous mix­ture of hy­dro­gen sul­fide with oxy­gen, an ex­plo­sion takes place. The re­ac­tion of hy­dro­gen sul­fide and sul­fur ox­ide 4 also makes it pos­si­ble to ob­tain free sul­fur:

2H₂S + SО₂ → 2Н₂О + 3S

Hy­dro­gen sul­fide is sol­u­ble in wa­ter, and three vol­umes of this gas can dis­solve in one vol­ume of wa­ter, form­ing weak and un­sta­ble hy­dro­sul­fu­ric acid (H₂S). This acid is also called hy­dro­gen sul­fide wa­ter. As you can see, the for­mu­la of hy­dro­gen sul­fide gas and hy­dro­sul­fu­ric acid are writ­ten in the same way.

If you add a so­lu­tion of lead salt to hy­dro­sul­fu­ric acid, a black sed­i­ment of lead sul­fide forms:

H₂S + Pb(NO₃)₂ → PbS + 2H­NO₃

This is the qual­i­ta­tive re­ac­tion for de­tect­ing hy­dro­gen sul­fide. It demon­strates the abil­i­ty of hy­dro­sul­fu­ric acid to en­ter into an ex­change re­ac­tion with so­lu­tions of salts. Thus, any sol­u­ble lead salt is a reagent for hy­dro­gen sul­fide. Some oth­er met­al sul­fides also have a char­ac­ter­is­tic col­or, for ex­am­ple: zinc sul­fide (ZnS) is white, cad­mi­um sul­fide (CdS) is yel­low, cop­per sul­fide (CuS) is black, and an­ti­mo­ny sul­fide (Sb₂S₃) is red.

In­ci­den­tal­ly, hy­dro­gen sul­fide is an un­sta­ble gas, and it breaks down al­most com­plete­ly into hy­dro­gen and free sul­fur on heat­ing:

H₂S → Н₂ + S

Hy­dro­gen sul­fide in­ter­acts in­tense­ly with aque­ous so­lu­tions of halo­gens:

H₂S + 4Cl₂ + 4H₂O→ H₂­SO₄ + 8HCl

Hy­dro­gen sul­fide in na­ture and prac­ti­cal ap­pli­ca­tion

Hy­dro­gen sul­fide is part of vol­canic gas­es, nat­u­ral gas and gas­es found in oil fields. There is a great deal of it in nat­u­ral min­er­al wa­ters, for ex­am­ple in the Black Sea it lies at a depth from 150 me­ters and be­low.

Hy­dro­gen sul­fide is used:

• in medicine – treat­ment with hy­dro­gen sul­fide baths and min­er­al wa­ters;
• in in­dus­try – ob­tain­ing sul­fur, sul­fu­ric acid and sul­fides;
• in an­a­lyt­i­cal chem­istry – for the pre­cip­i­ta­tion of sul­fides of heavy met­als, which are usu­al­ly in­sol­u­ble;
• in or­gan­ic syn­the­sis – to ob­tain sul­furous equiv­a­lents of or­gan­ic al­co­hols – mer­cap­tans, and thio­phen – an aro­mat­ic hy­dro­car­bon con­tain­ing sul­fur.

An­oth­er di­rec­tion in sci­ence that has ap­peared re­cent­ly is hy­dro­gen sul­fide en­er­gy. At present se­ri­ous re­search is be­ing con­duct­ed on ob­tain­ing en­er­gy from de­posits of hy­dro­gen sul­fide at the bot­tom of the Black Sea.

The na­ture of the ox­i­da­tion-re­duc­tion re­ac­tions of sul­fur and hy­dro­gen

Hy­dro­gen sul­fide is formed by sul­fur and hy­dro­gen. The re­ac­tion of its for­ma­tion is an ox­i­da­tion-re­duc­tion re­ac­tion:

Н₂⁰ + S⁰→ H₂⁺S²⁻

The process of the in­ter­ac­tion of sul­fur with hy­dro­gen can be eas­i­ly ex­plained by the struc­ture of their atoms: hy­dro­gen holds first place on the pe­ri­od­ic ta­ble, and ac­cord­ing­ly the charge of its atom­ic nu­cle­us is equal to (+1), and 1 elec­tron re­volves around its nu­cle­us. Hy­dro­gen eas­i­ly gives up its elec­tron to atoms of oth­er el­e­ments, turn­ing into a pos­i­tive­ly charged hy­dro­gen atom – a pro­ton.

Н⁰ -1е⁻= Н⁺

Sul­fur holds 16th place on the pe­ri­od ta­ble. So the charge of its nu­cle­us is equal to (+16), and the num­ber of elec­trons in each atom is also 16е-. The po­si­tion of sul­fur in the third pe­ri­od shows that its 16 elec­trons re­volve around the nu­cle­us, form­ing 3 lay­ers, and in the out­er lay­er there are 6 va­lent elec­trons. The num­ber of va­lent elec­trons of sul­fur cor­re­sponds to group VI, where it is lo­cat­ed in the pe­ri­od­ic sys­tem.

So, sul­fur can give up all six of its va­lent elec­trons, as in the case of the for­ma­tion of sul­fur ox­ide (VI):

2S⁰ + 3O₂⁰ → 2S⁺⁶O₃⁻²

Ad­di­tion­al­ly, as a re­sult of the ox­i­da­tion of sul­fur, 4е ̅ can be giv­en by the atom to an­oth­er el­e­ment with the for­ma­tion of sul­fur ox­ide (IV):

S⁰ + О₂⁰ → S⁺₄ O₂⁻²

Sul­fur can also give up two elec­trons with the for­ma­tion of sul­fur chlo­ride (II):

S⁰ + Cl₂⁰ → S⁺² Cl₂⁻

In all three of the above re­ac­tions, sul­fur gives up elec­trons, and ac­cord­ing­ly it ox­i­dizes, but it acts as a re­duc­er for oxy­gen (O) and chlo­rine (Cl) atoms.

How­ev­er, in the case of the for­ma­tion of H₂S, it is the hy­dro­gen atoms that ox­i­dize, as they lose elec­trons, re­duc­ing the out­er en­er­gy lev­el of sul­fur from six elec­trons to eight. As a re­sult, each hy­dro­gen atom in its mol­e­cule be­comes a pro­ton:

Н₂⁰-2е⁻ → 2Н⁺,

and the sul­fur mol­e­cule, on the con­trary, is re­duced, turn­ing into a neg­a­tive­ly charged an­ion (S2):

S⁰ + 2е⁻ → S⁻²

Thus, in the chem­i­cal re­ac­tion of the for­ma­tion of hy­dro­gen sul­fide, it is sul­fur that is the ox­i­diz­er. From the stand­point of sul­fur dis­play­ing dif­fer­ing de­grees of ox­i­da­tion, it is also in­ter­est­ing to see the in­ter­ac­tion of sul­fur ox­ide 4 and hy­dro­gen sul­fide – the re­ac­tion for ob­tain­ing free sul­fur:

2H₂⁺S-²+ S⁺⁴О₂-²→ 2H₂⁺O-²+ 3S⁰

As we can see from the equa­tion of the re­ac­tion, both the ox­i­diz­er and the re­duc­er in it are sul­fur ions, and name­ly two sul­fur an­ions ( 2-) give two of their elec­trons each to the sul­fur atom in the mol­e­cule of sul­fur ox­ide (II), as a re­sult of which all three sul­fur atoms are re­duced to free sul­fur.

S-² - 4е⁻→ 2S⁰ — re­duc­er, ox­i­dizes

S⁺⁴ + 4е⁻→ S⁰ — ox­i­diz­er, re­duces.

Here you'll find safe ex­per­i­ments with oth­er gas­es.