Sulfuric acid and reactions with it

Sul­fu­ric acid is one of the strong­est diba­sic acids, which has the for­mu­la H₂­SO₄.

As for its phys­i­cal prop­er­ties, sul­fu­ric acid looks like a thick trans­par­ent oily liq­uid with no smell. H₂­SO₄ has found wide use in in­dus­try, and de­pend­ing on the con­cen­tra­tion of sul­fu­ric acid, the so­lu­tion has many dif­fer­ent prop­er­ties and spheres of ap­pli­ca­tion. Sul­fu­ric acid is used in the fol­low­ing cas­es:

• treat­ing met­als;
• treat­ing ores;
• man­u­fac­ture of min­er­al fer­til­iz­ers;
• chem­i­cal syn­the­sis.

His­to­ry of the dis­cov­ery of sul­fu­ric acid

Sul­fu­ric acid has been known to hu­mans since an­cient times, and it was main­ly found in na­ture, for ex­am­ple in vol­canic lakes.

In the 9th cen­tu­ry, the al­chemist from Per­sia Muham­mad Ar-Razzi ob­tained a so­lu­tion of sul­fu­ric acid by the method of burn­ing cop­per and iron sul­fate.

But the method of the Per­sian al­chemist was im­proved four cen­turies lat­er by the Eu­ro­pean sci­en­tist Al­bert Mag­nus.

The mod­ern in­dus­tri­al (con­tact) method of ob­tain­ing sul­fu­ric acid in­volves ox­i­diz­ing sul­fur diox­ide, a gas which forms on the com­bus­tion of sul­fur or sul­fur pyrite. Sul­fur tri­ox­ide forms, and in­ter­acts with wa­ter.

Con­tact sul­fu­ric acid has a con­cen­tra­tion from 92 to 94 per­cent:

2SO₂ + O₂ = 2SO₂;

H₂O + SO₃ = H₂­SO₄.

Phys­i­cal and physi­co-chem­i­cal prop­er­ties of sul­fu­ric acid

H₂­SO₄ mix­es with wa­ter and SO₃ in all pro­por­tions.

In aque­ous so­lu­tions of H₂­SO₄ hy­drates form of the type H₂­SO₄∙nH₂O

The boil­ing point of sul­fu­ric acid de­pends on the lev­el of con­cen­tra­tion of the so­lu­tion and reach­es its max­i­mum at a con­cen­tra­tion of over 98 %.

The best-known com­pound in in­dus­try is the caus­tic com­pound oleum, which is a so­lu­tion of SO₃ in sul­fu­ric acid.

When the con­cen­tra­tion of sul­fur tri­ox­ide in oleum is high­er, the boil­ing point drops.

Chem­i­cal prop­er­ties of sul­fu­ric acid

When heat­ed, con­cen­trat­ed sul­fu­ric acid is a strong ox­i­diz­er, which can ox­i­dize many met­als, with the ex­cep­tion of:

• Gold (Au);
• Plat­inum (Pt);
• Irid­i­um (Ir);
• Rhodi­um (Rh);
• Tan­ta­lum (Та).

When con­cen­trat­ed sul­fu­ric acid ox­i­dizes met­als, it can re­duce to H₂S, S and SO₂.

Ac­tive met­al:

8 Al + 15H₂­SO₄(conc.)→4Al₂(SO₄)₃ + 12H₂O + 3H₂S

Met­al of medi­um ac­tiv­i­ty:

2Cr + 4 H2­SO4(conc.)→ Cr2(SO4)3 + 4 H2O + S

Met­al of low ac­tiv­i­ty

2Bi + 6H₂­SO₄(conc.)→ Bi₂(SO₄)₃ + 6H₂O + 3SO₂

With cold con­cen­trat­ed sul­fu­ric acid, such met­als as iron and alu­minum do not re­act, as they are cov­ered with an ox­ide film. This process is called pas­si­va­tion.

Re­ac­tion of sul­fu­ric acid and H₂O

When H₂­SO₄ is mixed with wa­ter an exother­mic process takes place, i.e. a large amount of heat is re­leased and the so­lu­tion may even boil. When con­duct­ing chem­i­cal ex­per­i­ments, one must al­ways add sul­fu­ric acid to wa­ter, not the oth­er way around.

Sul­fu­ric acid is a strong de­hy­drat­ing sub­stance, and con­cen­trat­ed sul­fu­ric acid forces wa­ter out of var­i­ous com­pounds. It is of­ten used as a dry­ing agent.

Re­ac­tion of sul­fu­ric acid and sug­ar

The affin­i­ty of sul­fu­ric acid for wa­ter can be demon­strat­ed by a clas­sic ex­per­i­ment, by mix­ing con­cen­trat­ed and sug­ar, which is an or­gan­ic com­pound – a car­bo­hy­drate. In or­der to re­move wa­ter from a sub­stance, sul­fu­ric acid is ca­pa­ble of de­stroy­ing mol­e­cules.

To con­duct the ex­per­i­ment, add a few drops of wa­ter to sug­ar and mix, then care­ful­ly add sul­fu­ric acid. Af­ter a short pe­ri­od, one can ob­serve a vi­o­lent re­ac­tion, with the for­ma­tion of car­bon and the re­lease of gas­es, sul­fur diox­ide and car­bon diox­ide.

Sul­fu­ric acid and sug­ar cube:

Re­mem­ber that work­ing with sul­fu­ric acid is very dan­ger­ous with­out ob­serv­ing safe­ty rules, as sul­fu­ric acid is a caus­tic sub­stance that can in­stant­ly leave se­ri­ous burns on the skin.

Here you’ll find safe chem­i­cal ex­per­i­ments with sug­ar

Re­ac­tion of sul­fu­ric acid and zinc

This re­ac­tion is quite pop­u­lar, and are one of the most wide­spread lab­o­ra­to­ry meth­ods for ob­tain­ing hy­dro­gen: if you add zinc gran­ules to di­lut­ed sul­fu­ric acid, the met­al will dis­solve with the re­lease of gas:

Zn + H₂­SO₄ → Zn­SO₄ + H₂

Di­lut­ed sul­fu­ric acid re­acts with met­als which are to the left of hy­dro­gen in the row of ac­tiv­i­ty, ac­cord­ing to the gen­er­al scheme:

Ме + H₂­SO₄(di­lut­ed) → salt + H₂↑

Re­ac­tion of sul­fu­ric acid and bar­i­um

The qual­i­ta­tive re­ac­tion to sul­fu­ric acid and its salt is the re­ac­tion with bar­i­um ions. This re­ac­tion is wide­ly used in quan­ti­ta­tive anal­y­sis, in par­tic­u­lar in gravime­try.

H₂­SO₄ + Ba­Cl₂ → Ba­SO₄ + 2HCl

Zn­SO₄ + Ba­Cl₂ → Ba­SO₄ + Zn­Cl₂

Warn­ing! Don’t try to re­peat these ex­per­i­ments with­out a pro­fes­sion­al su­per­vi­sion!