Properties of zinc

Reactions with zinc and its compounds


Zinc is an el­e­ment of the side sub­group of the pe­ri­od­ic ta­ble (in the 4th pe­ri­od and the 12th group (ac­cord­ing to the new clas­si­fi­ca­tion). It is usu­al­ly en­coun­tered in na­ture in the form of com­pounds (the most im­por­tant nat­u­ral min­er­als con­tain­ing zinc in­clude spha­lerite ZnS, smith­sonite Zn­CO₃, zincite ZnO, calamine 2ZnO·SiO₂·H₂O). Al­though the ox­i­da­tion state of zinc in com­pounds is +2 and it is in the 2nd group of the pe­ri­od­ic ta­ble, the met­al (both in free and bond­ed form) dis­plays am­pho­ter­ic prop­er­ties (i.e. de­pend­ing on con­di­tions it may dis­play acidic or base prop­er­ties).

Phys­i­cal prop­er­ties of zinc, how zinc is ob­tained

Metal­lic zinc is a sil­very-gray met­al with a melt­ing tem­per­a­ture of around 419.5 °C (787.2 de­grees °F). At 100 °C (or 212 °F), the met­al is mal­leable. It is rather brit­tle at room tem­per­a­ture.

Metal­lic zinc is ob­tained from en­riched ore (usu­al­ly spha­lerite ZnS is used for these pur­pos­es).

Sphalerite [Flickr/James St. John]

Zinc sul­fide is baked in ovens, chang­ing from a salt to an ox­ide:

2ZnS + 3O₂ = 2ZnO + 2SO₂

When bak­ing ends, zinc is re­duced from its ox­ide. This can be car­ried out by the py­romet­al­lur­gi­cal method (the met­al is re­duced from ox­ide by coal or coke):

ZnO + C = Zn + CO

at 1200-1300 °C (2192-2372 °F) metal­lic zinc and car­bon monox­ide СО

Elec­trol­y­sis of so­lu­tions of zinc salts may also serve as a method for ob­tain­ing the met­al (this is the hy­dromet­al­lur­gi­cal method for ob­tain­ing zinc). Af­ter treat­ing zinc ox­ide with sul­fu­ric acid, elec­trol­y­sis of zinc sul­fate is car­ried out. The fol­low­ing pro­cess­es take place on the cath­ode and an­ode (car­bon elec­trodes), the fol­low­ing pro­cess­es take place:

A⁺: 2H₂O - 4e = 4H⁺ + O₂

K⁻: Zn²⁺ + 2e = Zn⁰

2H₂O + 2e = H₂⁰ + 2OH⁻

The sum­ma­ry equa­tion of elec­trol­y­sis of the so­lu­tion of zinc sul­fate looks as fol­lows (with this re­ac­tion, met­al of high pu­ri­ty can be ob­tained):

Zn­SO₄ + 2H₂O = Zn + O₂ + H₂ + H₂­SO₄

Chem­i­cal prop­er­ties of zinc

As zinc is an el­e­ment of the side sub­group, we may say that it is in­clined to form com­plex­es (it is the cen­tral atom of a com­plex par­ti­cle). By its prop­er­ties, it is an am­pho­ter­ic met­al – i.e. it may re­act both with acids and al­ka­lis. In com­pounds, zinc al­ways dis­plays an ox­i­da­tion state of ⁺².

Prop­er­ties of metal­lic zinc

Zinc bears [Flickr/Matt Brown]

For metal­lic zinc the fol­low­ing re­ac­tions are char­ac­ter­is­tic

  • with oxy­gen (ox­ides are formed):

2Zn + O₂ = 2ZnO the re­ac­tion takes place at a high tem­per­a­ture, at room tem­per­a­ture only a thin ox­ide film may form on the sur­face of the met­al;

  • with sul­fur (sul­fides are formed):

Zn + S = ZnS (in­tense heat­ing is re­quired);

  • with halo­gens (halo­genides are formed):

Zn + Cl₂ = Zn­Cl₂;

Zn + Br₂ = Zn­Br₂ (re­ac­tion is con­duct­ed at high tem­per­a­tures);

  • with phos­pho­rus with for­ma­tion of phos­phides:

Zn + 2P = ZnP₂;

3Zn + 2P = Zn₃P₂ (heat­ing to high tem­per­a­tures is re­quired);

  • with am­mo­nia with (ni­trides are formed):

3Zn + 2NH₃ = Zn₃N₂ + 3H₂ (re­ac­tion takes place at 550-600 °C (or 1022-1112 °F), with molec­u­lar ni­tro­gen N₂ zinc does not re­act);

  • with so­lu­tions of acids:

Zn + 2HCl = Zn­Cl₂ + H₂ (with acids zinc re­acts like oth­er met­als which do not have an am­pho­ter­ic na­ture);

  • with so­lu­tions of al­ka­lis:

Zn + 2NaOH + 2H₂O = Na₂[Zn(OH)4] + H₂

With hy­dro­gen, car­bon, sil­i­con, ni­tro­gen and boron zinc does not re­act di­rect­ly.

Prop­er­ties of zinc ox­ide

Nanoscale-level coating of zinc oxide on top of a copper plate [Flickr/Oregon State University]

Zinc ox­ide ZnO is a hard white sub­stance which also has am­pho­ter­ic prop­er­ties (it re­acts with both acids and al­ka­lis):

  • ZnO + 2HCl = Zn­Cl₂ + H₂O (zinc ox­ides acts as a base);

  • ZnO + 2NaOH = Na₂ZnO₂ + H₂O (sodi­um zin­cate Na₂ZnO₂ forms in the al­loy­ing of zinc ox­ide and sodi­um hy­drox­ide);

  • ZnO + 2NaOH + H₂O = Na₂[Zn(OH)₄] (sodi­um tetrahy­drox­oz­in­cate forms when the re­ac­tion in the so­lu­tion is car­ried out with heat­ing up to 90 °C (194 °F);

  • ZnO + SiO₂ = Zn­SiO₃ (in the al­loy­ing of zinc ox­ide with sil­i­con ox­ides, glassy zinc sil­i­cate forms).

Prop­er­ties of zinc hy­drox­ide

Zinc hydroxide [Wikkimedia]

Zinc hy­drox­ide Zn(OH)₂ prac­ti­cal­ly does not dis­solve in wa­ter. It is a sol­id sub­stance of a white col­or, in­clined to break down when heat­ed. The chem­i­cal prop­er­ties of zinc hy­drox­ide are the fol­low­ing (the com­pound is also am­pho­ter­ic):

  • Zn(OH)₂ = ZnO + H₂O (when heat­ed, zinc hy­drox­ide breaks down into ox­ide and wa­ter);

  • Zn(OH)₂ + H₂­SO₄ = Zn­SO₄ + 2H₂O;

  • Zn(OH)₂ + CO₂ = Zn­CO₃ + H₂O;

  • Zn(OH)₂ + 2NaOH = Na₂ZnO₂ + 2H₂O (sodi­um zin­cate Na₂ZnO₂ forms when the re­ac­tion is car­ried out in a flux);

  • Zn(OН)₂ + 2NaOH = Na₂[Zn(OH)₄] (in the so­lu­tion, sodi­um tetrahy­drox­oz­in­cate forms);

  • Zn(OH)₂ + 4NH3 = Zn(NH₃)₄₂ (an am­mo­nia com­plex of zinc forms in an abun­dance of am­mo­nia).

Click here for mak­ing amaz­ing ex­per­i­ments with zinc and learn­ing its prop­er­ties.

Prop­er­ties of zinc salts

Zinc salts are prone to hy­drol­y­sis, as when dis­solves in wa­ter, zinc ions re­acts with OH⁻ from the wa­ter. The wa­ter al­most does not dis­so­ci­ate into ions, but nev­er­the­less it is in­clined to au­to­pro­tol­y­sis – the for­ma­tion of an equal num­ber of cations and an­ions as a re­sult of the trans­fer of the pro­ton Н⁺ from one mol­e­cule to an­oth­er:

H₂O + H₂O= H₃O⁺ + OH⁻ – hence hy­drox­ide ions OH⁻ ap­pear, which sub­se­quent­ly re­act with Zn²⁺. In full hy­drol­y­sis, in­sol­u­ble zinc hy­drox­ide Zn(OH)₂ forms.

Zn­Cl₂ is zinc chlo­ride (the salt is formed by the weak base Zn(OH)₂ and the strong acid HCl; hy­drol­y­sis takes place on the cation, as this salt has the cation of a weak base)

Zinc chloride hydrate [Wikimedia]
  • Zn­Cl₂ + H₂O = ZnO­HCl + HCl (in the first stage, the main salt forms, zinc hy­drox­ochlo­ride);

  • ZnO­HCl + H₂O = Zn(OH)₂ + HCl (in the sec­ond stage of hy­drol­y­sis, the in­sol­u­ble weak base Zn(OH)₂ and strong acid HCl form; this is why the re­ac­tion of the medi­um in the so­lu­tion of zinc chlo­ride is acidic – the acid formed in hy­drol­y­sis is stronger than the base ob­tained).

Oth­er­wise the prop­er­ties of zinc salts are typ­i­cal:

  • Zn­Cl₂ + 2NaOH = Zn(OH)₂ + 2Na­Cl (zinc hy­drox­ide forms as a white sed­i­ment);

  • Zn­Cl₂ + 2Ag­NO₃ = Zn(NO₃)₂ + 2Ag­Cl (sil­ver chlo­ride forms as a white cot­tage-cheese-like sed­i­ment);

  • Mg + Zn­SO₄ = Mg­SO4 + Zn (the more re­ac­tive met­al mag­ne­sium forces zinc out of so­lu­tions of its salt);

  • Zn­CO₃ = ZnO + CO₂ (when heat­ed, the ther­mic break­down of zinc car­bon­ate takes place);

  • ZnO­HCl + HCl = Zn­Cl₂ + H₂O

Zinc is used wide­ly in medicine as an an­ti­sep­tic and anti-in­flam­ma­to­ry agent. In in­dus­try, al­loys and oil paints are man­u­fac­tured on the ba­sis of zinc. The met­al is used in pro­tect­ing steel from cor­ro­sion (this method for pre­vent­ing met­al dam­age is called zinc plat­ing).