Reactions of potassium and potassium hydroxide

Main properties of potassium and potassium hydroxide

Potash [Wikimedia]

Potas­si­um is a mem­ber of the first group of the pe­ri­od­ic ta­ble (the el­e­ment is 19ᵗʰ in the pe­ri­od­ic ta­ble). By its phys­i­cal prop­er­ties, metal­lic potas­si­um is a soft met­al of a sil­very white col­or. Like oth­er al­ka­line met­als, potas­si­um is quite re­ac­tive – so it can­not be en­coun­tered in a free state in na­ture. It re­acts read­i­ly with many sub­stances, es­pe­cial­ly with wa­ter (potas­si­um hy­drox­ide forms – “caus­tic potash”). In air, the met­al re­acts very rapid­ly – be­cause of its high re­ac­tiv­i­ty, the met­al is stored un­der a lay­er of kerosene or Vase­line. We can name many dif­fer­ent prop­er­ties and pos­si­ble chem­i­cal re­ac­tions that are char­ac­ter­is­tic for potas­si­um and its com­pounds.

Potassium shell structure [Wikimedia]

Sim­ple sub­stances that metal­lic potas­si­um re­acts with

Metal­lic potas­si­um is a good re­duc­er. It eas­i­ly re­acts with many sim­ple sub­stances:

  • it re­acts vi­o­lent­ly with wa­ter, form­ing an al­ka­li – potas­si­um hy­drox­ide:

2K + 2H₂O = 2KOH + H₂.

  • it burns in air, form­ing the potas­si­um su­per­ox­ide KO₂ (nev­er­the­less, in the main prod­uct potas­si­um per­ox­ide K₂O₂ is present):

K + O₂ = KO₂ (the ox­ide can be ob­tained from the su­per­ox­ide by heat­ing it with metal­lic potas­si­um: KO₂ + K = K₂O).

  • it also re­acts with oth­er chalco­gens (ex­cept oxy­gen):

2K + S = K₂S (in a sim­i­lar way it re­acts with se­le­ni­um and tel­luri­um, to con­duct the re­ac­tion heat­ing up to 100-200 ᵒC (also 212-392 ᵒF is re­quired).

Potassium sulfide [Wikimedia]
  • it forms potas­si­um hy­dride – an ion­ic com­pound of met­al with hy­dro­gen – when heat­ed up to 200-300 ᵒC (also 392-572 ᵒF) with hy­dro­gen:

2K + H₂ = 2KH.

  • it eas­i­ly re­acts with halo­gens (heat­ing is not re­quired to car­ry out the re­ac­tion):

2K + I₂ = 2KI.

2K + Cl₂ = 2KCl.

Click here for in­ter­est­ing ex­per­i­ments with chlo­rine.

  • it forms phos­phides with phos­pho­rus (an in­ert at­mos­phere and heat­ing up to 200 ᵒC (also 392 ᵒF) is re­quired):

3K + P = K₃P.

At room tem­per­a­ture, potas­si­um bare­ly re­acts with gaseous ni­tro­gen (when potas­si­um hy­dride is heat­ed in a cur­rent of ni­tro­gen, the for­ma­tion of K₃N ni­trides is pos­si­ble). With some met­als (for ex­am­ple sodi­um or lead), potas­si­um forms in­ter­metal­lic com­pounds – com­pounds of met­als be­tween each oth­er.

Re­ac­tion of potas­si­um with com­plex sub­stances

Potas­si­um re­acts with am­mo­ni­um both on heat­ing and on cool­ing – but dif­fer­ent sub­stances form:

  1. K + 6NH₃ = [K(NH₃)₆] (the com­plex only forms in liq­uid am­mo­ni­um at a tem­per­a­ture of -50 ᵒC (also -58 ᵒF) – the met­al dis­solves);

  2. 2K + 2NH₃ = 2KN­H₂ + H₂ (with gen­tle heat­ing – 65-105 ᵒC (149-221 ᵒF) – potas­si­um amide forms).

As al­ka­line met­als are in lo­cat­ed in the elec­tro­chem­i­cal se­ries be­fore hy­dro­gen, they re­act with acids by a dis­place­ment re­ac­tion:

K + 2HCl = 2KCl + H₂.

With sul­fur and ni­tric acids (di­lut­ed) potas­si­um re­acts dif­fer­ent­ly, dis­play­ing prom­i­nent re­duc­ing prop­er­ties:

  • 8K + 6H₂­SO₄ = 4K₂­SO₄ + SO₂ + S + 6H₂O;
  • 21K + 26H­NO₃ = 21­KNO₃ + NO + N₂O + N₂ + 13H₂O.
Potassium nitrate [Wikimedia]

With con­cen­trat­ed sul­fu­ric acid the re­ac­tion takes place as fol­lows:

8K + 5H₂­SO₄ = 4K₂­SO₄ + H₂S + 4H₂O.

Metal­lic potas­si­um can be al­loyed with its al­ka­li – then potas­si­um ox­ide forms:

2K + 2KOH = 2K₂O + H₂ (heat­ing takes place at a tem­per­a­ture of around 450 ᵒC or 842 ᵒF).

Re­ac­tion of potas­si­um hy­drox­ide

Potas­si­um hy­drox­ide KOH is a strong al­ka­li that dis­plays strong ba­sic prop­er­ties – ac­cord­ing­ly, re­ac­tions with potas­si­um hy­drox­ide take place ac­cord­ing to a typ­i­cal­ly ba­sic form. It serves as the main raw ma­te­ri­al for ob­tain­ing metal­lic potas­si­um with­out al­loys (elec­trol­y­sis of an al­loy of potas­si­um hy­drox­ide is car­ried out ac­cord­ing to a sum­ma­ry equa­tion):

4KOH = 4K + O₂ + 2H₂O.

Potassium hydroxide [Wikimedia]

Like all oth­er bases, KOH re­acts with acids by a neu­tral­iza­tion re­ac­tion, form­ing salt and wa­ter:

KOH + HCl = KCl + H₂O (with hy­drochlo­ric acid).

KOH + HI = KI + H₂O (with hy­droiod­ic acid).

If an acid is diba­sic (for ex­am­ple H₂­SO4) or more, dif­fer­ent salts can be ob­tained de­pend­ing on the ra­tio of the reagents:

  • KOH + H₂­SO₄ = KHSO₄ + H₂O (with a ra­tio of reagents of 1:1 potas­si­um hy­dro­sul­fate forms);
  • 2KOH + H₂­SO₄ = K₂­SO₄ + H₂O (with a ra­tio of al­ka­li and acid of 2:1 potas­si­um sul­fate forms).

Salts also form in the re­ac­tion of potas­si­um hy­drox­ide with acidic ox­ides:

2KOH + SO₃ = K₂­SO₄ + H₂O (potas­si­um sul­fate).

A re­ac­tion with car­bon diox­ide is pos­si­ble:

2KOH + CO₂ = K₂­CO₃ + H₂O.

With am­pho­ter­ic ox­ides, potas­si­um hy­drox­ide forms dou­ble salts:

  • 2KOH + ZnO = K₂ZnO₂ + H₂O (potas­si­um zin­cate is formed with zinc ox­ide);
  • 2KOH + Al₂O₃ = 2KAlO₂ + H₂O (the prod­uct of re­ac­tion with alu­minum ox­ide is potas­si­um alu­mi­nate).

With am­pho­ter­ic hy­drox­ides, the re­ac­tion may take place in a so­lu­tion (com­plex com­pounds are formed) or in a flux (the prod­uct of re­ac­tion is a dou­ble salt):

  • Be(OH)₂ + 2KOH = K₂BeO₂ + 2H₂O (potas­si­um beryl­late forms);
  • Fe(OH)₃ + KOH = K[Fe(OH)₄] (potas­si­um tetrahy­drox­o­fer­rate (III) forms);
  • Al(OH)₃ + 3KOH = K3[Al(OH)₆] (in an abun­dance of al­ka­li – potas­si­um hex­ahy­drox­oa­lu­mi­nate (III) forms).

Dou­ble and com­plex salts are also formed in the re­ac­tion of potas­si­um hy­drox­ide with am­pho­ter­ic met­als:

  1. 2KOH + Zn = K₂ZnO₂ + H₂;
  2. 2KOH + 2Al + 2H₂O = 2KAlO₂ + 3H₂;
  3. 2KOH + 2Al + 6H₂O = 2K[Al(OH)₄] + 3H₂.

If as a re­sult of the re­ac­tion, an in­sol­u­ble com­pound forms, potas­si­um hy­drox­ide can also re­act with salts:

2KOH + Cu­SO₄ = K₂­SO₄ + Cu(OH)₂ (cop­per (II) hy­drox­ide – a base that is in­sol­u­ble in wa­ter).

Potas­si­um salts be­have in a sim­i­lar way – with oth­er salts the re­ac­tion takes place if an in­sol­u­ble com­pound forms, as in the re­ac­tion of potas­si­um hy­drox­ide with potas­si­um phos­phate: 2K₃PO₄ + 3Ca(OH)₂ = Ca₃(PO₄)₂ + 6KOH.

Tripotassium phosphate [Wikimedia]

Al­ka­lis also re­act with halo­gens ac­cord­ing to the equa­tion 2KOH + Cl₂ = KClO + KCl + H₂O (KClO – potas­si­um hypochlo­rite). This re­ac­tion takes place at a cold tem­per­a­ture. On heat­ing, potas­si­um chlo­rate forms:

6KOH + 3Cl₂ = KClO₃ + 5KCl + 3H₂O.

With phos­pho­rus the fol­low­ing re­ac­tion takes place:

4Р + 3KOH + 3Н₂О = РН₃ + 3KН₂РО₂ (a dis­pro­por­tion­a­tion re­ac­tion takes place – phos­pho­rus both ox­i­dizes and re­duces).

In some cas­es, potas­si­um hy­drox­ide serves as a medi­um for con­duct­ing a re­ac­tion with potas­si­um per­man­ganate:

2KM­nO₄ + K₂­SO₃ + 2KOH = K₂­SO₄ + 2K₂M­nO₄ + H₂O (potas­si­um man­ganate with the for­mu­la K₂M­nO₄ can only be ob­tained in the pres­ence of an al­ka­li).

Re­ac­tions of metal­lic potas­si­um and its hy­drox­ide with or­gan­ic com­pounds

Met­al is ca­pa­ble of re­act­ing with some or­gan­ic com­pounds. For ex­am­ple, the ba­sis of the Wurtz syn­the­sis is the re­ac­tion be­tween haloalkenes and al­ka­line met­als. With this re­ac­tion, we can ob­tain longer chains of hy­dro­car­bons from short­er ones:

2K + 2CH₃­Cl = C₂H₆ (CH₃-CH₃) + 2KCl (this is an ex­am­ple of the sym­met­ri­cal Wurtz syn­the­sis, as the same haloalkene is used as the reagent – ac­cord­ing­ly, the prod­uct has a sym­met­ri­cal struc­ture).

When dif­fer­ent haloalka­nes are “linked” to­geth­er the prod­ucts are dif­fer­ent – the Wurtz syn­the­sis can take place both sym­met­ri­cal­ly and asym­met­ri­cal­ly):

6K + 3CH₃­Cl + 3C₂H₅­Cl = CH₃-CH₃ + CH₃-C₂H₅ (asym­met­ric prod­uct) + C₂H₅-C₂H₅ + 6KCl.

With al­co­hols, metal­lic potas­si­um re­acts with the for­ma­tion of al­co­ho­lates:

2C₂H₅OH + 2K = 2C₂H₅OK + H₂ (potas­si­um ethy­late).

Potas­si­um hy­drox­ide has found wide ap­pli­ca­tion in re­ac­tions of nu­cle­ophilic sub­sti­tu­tion and elim­i­na­tion of halo­gen de­riv­a­tives:

  • with an aque­ous so­lu­tion of potas­si­um hy­drox­ide, a nu­cle­ophilic sub­sti­tu­tion takes place, when an atom of halo­gen in a halo­gen de­riv­a­tive is sub­sti­tut­ed to the OH-group:

CH₃-CH₂-Br + KOH(aque­ous) = CH₃-CH₂-OH + KBr (ethyl al­co­hol is ob­tained from bromic ether);

  • with an al­co­hol so­lu­tion of al­ka­li, the re­ac­tion takes place dif­fer­ent­ly: ow­ing to the break­ing off of atoms of the halo­gen and hy­dro­gen from neigh­bor­ing (fre­quent­ly) atoms of car­bon, an alkene is formed – a hy­dro­car­bon with a dou­ble bond:

CH₃-CH₂-Br + KOH(al­co­hol) = CH₂=CH₂ + KBr + H₂O (eth­yl­ene forms). This is an elim­i­na­tion re­ac­tion.

Potas­si­um has found wide ap­pli­ca­tion in or­gan­ic syn­the­sis, for the man­u­fac­ture of fer­til­iz­ers and in­su­la­tion ma­te­ri­als. Potas­si­um com­pounds (main­ly hy­drox­ide and salts) are used in elec­tro­plat­ing, catal­y­sis, man­u­fac­ture of food prod­ucts, de­ter­gent bases and dyes.