Reactions of potassium and potassium hydroxide
Main properties of potassium and potassium hydroxide
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.
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).
- 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:
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K + 6NH₃ = [K(NH₃)₆] (the complex only forms in liquid ammonium at a temperature of -50 ᵒC (also -58 ᵒF) – the metal dissolves);
-
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.
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.
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:
- 2KOH + Zn = K₂ZnO₂ + H₂;
- 2KOH + 2Al + 2H₂O = 2KAlO₂ + 3H₂;
- 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.
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.
