Chemical reactions of the elements

Reaction of potassium with air

Potassium is very soft and easily cut. The resulting surface is bright and shiny. However, this surface soon tarnishes because of reaction with oxygen and moisture from the air. If potassium is burned in air, the result is mainly formation of orange potassium superoxide, KO2.

K(s) + O2(g) → KO2(s)

Reaction of potassium with water

Potassium metal reacts very rapidly with water to form a colourless solution of potassium hydroxide (KOH) and hydrogen gas (H2). The resulting solution is basic because of the dissolved hydroxide. The reaction is exothermic. Early in the reaction, the potassium metal becomes so hot that it catches fire and burns with a characteristic pale lilac colour. The reaction is slower than that of rubidium (immediately below potassium in the periodic table), but faster than that of sodium (immediately above potassium in the periodic table).

2K(s) + 2H2O → 2KOH(aq) + H2(g)

Reaction of potassium with the halogens

Potassium metal reacts vigorously with all the halogens to form potassium halides. So, it reacts with fluorine, F2, chlorine, Cl2, bromine, I2, and iodine, I2, to form respectively potassium(I) bromide, KF, potassium(I) chloride, KCl, potassium(I) bromide, KBr, and potassium(I) iodide, KI.

2K(s) + F2(g) → KF(s)

2K(s) + Cl2(g) → KCl(s)

2K(s) + Br2(g) → KBr(s)

2K(s) + I2(g) → KI(s)

Reaction of potassium with acids

Potassium metal dissolves readily in dilute sulphuric acid to form solutions containing the aquated K(I) ion together with hydrogen gas, H2.

2K(s) + H2SO4(aq) → 2K+(aq) + SO42-(aq) + H2(g)

Reaction of potassium with bases

Potassium metal reacts very rapidly with water to form a colourless basic solution of potassium hydroxide (KOH) and hydrogen gas (H2). The reaction continues even when the solution becomes basic. The resulting solution is basic because of the dissolved hydroxide. The reaction is exothermic. Early in the reaction, the potassium metal becomes so hot that it catches fire and burns with a characteristic pale lilac colour. The reaction is slower than that of rubidium (immediately below potassium in the periodic table), but faster than that of sodium (immediately above potassium in the periodic table). As the reaction continus, the concentration of base increases.

2K(s) + 2H2O → 2KOH(aq) + H2(g)

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