|Molar mass||94.20 g/mol|
|Appearance||pale yellow solid|
>350 °C decomp.
|Solubility in water||Reacts forming KOH|
|Crystal structure||Antifluorite (cubic), cF12|
|Space group||Fm3m, No. 225|
|Tetrahedral (K+); cubic (O2–)|
|Std enthalpy of
|EU Index||Not listed|
|Main hazards||Corrosive, reacts violently with water|
|Other anions||Potassium sulfide|
|Other cations||Lithium oxide
|Related potassium oxides||Potassium peroxide
|Related compounds||Potassium hydroxide|
| (what is: / ?)
Except where noted otherwise, data are given for materials in their standard state (at 25 °C, 100 kPa)
Potassium oxide (K2O) is an ionic compound of potassium and oxygen. This pale yellow solid, the simplest oxide of potassium, is a rarely encountered, highly reactive compound. Some materials of commerce, such as fertilizers and cements, are assayed assuming the percent composition that would be equivalent to K2O.
- K2O2 + 2 K → 2 K2O
Alternatively and more conveniently, K2O is synthesized by heating potassium nitrate with metallic potassium:
- 2 KNO3 + 10 K → 6 K2O + N2
Potassium hydroxide cannot be further dehydrated to the oxide.
Potassium oxide can also be made by heating potassium metabisulfite crystals above 180°C:
- K2S2O5 → K2O + 2SO2
Properties and reactions
K2O crystallises in the antifluorite structure. In this motif the positions of the anions and cations are reversed relative to their positions in CaF2, with potassium ions coordinated to 4 oxide ions and oxide ions coordinated to 8 potassium. K2O is a basic oxide and reacts with water violently to produce the caustic potassium hydroxide. It is deliquescent and will absorb water from the atmosphere, initiating this vigorous reaction.
The chemical formula K2O is used in the N-P-K (nitrogen-phosphorus-potassium) numbers on the labels of fertilizers. Although K2O is the correct formula for potassium oxide, potassium oxide is not used as a fertilizer in these products. Normally, potassium chloride, potassium sulfate, or potassium carbonate is used as a fertilizer source for potassium. The percentage of K2O given on the label only represents the amount of potassium in the fertilizer if it was in the form of potassium oxide. Potassium oxide is about 83% potassium by weight, but potassium chloride, for instance, is only 60% potassium by weight. Potassium chloride provides less potassium than an equal amount of potassium oxide. Thus, if a fertilizer is 30% potassium chloride by weight, its standard potassium rating, based on potassium oxide, would be only 21.7%.
- Zumdahl, Steven S. (2009). Chemical Principles 6th Ed. Houghton Mifflin Company. p. A22. ISBN 0-618-94690-X.
- Holleman, A. F.; Wiberg, E. "Inorganic Chemistry" Academic Press: San Diego, 2001. ISBN 0-12-352651-5.
- Zintl, E.; Harder, A.; Dauth B. (1934), "Gitterstruktur der oxyde, sulfide, selenide und telluride des lithiums, natriums und kaliums", Zeitschrift für Elektrochemie und Angewandte Physikalische Chemie 40: 588–93
- Wells, A.F. (1984) Structural Inorganic Chemistry, Oxford: Clarendon Press. ISBN 0-19-855370-6.