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Chemistry of Potassium (Z=19)

Potassium is a group 1 metal, abbreviated as K on the periodic table. In its pure form, potassium has a white-sliver color, but quickly oxidizes upon exposure to air and tarnishing in minutes if it is not stored under oil or grease. Potassium is essential to several aspects of plant, animal, and human life and is thus mined, manufactured, and consumed in huge quantities around the world.

Introduction

The seventh most abundant element, potassium was discovered and isolated in 1807 by Sir Humphry Davy. Important compounds of potassium include potassium hydroxide (used in some drain cleaners), potassium superoxide, \(KO_2\), which is used in respiratory equipment and potassium nitrate, used in fertilizers and pyrotechnics.

Potassium, like sodium, melts below the boiling point of water (63oC) and is less dense than water also. Like most of the alkali metals, potassium compounds impart a characteristic color to flames. In the case of the 19th element, the color is pale lavender.

Like sodium ions, the presence of potassium ions in the body is essential for the correct function of many cells.

Basic Chemical and Physical Properties

Atomic Number 19
Atomic Mass 39.098 g/mol
Electronegativity 0.8
Density 0.862 g/cm3 (at 0o C) (floats on water)
Melting Point 63.65o C
Boiling Point 773.9o C
Atomic Radius 227 pm
Ionic Radius 0.133 (+1)
Isotopes 5
Electronic Shell [Ar] 4s1
1st Ionization Energy 418.8 kJ/mol
Electrode Potential -2.924
Hardness 0.5
Crystal Lattice body-centered cubic
Specific Heat 0.741 J/gK
Heat of Fusion 59.591 J/g
Heat of Vaporization 2075 kJ/g
Electron Configuration 1s22s22p63s23p64s1

Notable Reactions with Phosphorus 

Potassium reacts so violently with water that it bursts into flame. The silvery white metal is very soft and reacts rapidly with the oxygen in air. Its chemical symbol is derived from the Latin word kalium which means "alkali". Its English name is from potash which is the common name for a compound containing it.

Table 1.1.1: Key reactions of Potassium
Reactant Reaction Product
H2 begins slowly at ca 200°C; rapid above 300°C KH
O2 begins slowly with solid; fairly rapid with liquid K2O, K2O2, KO2
H2O extremely vigorous and frequently results in hydrogen–air explosions KOH, H2
C(graphite) 150–400°C KC4, KC8, KC24
CO forms unstable carbonyls (KCO)
NH3 dissolves as K; iron, nickel, and other metals catalyze in gas and liquid phase KNH2
S molten state in liquid ammonia K2S, K2S2, K2S4
F2, Cl2, Br2 violent to explosive KF, KCl, KBr
I2 ignition KI
CO2 occurs readily, but is sometimes explosive CO, C, K2CO3

Potassium in the Environment

Potassium has a 2.6% abundance by mass in the earth's crust and is found mostly in mineral form as part of feldspars (groups of minerals) and clays. Potassium easily leaches out of these minerals over time and thus has a relatively high concentration in sea water as well (.75g/L). Today, most of the world's potassium is mined in Canada, the U.S., and Chile but was originally monopolized by Germany.

Potassium and Living Organisms

Plants, animals and humans all depend on potassium for survival and good health. The element is part of many bodily fluids and assists related functions of the human body. Most notably, potassium aids nerve functions and is found in several cell types (including skeletal cells, smooth muscle cells, endocrine cells, cardiac cells, and central neurons). Plants depend on potassium for healthy growth. Potassium found in animal excretions and dead plants easily binds to clay in the soil they fall on and is thus utilized by plants.  The element helps maintain osmotic pressure and cell size and plays a role in photosynthesis and energy production.

Applications

95% of manufactured potassium is used in fertilizers and the rest is used to produce specific compounds of potassium, such as potassium hydroxide (\(KOH\)), which can then be turned into potassium carbonate (\(K_2CO_3\)). Potassium carbonate is used in glass manufacturing and potassium hydroxide is found in liquid soaps and detergents. Potassium chloride is used in many pharmaceuticals  and other salts of potassium are used in baking, photography, tanning leather, and iodized salt.  In these cases, potassium is utilized for its negative anion.

Potassium can be obtained through various known reactions, all of which require heat treatment:

\[K_2CO_3+2C \overset{\Delta}{\longrightarrow} 3CO+2K \tag{1}\]

\[2KCl+CaC_2 \overset{\Delta}{\longrightarrow} CaCl_2+2C+K \tag{2}\]

\[2KN_3 \overset{\Delta}{\longrightarrow} 3N_2+2K \tag{3}\]

Due to expenses, these processes are not commercially adaptable. Therefore the element is commonly obtained through reduction. Sodium is often combined with \(KCl\), \(KOH\), or \(K_2CO_3\) to produce potassium sodium alloys and in the 1950's the Mine Safety Appliances Company developed a reduction process that yields high purity potassium:

\[KCl+Na \overset{\Delta}{\longrightarrow} K+NaCl \tag{4}\]

The reaction is heated in a special device equipped with a furnace, heat-exchanger tubes, a fractionating column, a \(KCl\) feed, a waste removal system, and a vapor condensing system. Because the reaction attains equilibrium quickly, potassium can be removed continuously as a product in order to shift equilibrium to the right and produce even more potassium in its place.

Alloys of potassium include \(NaK\) (Sodium) and \(KLi\) (Lithium). Both of these alloys produce metals of low vapor pressure and melting points.

References

  1. Chiu, Kuen-Wai, "Potassium." Kirk-Othmer Encyclopedia of Chemical Technology. John Wiley & Sons, Inc. 2005.
  2. Freilich, Mark and Richard Peterson. "Potassium Compounds." Kirk-Othermer Encyclopedia of Chemical Technology. John Wiley & Sons, Inc. 2000.
  3. UCD University Library Subject Guide, Combined Chemical Dictionary. "Potassium." Chem Net Base. <http://ccd.chemnetbase.com/AAA00.ent...no=HSB87#HSB87>

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Problems

  1. Why is Potassium never found pure in nature?
  2. Write out the chemical reaction between potassium and water.
  3. Name 3 uses of potassium.
  4. Where is Potassium on the periodic table. What are a few things you can deduce just from this location?
  5. Name a common alloy of Potassium. What are the beneficial properties of this alloy?

Answers

  1. It is too reactive. Potassium is a very strong reducing agent because of its desire to achieve an inert gas electron configuration (like the other alkali metals). This means that it will easily give up electrons, giving it the ability to reduce numerous other elements.
  2. \(K+H_2O \rightarrow KOH + H_{2(g)}\):  Like other group 1 metals, potassium reacts readily with water to generate hydrogen gas.
  3. Potassium is used in glass making and is found in fertilizers and soaps.
  4. Potassium is in group one, and is the 4th element down in it's column. This tells us that it is an alkali metal. It is very reactive, has a low density, and is a good reducing agent.
  5. Potassium can form an alloy with \(Na\) that has a low vapor pressure and melting point.