4.1.1 Alkali Metals
From ChemEd Collaborative
Alkali Metals
The element potassium combines violently and spectacularly with water, as shown in Plate l. The flame is due to combustion of hydrogen gas which is given off, and if the excess water is evaporated, the compound potassium hydroxide (KOH) remains behind. Thus the equation for this reaction is
2K(s) + 2H2O(l) → 2KOH(aq) + H2(g) (4.1)
The elements lithium, sodium, rubidium, and cesium also combine violently with water to form hydroxides. The equations for their reactions are
2Li(s) + 2H2O(l) → 2LiOH(aq) + H2 (g)
2Na(s) + 2H2O(l) → 2NaOH(aq) + H2 (g)
2Rb(s) + 2H2O(l) → 2RbOH(aq) + H2 (g)
2Cs(s) + 2H2O(l) → 2CsOH(aq) + H2 (g)
Since potassium and these four elements all react with water in the same way, a general equation may be written:
2M(s) + 2H2O(l) → 2MOH(aq) + H2(g) M = K, Li, Na, Rb, or Cs
The symbol M represents any one of the five elements.
In addition to their behavior when added to water, lithium, sodium, potassium, rubidium, and cesium have a great many other properties in common. All are solids at 0°C and melt below 200°C. Each is silvery in color and has metallic properties such as good conduction of heat and electricity, malleability (the ability to be hammered into sheets), and ductility (the ability to be drawn into wires). The high thermal (heat) conductivity and the relatively low melting point (for a metal) of sodium make it an ideal heat-transfer fluid. It is used to cool certain types of nuclear reactors (liquid-metal fast breeder reactors, LMFBRs) and to cool the valves of high-powered automobile engines for this reason.
Because of their similarities, lithium, sodium, potassium, rubidium, and cesium are grouped together and called the alkali metals. (The term alkali is derived from an Arabic word meaning “ashes.” Compounds of potassium as well as other alkali metals were obtained from wood ashes by, early chemists.) The alkali metals all react directly with oxygen from the atmosphere, forming oxides, M2O:
4M(s) + O2(g) → 2M2O(s) M = Li, Na, K, Rb, or Cs
(Li2O is lithium oxide, Na2O is sodium oxide, etc.) All except lithium react further to form peroxides, M2O2:
2M2O(s) + O2(g) → 2M2O2(s) M = Na, K, Rb, or Cs
(Na2O2 is sodium peroxide, etc.) Potassium, rubidium, and cesium are sufficiently reactive that superoxides (whose general formula is MO2) can be formed:
M2O2(s) + O2(g) → 2MO2(s) M = K, Rb, or Cs
Unless the surface of a sample of an alkali metal is scraped clean, it will appear white instead of having a silvery metallic luster. This is due to the oxide, peroxide, or superoxide coating that forms after a few seconds of exposure to air.
The alkali metals react with most of the other chemical elements as well. For example, all combine directly with hydrogen gas to form com- pounds known as hydrides, MH:
2M(s) + H2(g) → 2MH(s) M = Li, Na, K, Rb, or Cs
They react with sulfur to form sulfides, M2S:
2M(s) + S(g) → M2S(s) M = Li, Na, K, Rb, or Cs
They also react directly with chlorine, forming chlorides,
2M(s) + Cl2(g) → 2MCl(s) M = Li, Na, K, Rb, or Cs (4.2a)
They react with fluorine to form fluorides, MF:
2M(s) + F2(g) → 2MF(s) M = Li, Na, K, Rb, or Cs (4.2b)
They react with bromine to form bromides, MBr:
2M(s) + Br2(g) → 2MBr(s) M = Li, Na, K, Rb, or Cs (4.2c)
Notice that each member of the chemical family of alkali metals has physical and chemical properties very similar to all the others. In most cases all alkali metals behave the same with regard to the formulas of their compounds. The peroxides and superoxides are exceptions to this rule, but formulas for oxides and each of the other types of compounds we have described are identical except for the chemical symbol of each alkali metal.
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