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Alkali Metals

All elements of group I are typical metals. These are referred to as alkali metals since they react with water to form highly alkaline substances. Francium is a radioactive element, and its longest isotope has a half-life of 21 min. So very little is known about this element, but in its properties it resembles with Cs and therefore has been placed in group I.

Physical properties

  1. Melting point and boiling point: All these elements are soft and have low melting point. The melting points invariably decrease in moving down the group from Li to Cs. The boiling point also decreases in the same order as the melting point.
  2. Ionization energy: The first ionization energies for the atoms in this group are appreciably lower than those for any other group in the periodic table. On descending the group from Li to Cs, the size of the atoms increases, the outermost electrons become less strongly held, and so the ionization energy decreases.
  3. Electronegativity: The electronegativity values for the elements in this group are very small. In fact the smallest values of any element. Thus, when these elements react with other elements to form compounds, a large electronegativity difference between the two atoms is probable and ionic bond is formed.
  4. Photoelectric effect and coloration to the flame: Electrons may also be quite readily excited to a higher energy level, for example in the flame test. To perform this test, a sample of the metal chloride, or any salt of the metal moistened with concentrated HCl, is heated on a platinum or nichrome wire in a Bunsen burner flame. In general, chlorides are more volatile than most of other solids. Thus, the color of the flame is crimson red in the case of Li, yellow in the case of Na, violet in the case of K, and almost same in the case of Rb and Cs.
  5. Color of compounds: Color arises because the energy absorbed or emitted in electronic transitions corresponds to a wavelength in the visible region. The compounds are typically white. Any transitions, which do occur, will be of high energy and will appear in the ultraviolet region rather than in the visible region and will be invisible to the human eye.
  6. Lattice enthalpy of alkali metal compounds: Lattice enthalpies (ΔLH°) of alkali metal salts are very high. The lattice enthalpy decreases (in magnitude) down the group from Li to Cs.
    The change in enthalpy when one mole of any crystalline solid is completely separated into its constituent units (ions if the solid is ionic, and molecule if the solid is molecular) in the gaseous form under standard conditions is called lattice enthalpy (ΔLH°). For example, the enthalpy change for the reaction,
    M+X(s) M+(g) + X(g); Δr H = ΔLH°
    is called the lattice enthalpy of M+X(s). As per definition, lattice enthalpy is a positive quantity.
    Compounds of alkali metals are ionic in nature. These compounds consist of cations and anions arranged in space around each other, which are held together by strong coulombic forces. Therefore, lattice enthalpies of alkali metal salts are very high.
    Lattice enthalpy of any salt depends directly on the product of the charges on the cation and anion and inversely on the sum of ionic radii (r+ + r). So, for salt of particular type, the lattice enthalpy will be lower for bigger ions. That is why, the magnitude of the lattice enthalpy decreases in going from Li to Cs.

Chemical properties

  1. Action of air
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  2. Solution of metals in liquid ammonia: If a small amount of an alkali metal is dissolved in liquefied ammonia, the latter becomes light blue in color. If more alkali metal is added in the ammonia, the latter becomes dark blue in color. If more alkali metal is added to the ammonia, a point is reached when a bronze-colored phase separates out and floats on the blue solution. Further addition of alkali metal results in the disappearance of the blue solution, and its complete conversion to bronze solution. The evaporation of ammonia from the bronze solution allows one to recover unchanged alkali metal. This unusual behavior has fascinated chemists since its discovery in 1864. The interpretation is as follows:
    The blue solutions exhibit the following characteristics:
    • Its color, which is independent of the metal.
    • Its density, which is similar to that at pure NH3.
    • Its conductivity, which is in the same range as those of other electrolytes in NH3.
    • Its paramagnetism, which indicates unpaired electrons.
    • Its reversible nature.
    • Its strong reducing nature. This has been interpreted in terms of ionization of alkali metal to form alkali metal cations and electrons which are solvated by ammonia.
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      The dissociation into cation and electron accounts for the electrical conductivity. The dilute solutions thus consist of free electrons (thus showing reducing behavior). Such solutions are metastable and when catalyzed give hydrogen and amide.

Manufacture of sodium carbonate

Sodium carbonate is manufactured by the Solvay or ammonia–soda process.
Principle: When carbon dioxide is passed into a concentrated solution of brine saturated with ammonia, ammonium bicarbonate is produced,
CO2 + H2O H2CO3
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The ammonium bicarbonate then reacts with common salt forming sodium bicarbonate.
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Sodium bicarbonate being slightly soluble (in the presence of sodium ions) gets precipitated. The precipitated sodium bicarbonate is removed by filtration and changed into sodium carbonate by heating.
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The mother liquor remaining after the precipitation of sodium bicarbonate contains ammonium chloride. This is then heated by steam with milk of lime to regenerate ammonia, which can be used as one of the raw materials.
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  • Saturating the ammoniating tank
    In this tank, ammonia gas mixed with a little carbon dioxide gas (from ammonia recovery tower) is bubbled through a 20% sodium chloride solution (brine). Impurities of calcium and magnesium salts present in brine are precipitated as carbonates or hydroxides.
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    The ammonia that escapes absorption in the saturating tank is absorbed by ammonia absorption tower fitted at the top of the saturating tank.
  • Carbonating tower
    In this tower, carbonation of ammoniacal brine is carried out on the principle of countercurrent. The clear ammoniacal brine solution is pumped to the top of the tower which flows downward and meets a current of carbon dioxide (obtained from a lime kiln) introduced from the bottom of the tower at a pressure of 1–2 atmosphere. As a result of this reaction, ammonium chloride and crystals of sodium bicarbonate are formed.
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    These crystals remain suspended in the mother liquor giving rise to thick milky liquid.

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