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Chemical Properties Of S-Block Elements

Chemical Properties of S-Block Elements Assignment / Homework Help
From the understanding of physical properties of alkali metals, it can be said that they are highly reactive metals. In this session we shall focus on the chemical properties of alkali metals i.e. how they react with air, oxygen etc. The reactivity of alkali metals increases from Li to Cs.

  • Action of Air:

    Alkali metals lose their lustre very easily upon exposure to air. To prevent the alkali metals from tarnishing, they have to be preserved under paraffin oil or kerosene.

    Lithium readily forms lithium oxide when burnt with oxygen. Sodium forms peroxide while other alkali metals i.e. K, Rb and Cs form superoxide.
  • Reaction with water:

    Alkali metals react with water and form hydroxides along with liberation of hydrogen. This can be given as

    2M + 2H2O --- => 2MOH + H2

    Where M represents any alkali metal, MOH represents the corresponding hydroxide such as lithium hydroxide.

    As we know, the reactivity with water increases as we move down the group evolving more amount of hydrogen. As a result, the hydrogen liberated in the reactions involving the alkali metals K, Rb and Cs catches fire immediately.
  • Reaction with hydrogen:

    Alkali metals react with hydrogen and form ionic hydrides of the form M+ H-. These metal hydrides in turn react with water to give back the hydrogen. These two reactions are as given below:

    2M + H2 --- => 2MH

    MH + H2O --- => MOH + H2

    All metal halides are easily soluble in water, good conductors of electricity in fused state and have high melting and boiling. Due to high lattice energy alkali metal halides are highly stable.
  • Reaction with halogens:

    Alkali metals react with halogens very easily forming halides. The reactivity of alkali metals with halogen increases Li to Cs.
  • Solubility in liquid Ammonia:

    All alkali metals are readily soluble in liquid ammonia. When the concentration of the metal is less in the alkali-metal ammonia solutions, light blue colour solution is formed. As the concentration of the metal increases, the solution turns to copper blue in colour. After that the metals does not dissolve in the solution and solution is said to be saturated with the metal.

    The metal ammonia solution exhibits certain properties such as conductivity, paramagnetism and stability due to presence of metal cations and ammoniated electrons.
  • Reducing property:

    It is the ability of the metal to lose electrons. As the ionization energies of the alkali metals decreases from Li to Cs, the reducing property increases from Li to Cs. However, Li shows an anomalous behaviour and is the strongest reducing agent.

    The reducing power of alkali metals is expressed as oxidation potential. Thus, the oxidation potential increases as the reducing power of an element increases. Lithium, a strong reducing agent has highest oxidation potential.
  • Ability to form alloys:

    Alloys are mixtures of metals. Alkali metals can form alloys with other elements in the same period or with metals in other groups. Alkali metals form amalgams very easily by dissolving in mercury.
  • Ability to form Complexes:

    Alkali metals do not have the ability to form complexes. This is due to large size, low nuclear charge and poor ability to attract electrons. These properties are opposite to the requisites for formation of complexes. As lithium is smallest element in the group, it has the ability to form certain complexes. The complex forming ability decreases as we move down the group.

Solved problems
  • Hydrides are formed when alkali metals react with
    • Water
    • Hydrogen
    • Ammonia
    • None
    Answer: b
  • Which of the following is highly reactive with water
    • Potassium
    • Sodium
    • Rubidium
    • Caesium
    Answer: d
  • Which of the following is the strong reducing agent
    • Lithium
    • Caesium
    • Sodium
    • None
    Answer: a
  • The reducing power of a metal is expressed in terms of
    • Ionization energy
    • Electron affinity
    • Hydration energy
    • Oxidation potential
    Answer: d

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