chemical properties of silicon, germanium, tin and lead Structures

Chemical properties Silicon is much more reactive than carbon. At high temperatures, Si combines with O₂, F₂, Cl₂, Br₂, I₂, S₈, N₂, P₄, C and B to give binary compounds. Silicon liberates H₂ from aqueous alkali (equation 1.1), but is insoluble in acids other than a mixture of concentrated HNO₃ and HF.

                                      (1.1)

On descending group 14, the electropositivity and reactivity of the elements increase. In general, Ge behaves in a similar manner to Si, but, being more electropositive, reacts with concentrated HNO₃ (forming GeO₂), and does not react with aqueous alkali. Reactions between Ge and HCl or H₂S yield GeCl₄ or GeS₂ respectively. Although high temperatures are needed for reactions between Sn and O₂ (to give SnO₂) or sulfur (giving SnS₂), the metal reacts readily with halogens to yield SnX₄. Tin is little affected by dilute HCl or H₂SO₄, but reacts with dilute HNO₃ (to give Sn(NO₃)₂ and NH₄NO₃) and with concentrated acids yielding SnCl₂ (from HCl) and SnSO₄ and SO₂ (from H₂SO₄). Hot aqueous alkali oxidizes the metal to Sn(IV) according to equation 1.2.

       (1.2)

A pyrophoric material is spontaneously inflammable. When finely divided, Pb is pyrophoric, but bulk pieces are passivated by coatings of, for example, PbO, and reaction with O₂ in air occurs only above ≈ 900 K. Lead reacts very slowly with dilute mineral acids, slowly evolves H₂ from hot concentrated HCl, and reacts with concentrated HNO₃ to give Pb(NO₃)₂ and oxides of nitrogen. For reactions of Pb with halogens.

Worked example : Reactivity of the group 14 elements with halogens

Write an equation for the reaction that takes place when Si is heated in F₂. The product of the reaction is a gas for which Δ_fH^o(298 K) = ^_1615 kJ mol^-1. Use this value and appropriate data from the Appendices in the book to calculate a value for the Si–F bond enthalpy.

To find the bond enthalpy term, start by writing an equation for the dissociation of gaseous SiF₄ into gaseous atoms, and then set up an appropriate thermochemical cycle that incorporates Δ_fH^o (SiF₄,g).

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