Lecture8_AlkaliMetals_bandw

Formation chmb31h3 of h3o in consideration the

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Unformatted text preview: + e- ∆H = 439 kJ/mol → ∆H = 439 kJ/mol is actually underestimated since it takes only the formation CHMB31H3 of H3O+ in consideration! The Chemistry of Group 1 8 Reduction potentials (cont.) M+ (aq) −∆Hhyd −∆Ha −∆HEI M+ (g) M (g) M (s) Li+ has a high hydration energy (due to very small cation size and high charge-to-radius ratio) which is sufficient to reverse the position of Li (the most positive instead of the least positive reduction potential) All values in kJ/mol CHMB31H3 The Chemistry of Group 1 9 Hydrides (MH) • Synthesis: commonly direct reaction with H2 at elevated temp. • Structure: All are saline (salt- like) with NaCl structure. • Thermodynamics: As the radius of M+ increases ∆Hlatt becomes more positive (because ∆Hlatt ∝ 1/(r++r-)) and as a result ∆Hf becomes more positive (less exothermic reaction). Thus, although ∆Ha and EI1 for Li are the highest, ∆Hlatt is sufficiently negative and compensates this ‘energy cost’ making LiH the most exothermic hydride of the Group 1. • Properties: All hydrides are strong bases and strong reducing reagents, react with water giving H2 and MOH. CHMB31H3 The Chemistry of Group 1 10 Halides (MX) • All halides are known and can obtained by direct reaction M + ½ X2 (there are other ways…i.e. NaOH + HCl and alike) • Thermodynamic data show trend in ∆Hlatt but no clear trend in ∆Hf: • Remember that: ∆H latt ∝ 1 (r+ + r− ) CHMB31H3 The Chemistry of Group 1 11 ∆Hlatt and ∆Hf for the Group 1 Halides Decreases from Cl2 to I2 (but is the lowest for F2) Increases from F to I (as expected) <0 Change with M following the periodic trends ∆H latt ∝ 1 (r+ + r− ) • Keep in mind that ∆Hlatt is only one component of ∆Hf: ∆Hlat...
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