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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.
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
Cl2 to I2 (but is
the lowest for F2) Increases from F
to I (as expected)
<0 Change with M
periodic trends ∆H latt ∝ 1 (r+ + r− ) • Keep in mind that ∆Hlatt is only one component of ∆Hf: ∆Hlat...
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