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farrah sharp Grade: Upto college level

Can somebody explain to me about the negative sign and magnitude for the values of standard reduction potentials for the alkali metals?

8 years ago

Answers : (1)

Badiuddin askIITians.ismu Expert
147 Points

Dear farrah

The alkali metals are strong reducing agents. The standard electrode potentials all lie between -2.7V and -3.0V, indicating a strong tendency to form cations in solution. They can reduce oxygen, chlorine, ammonia and hydrogen. The reaction with oxygen tarnishes the metals in air, so they are stored under oil. They cannot be stored under water because they react with it to produce hydrogen and alkali hydroxides:

2M(s) + 2H2O(l) -------> 2M+(aq) + 2OH-(aq) + H2(g)

eg 2Na(s) + 2H2O(I) ------>2NaOH(aq) + H2(g)

This reaction illustrates the increasing reactivity on descending the Group. Li reacts steadily with water, with effervescence; sodium reacts more violently and can burn with an orange flame; K ignites on contact with water and burns with a lilac flame; Cs sinks in water, and the rapid generation of hydrogen gas under water produces a shock wave that can shatter a glass container.

Na dissolves in liquid ammonia to give a deep blue solution of sodium cations and solvated electrons. This solution is used as a reducing agent. At higher concentrations the colour of the solution changes to bronze and it conducts electricity like a metal.

The chemistry of Li shows some anomalies, as the cation Li+ is so small it polarises anions and so introduces a covalent character to its compounds. Li has a diagonal relationship with magnesium.

      Standard Electrode Potentials/V
Li          -3.04
Na          -2.71
K          -2.92
Rb          -2.92
Cs          -2.92 

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8 years ago
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