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Activity Series - Alkali Metals in Water

Summary

Lithium, sodium and potassium metals are added to water containing phenolphthalein indicator. The resulting metal hydroxide solutions change the indicator from clear to pink. Increasing reactivity of group one metals is demonstrated.

Element Ionization Potential (eV)
Lithium 5.3917
Sodium 5.1391
Potassium 4.3407
Rubidium 4.1771
Cesium 3.8939

Hazards

  • Store all metals under oil to protect from moisture.
  • The metals react vigorously with water such that even water in the air is enough to ignite sodium and potassium.
  • Do not do this experiment on the overhead. It is not safe. There are often microscopic pits in the metal causing the metal to launch.

Chemicals and Solutions

  • Small pieces of Li, Na and K
  • Phenolphthalein solution
  • Hexane

Materials

  • Three crystallizing dishes filled with water
  • Overhead projector
  • Forceps
  • Do not do this demo on the overhead

Procedure

Drop a small piece of Li, Na or K into each dish. The metal will scoot around on the surface of the water, producing bubbles of hydrogen and causing the phenolphthalein indicator to turn pink.

Optional: Use hexane to rinse mineral oil from metal surface. This produces a faster reaction. Be careful of larger chunks, which, due to internal air pockets, may take off like rockets from dish. Lithium reacts the least vigorously.

Potassium is the most reactive, immediately produces purple sparks and flames.

Be sure to use small pea-sized pieces of metal.

\( \ce{2Li_{(s)} + 2H2O_{(l)} -> 2LiOH_{(aq)} + H2_{(g)}} \)

\( \ce{2Na_{(s)} + 2H2O_{(l)} -> 2NaOH_{(aq)} + H2_{(g)}} \)

\( \ce{2K_{(s)} + 2H2O_{(l)} -> 2KOH_{(aq)} + H2_{(g)}} \)

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