Demo Notes:
Lithium, Sodium, Potassium Reactivity
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Performing the Demonstration
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Obtain samples of the three metals in the title, a
gram or less of each will suffice
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Ensure that safety procedures are followed: use
safety glasses, gloves and a lab apron, require
students to stand back at least 3 meters
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Turn out the lights so that any flame will be clearly
visible
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Add a very small amount of each metal to the water in
this order: Li, Na, K using a separate 500 mL beaker
for each metal; this will demonstrate the increase in
reactivity as elements farther toward the bottom of
the group are dcemonstrated
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Show the basicity of the solution by adding an
indicator that is clear in neutral solution but
colored in basic solution (good indicators include:
o-Cresolphthalein, Phenolphthalein,
m-Nitrophenol, or Thymolphthalein)
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Notes About What's Going On
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The chemical equations for these reactions, in order
of reactivity and in order of the amount of heat
evolved are as follows:
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2Li + 2H2O -> 2LiOH + H2
+ heat
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2Na + 2H2O -> 2NaOH + H2
+ heat
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2K + 2H2O -> 2KOH + H2 +
heat
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These elements belong to group 1, the alkali metals,
and therefore exhibit a similar reaction upon
immersion in water
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The reactivity of these elements as you go down the
group to higher atomic numbers
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Contrast this with the reactivity order of the
halogens, where under some circumstances,
F>Cl>Br>I.
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Contrast this also with the reactivity of the noble
gases, none of which typically undergoes chemical
reactions
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Note: when Li/Na/K are done reacting they are all +1
cations and have noble gas electron configurations
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This is why we study electron configurations
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The products of these reactions are hydroxide ion
(OH-) and hydrogen gas (H2)
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The hydroxide ion causes the water to be basic
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The hydrogen gas can be lit on fire because the
reaction is exothermic
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As the hydrogen burns, it becomes water:
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Safety
- As always, no one but a trained chemist should perform this demonstration.
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Adding alkali metals to water is very dangerous
because the hydrogen gas that is released can burst
into flame and even explode violently
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Handle the metals carefully and ensure that the bulk
of the metal stays covered with the paraffin oil
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Only add a very small amount of each alkali metal to
the water, and use a large excess of water so that a
complete reaction can be assured
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Cleanup
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Simply neutralize the solutions with 1 M acid and
pour down the drain
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Check for neutral pH using litmus paper or an
appropriate pH indicator solution