Periodic Activity of Metals continued
3
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Tips
• We strongly recommend the purchase of small quantities only of sodium and lithium metal. Purchase only the
amounts that will be used in one academic year. Sodium and lithium are available in small, precut pieces that are
suitable for demonstrations (Flinn Catalog Nos. S0329 and L0057, respectively).
• We do not recommend the use of potassium in the high school science laboratory. Potassium is considerably more water-
reactive than sodium and is a serious fire and explosion hazard. There is a significant and often undetectable explosion
risk because of the propensity of potassium to form a superoxide (peroxide) coating on its surface. Potassium reacts with
oxygen in air to form a coating of yellow potassium superoxide (KO
2
), even when the metal is stored under dry mineral
oil. Old pieces of potassium are thus extremely dangerous. When the metal is cut, the pressure of the knife may touch off
a violent, uncontrollable, explosive reaction between the superoxide coating and the underlying metal.
• Calcium metal must be reasonably fresh to react with water. Old (dull) calcium metal will not react with water.
• The reactions of sodium and lithium with water may be quite vigorous—we recommend using tall-form (Berzelius)
beakers to contain any molten metal pieces that may splatter. Do NOT scale up this demonstration.
• The use of a ChemCam
™
video camera will make it easier for students to observe the appearance and properties of the
metal pieces.
• Demonstrate the softness of lithium and sodium by showing how the metal pieces can be cut with a dry spatula or knife.
• In ranking the metals in order of their activity, it is easier to begin with pairwise comparisons. Which is more active—
sodium or lithium? Calcium or magnesium? Magnesium or sodium?
• Is the activity of metals related to their hardness? Density? The answer, a firm “maybe,” depends on the comparisons
being made. The alkali metals as a group are softer and less dense than their nearest alkaline earth metal neighbors, and
also more reactive. Within the group of alkali metals, however, the opposite trend is observed. Lithium is less dense but
also less reactive than sodium.
Discussion
Sodium reacts with water to form hydrogen gas and sodium hydroxide, according to the following balanced chemical equation.
2Na(s) + 2H
2
O(l) → H
2
(g) + 2NaOH(aq) + Heat
As sodium metal reacts with water, a great deal of heat is generated. The sodium melts and seems to float or bob on the water
surface. The oxide coating that may have dulled the metal disappears and sodium’s silvery gray, metallic character is more
apparent. The evolution of hydrogen gas is evident in the production of a white smoke, which pops and ignites as it is heated
above its flash point. The formation of sodium hydroxide, a strong base, is inferred from the color change observed with
phenolphthalein, an acid–base indicator. Phenolphthalein is colorless in neutral or slightly basic solutions (pH < 8) and red in
more basic solutions (pH >10). Between pH 8 and 10 phenolphthalein appears various shades of pink.
The balanced chemical equations for reactions of other active metals with water are given below.
2Li(s) + 2H
2
O(l) → H
2
(g) + 2LiOH(aq)
Ca(s) + 2H
2
O(l) → H
2
(g) + Ca(OH)
2
(aq)
Of the four metals tested, sodium is the most active and magnesium is the least active. Magnesium does not react with water
under these conditions (it may react slightly in hot water). The order of metal reactivity is Na > Li, Ca >> Mg. Periodic trends
in the activity of metals are generally attributed to differences in their ionization energies. The activity of metals increases as
the value of their first ionization energy decreases. Within a vertical column (group or family) of elements in the periodic table,
ionization energy decreases from top to bottom. As a result, metal activity increases going down a column in the periodic table
(K > Na > Li; Ca > Mg). Across a horizontal row (period or series) in the periodic table, ionization energy also increases from
left to right. As a result, metal activity decreases from left to right across a row in the periodic table (Na >> Mg; K >> Ca).