Unit 9: Chemical Properties of s & p-block Metals
Exploring how the most reactive metals interact with air, water, and acids.
9.9 s-block Metals (Na, Ca): Reactivity with Air, Water, Acids
The s-block metals (Group 1 and Group 2) are defined by their extreme electropositivity. They hold their valence electrons very loosely, making them the most reactive metals. This reactivity increases down each group as the valence electrons are further from the nucleus and more shielded. In all reactions, these metals are oxidized (lose electrons).
Reaction with Air (Oxygen)
Most s-block metals react rapidly when exposed to air, quickly forming a dull oxide layer that tarnishes their shiny surface. The reactions are highly exothermic and can be violent if the metal is ignited. To prevent this, reactive metals like sodium are stored under oil.
- Sodium (Na): Burns in air with a yellow flame to form a mixture of
sodium oxide and sodium peroxide.
$4Na(s) + O_2(g) \rightarrow 2Na_2O(s)$ (Sodium Oxide)
$2Na(s) + O_2(g) \rightarrow Na_2O_2(s)$ (Sodium Peroxide) - Calcium (Ca): Burns in air with a red-orange flame to form calcium
oxide.
$2Ca(s) + O_2(g) \rightarrow 2CaO(s)$ (Calcium Oxide)
Reaction with Water
Group 1 metals and most Group 2 metals (from Ca down) react vigorously with cold water to produce a metal hydroxide and hydrogen gas. The metal fizzes as H₂ gas is evolved.
- Sodium (Na): Reacts explosively. It darts across the water surface,
melting into a silvery ball, producing sodium hydroxide and hydrogen gas.
$2Na(s) + 2H_2O(l) \rightarrow 2NaOH(aq) + H_2(g)$ - Calcium (Ca): Reacts steadily, producing calcium hydroxide and hydrogen
gas. The resulting solution turns cloudy because calcium hydroxide, $Ca(OH)_2$, is only
sparingly soluble in water and precipitates out.
$Ca(s) + 2H_2O(l) \rightarrow Ca(OH)_2(aq/s) + H_2(g)$
Reaction with Acids
The reaction with dilute acids is even more vigorous and dangerously explosive for Group 1 metals. The metal is oxidized, and the H⁺ ions from the acid are reduced to hydrogen gas, forming a salt.
- Sodium (Na) with HCl: $2Na(s) + 2HCl(aq) \rightarrow 2NaCl(aq) + H_2(g)$
- Calcium (Ca) with HCl: $Ca(s) + 2HCl(aq) \rightarrow CaCl_2(aq) + H_2(g)$
Solved Examples:
-
Why is potassium's reaction with water more violent than
sodium's?
Solution: Potassium is below sodium in Group 1. It has a larger atomic radius and more electron shielding, so its valence electron is lost more easily. This makes potassium more reactive than sodium, leading to a more violent reaction. -
Write a balanced equation for the reaction of magnesium (Mg) with
steam.
Solution: Magnesium is less reactive than calcium and reacts only with steam (not cold water) to form magnesium oxide and hydrogen.
$Mg(s) + H_2O(g) \rightarrow MgO(s) + H_2(g)$ -
What would you observe when a piece of calcium is dropped into a beaker of
water containing universal indicator?
Solution: You would observe fizzing as hydrogen gas is produced. The solution would turn blue or purple because the product, calcium hydroxide ($Ca(OH)_2$), is an alkali. A white precipitate of $Ca(OH)_2$ would also form.
9.10 p-block Metals (Al, Sn, Pb): Reactivity with Air, Water, Acids
The p-block metals are generally less reactive than the s-block metals because they are less electropositive. Their reactivity varies within the block, with aluminum being the most reactive of the common examples.
Reaction with Air and Water
Aluminum (Al) is a paradox. Based on its position in the periodic table, it
should be a very reactive metal. However, it is highly resistant to corrosion. This is
because a freshly exposed surface of aluminum reacts instantly with oxygen in the air to
form a very thin, tough, and transparent layer of aluminum oxide
($Al_2O_3$).
$4Al(s) + 3O_2(g) \rightarrow 2Al_2O_3(s)$
This oxide layer is non-porous and chemically inert, so it protects the metal underneath from any further reaction with air or water. If this layer is scratched off, a new one forms immediately. This property of passivation makes aluminum useful for window frames, drink cans, and aircraft. Tin (Sn) and Lead (Pb) also form protective oxide layers but are much less reactive than aluminum to begin with.
Reaction with Acids
The protective oxide layer can be removed by strong acids, after which the metal will react.
- Aluminum (Al): Reacts readily with dilute acids like HCl once the oxide
layer is penetrated, producing aluminum salts and hydrogen gas.
$2Al(s) + 6HCl(aq) \rightarrow 2AlCl_3(aq) + 3H_2(g)$ - Tin (Sn) and Lead (Pb): React slowly with warm dilute acids to form
salts and hydrogen.
$Sn(s) + 2HCl(aq) \rightarrow SnCl_2(aq) + H_2(g)$
$Pb(s) + 2H_2SO_4(conc) \rightarrow PbSO_4(s) + SO_2(g) + 2H_2O(l)$ (Note: Lead reacts with concentrated, not dilute, sulfuric acid, and forms an insoluble layer of $PbSO_4$ that stops the reaction).
Reaction with Alkalis
A unique property of aluminum is its ability to react with strong alkalis, such as sodium
hydroxide. This is because aluminum oxide is amphoteric (can act as both an
acid and a base).
$2Al(s) + 2NaOH(aq) + 2H_2O(l) \rightarrow 2NaAlO_2(aq) + 3H_2(g)$ (Sodium Aluminate)
Solved Examples:
-
Why are aluminum saucepans safe for cooking, despite aluminum being a
reactive metal?
Solution: The surface of the saucepan is protected by a tough, unreactive, and non-porous layer of aluminum oxide ($Al_2O_3$), which prevents the aluminum metal from reacting with food or water. -
A student places a strip of aluminum in dilute nitric acid and observes no
initial reaction. After a few minutes, bubbles start to form. Explain these
observations.
Solution: Initially, there is no reaction because the aluminum is protected by its inert oxide layer. The nitric acid slowly dissolves this protective layer. Once the layer is removed, the underlying aluminum metal is exposed and reacts with the acid to produce hydrogen gas bubbles. -
Write an equation for the reaction between tin and dilute sulfuric
acid.
Solution: $Sn(s) + H_2SO_4(aq) \rightarrow SnSO_4(aq) + H_2(g)$ -
Why would you not use a drain cleaner containing sodium hydroxide in an
aluminum sink?
Solution: Aluminum reacts with strong alkalis like sodium hydroxide. The drain cleaner would react with and dissolve the aluminum sink, causing damage and producing flammable hydrogen gas.