5.2 Reactivity with Oxygen, Water and Dilute Acids | Group 2 Elements | Chemistry 12

Group 2 elements (alkaline earth metals) show a clear trend of increasing reactivity down the group. This is due to decreasing ionization energies and increasing atomic radii as you descend from Beryllium ( $B e$ ) to Barium ( $B a$ ).

Reaction with Oxygen

All Group 2 metals burn in oxygen to form ionic oxides with the general formula $M O$ :

$2 M (s) + O_{2} (g) \to 2 M O (s)$

Metal	Product	Equation
$B e$	$B e O$	$2 B e + O_{2} \to 2 B e O$
$M g$	$M g O$	$2 M g + O_{2} \to 2 M g O$
$C a$	$C a O$	$2 C a + O_{2} \to 2 C a O$
$B a$	$B a O$	$2 B a + O_{2} \to 2 B a O$

Nature of Group 2 Oxides

All Group 2 oxides are basic — they react with acids to form salts and water.
Exception: $B e O$ is amphoteric — it reacts with both acids and bases due to the very high charge density of the small $B e^{2 +}$ ion.

Reaction of $B e O$ with acid: $B e O (s) + 2 H C l (a q) \to B e C l_{2} (a q) + H_{2} O (l)$

Reaction of $B e O$ with base (forming beryllate): $B e O (s) + 2 N a O H (a q) \to N a_{2} B e O_{2} (a q) + H_{2} O (l)$

Reaction of other Group 2 oxides with acid (basic nature): $M g O (s) + 2 H C l (a q) \to M g C l_{2} (a q) + H_{2} O (l)$ $C a O (s) + 2 H C l (a q) \to C a C l_{2} (a q) + H_{2} O (l)$

The basicity of Group 2 oxides increases down the group as the metal becomes more electropositive.

Reaction with Water

Reactivity with water increases down the group:

Metal	Reaction with Water	Products
$B e$	No reaction (even with steam)	—
$M g$	Very slow with cold water; rapid with steam	$M g O + H_{2}$ (steam); $M g (O H)_{2} + H_{2}$ (hot water)
$C a$	Reacts readily with cold water	$C a (O H)_{2} + H_{2}$
$S r$	Reacts vigorously with cold water	$S r (O H)_{2} + H_{2}$
$B a$	Reacts very vigorously with cold water	$B a (O H)_{2} + H_{2}$

Key equations:

$M g (s) + H_{2} O (g) steam M g O (s) + H_{2} (g)$

$C a (s) + 2 H_{2} O (l) \to C a (O H)_{2} (a q) + H_{2} (g)$

$B a (s) + 2 H_{2} O (l) \to B a (O H)_{2} (a q) + H_{2} (g)$

Why does $B e$ not react with water? Beryllium is protected by a tough, adherent layer of $B e O$ on its surface, which prevents water from reaching the metal.

Why does $M g$ react with steam but not cold water? The $M g O$ layer on magnesium's surface is impermeable at room temperature. Steam provides enough energy to break through this barrier.

Reaction with Dilute Acids

All Group 2 metals react with dilute acids ( $H C l$ or $H_{2} S O_{4}$ ) to produce a metal salt and hydrogen gas:

$M (s) + 2 H C l (a q) \to M C l_{2} (a q) + H_{2} (g)$

Examples:

$M g (s) + 2 H C l (a q) \to M g C l_{2} (a q) + H_{2} (g)$

$C a (s) + 2 H C l (a q) \to C a C l_{2} (a q) + H_{2} (g)$

$M g (s) + H_{2} S O_{4} (a q) \to M g S O_{4} (a q) + H_{2} (g)$

Anomalous Behaviour of Beryllium with Dilute Acids

Although $B e$ has a very negative electrode potential (suggesting high reactivity), it reacts slowly with dilute acids at room temperature. This is because:

A thin, tough layer of $B e O$ forms on the surface of beryllium.
This protective oxide layer acts as a kinetic barrier, preventing the acid from reaching the metal.
On heating or with concentrated acids, the oxide layer dissolves and reaction proceeds.

$B e (s) + 2 H C l (a q) \to B e C l_{2} (a q) + H_{2} (g)$

Trend: Reactivity with dilute acids increases down the group: $B e < M g < C a < S r < B a$

Summary of Reactivity Trends

Reaction	$B e$	$M g$	$C a$	$S r$	$B a$
With $O_{2}$	Slow (oxide layer)	Burns brightly	Burns	Burns	Burns
With cold water	No reaction	Very slow	Vigorous	More vigorous	Very vigorous
With steam	No reaction	Rapid	—	—	—
With dilute $H C l$	Slow (oxide layer)	Moderate	Vigorous	More vigorous	Very vigorous

Reason for increasing reactivity down the group:

Atomic radius increases → outer electrons are further from nucleus
Ionization energy decreases → electrons are more easily lost
Shielding effect increases → nuclear attraction on valence electrons decreases
Therefore, $B e^{2 +}$ is harder to form than $B a^{2 +}$

Reaction with Oxygen

All Group 2 metals burn in oxygen to form ionic oxides with the general formula $M O$ :

$2 M (s) + O_{2} (g) \to 2 M O (s)$

Metal	Product	Equation
$B e$	$B e O$	$2 B e + O_{2} \to 2 B e O$
$M g$	$M g O$	$2 M g + O_{2} \to 2 M g O$
$C a$	$C a O$	$2 C a + O_{2} \to 2 C a O$
$B a$	$B a O$	$2 B a + O_{2} \to 2 B a O$

Nature of Group 2 Oxides

All Group 2 oxides are basic — they react with acids to form salts and water.
Exception: $B e O$ is amphoteric — it reacts with both acids and bases due to the very high charge density of the small $B e^{2 +}$ ion.

Reaction of $B e O$ with acid: $B e O (s) + 2 H C l (a q) \to B e C l_{2} (a q) + H_{2} O (l)$

Reaction of $B e O$ with base (forming beryllate): $B e O (s) + 2 N a O H (a q) \to N a_{2} B e O_{2} (a q) + H_{2} O (l)$

Reaction of other Group 2 oxides with acid (basic nature): $M g O (s) + 2 H C l (a q) \to M g C l_{2} (a q) + H_{2} O (l)$ $C a O (s) + 2 H C l (a q) \to C a C l_{2} (a q) + H_{2} O (l)$

The basicity of Group 2 oxides increases down the group as the metal becomes more electropositive.

Reaction with Water

Reactivity with water increases down the group:

Metal	Reaction with Water	Products
$B e$	No reaction (even with steam)	—
$M g$	Very slow with cold water; rapid with steam	$M g O + H_{2}$ (steam); $M g (O H)_{2} + H_{2}$ (hot water)
$C a$	Reacts readily with cold water	$C a (O H)_{2} + H_{2}$
$S r$	Reacts vigorously with cold water	$S r (O H)_{2} + H_{2}$
$B a$	Reacts very vigorously with cold water	$B a (O H)_{2} + H_{2}$

Key equations:

$M g (s) + H_{2} O (g) steam M g O (s) + H_{2} (g)$

$C a (s) + 2 H_{2} O (l) \to C a (O H)_{2} (a q) + H_{2} (g)$

$B a (s) + 2 H_{2} O (l) \to B a (O H)_{2} (a q) + H_{2} (g)$

Why does $B e$ not react with water? Beryllium is protected by a tough, adherent layer of $B e O$ on its surface, which prevents water from reaching the metal.

Why does $M g$ react with steam but not cold water? The $M g O$ layer on magnesium's surface is impermeable at room temperature. Steam provides enough energy to break through this barrier.

Reaction with Dilute Acids

All Group 2 metals react with dilute acids ( $H C l$ or $H_{2} S O_{4}$ ) to produce a metal salt and hydrogen gas:

$M (s) + 2 H C l (a q) \to M C l_{2} (a q) + H_{2} (g)$

Examples:

$M g (s) + 2 H C l (a q) \to M g C l_{2} (a q) + H_{2} (g)$

$C a (s) + 2 H C l (a q) \to C a C l_{2} (a q) + H_{2} (g)$

$M g (s) + H_{2} S O_{4} (a q) \to M g S O_{4} (a q) + H_{2} (g)$

Anomalous Behaviour of Beryllium with Dilute Acids

Although $B e$ has a very negative electrode potential (suggesting high reactivity), it reacts slowly with dilute acids at room temperature. This is because:

A thin, tough layer of $B e O$ forms on the surface of beryllium.
This protective oxide layer acts as a kinetic barrier, preventing the acid from reaching the metal.
On heating or with concentrated acids, the oxide layer dissolves and reaction proceeds.

$B e (s) + 2 H C l (a q) \to B e C l_{2} (a q) + H_{2} (g)$

Trend: Reactivity with dilute acids increases down the group: $B e < M g < C a < S r < B a$

Summary of Reactivity Trends

Reaction	$B e$	$M g$	$C a$	$S r$	$B a$
With $O_{2}$	Slow (oxide layer)	Burns brightly	Burns	Burns	Burns
With cold water	No reaction	Very slow	Vigorous	More vigorous	Very vigorous
With steam	No reaction	Rapid	—	—	—
With dilute $H C l$	Slow (oxide layer)	Moderate	Vigorous	More vigorous	Very vigorous

Reason for increasing reactivity down the group:

Atomic radius increases → outer electrons are further from nucleus
Ionization energy decreases → electrons are more easily lost
Shielding effect increases → nuclear attraction on valence electrons decreases
Therefore, $B e^{2 +}$ is harder to form than $B a^{2 +}$