Reactivity with Acids
Hydrogen displacement, passivity, and amphoterism.
Alkaline earth metals are strong reducing agents. Because their standard reduction potentials ($E^\circ$) are highly negative (sitting well above Hydrogen in the electrochemical series), they readily displace hydrogen gas from dilute mineral acids. However, their reactivity is heavily marked by the unique behavior of Beryllium.
1. The General Reaction with Mineral Acids
All alkaline earth metals ($M$) react with dilute mineral acids (such as $HCl$ or $H_2SO_4$) to form their corresponding $+2$ metal salts and liberate Hydrogen gas ($H_2$).
For example, Magnesium reacting with dilute Hydrochloric acid:
Trend in Reactivity:
- The reactivity and vigor of the reaction increase down the group.
- $Be \lt Mg \lt Ca \lt Sr \lt Ba$
- This occurs because the total ionization enthalpy ($IE_1 + IE_2$) decreases down the group, making it progressively easier for the metal to donate its two electrons to the $H^+$ ions. Beryllium reacts very slowly, while Barium reacts violently.
2. The Beryllium Exception: Passivity with Nitric Acid
While Beryllium reacts slowly with dilute $HCl$ or $H_2SO_4$, its behavior with Concentrated Nitric Acid ($HNO_3$) is entirely different and is a highly tested concept in JEE/NEET.
Nitric acid is a very strong oxidizing agent. When Beryllium is placed in it, the acid instantly oxidizes the surface of the metal, forming a very thin, tough, and chemically inert layer of Beryllium Oxide ($BeO$).
This oxide film physically coats the metal, preventing the acid from reaching the bulk Beryllium underneath, thereby stopping the reaction completely.
Note: Due to their diagonal relationship, Aluminum ($Al$) also exhibits this exact same passivity with concentrated Nitric Acid.
3. The Amphoteric Nature of Beryllium
Metals normally react with acids, not bases. However, Beryllium is amphoteric. It is the only element in Group 2 that reacts with both acids AND strong alkalis (like $NaOH$) to liberate Hydrogen gas.
Reaction with Strong Base:
When Beryllium is heated with aqueous Sodium Hydroxide, it dissolves to form a complex ion called the Beryllate ion, releasing hydrogen.
- None of the other alkaline earth metals (Mg, Ca, Sr, Ba) react with alkalis.
- This amphoteric behavior is again shared by Aluminum, which reacts with $NaOH$ to form Sodium Aluminate ($Na[Al(OH)_4]$).
4. Comparison with Alkali Metals
How does the reaction of Group 2 metals with acids compare to Group 1 metals?
- Speed and Violence: The reactions of Group 2 metals are significantly less violent than those of Group 1 metals. While placing Sodium in acid causes a violent explosion, Magnesium merely fizzes rapidly without exploding.
- Thermodynamic Reason: Alkaline earth metals have a stable $ns^2$ configuration and a higher nuclear charge. Removing two electrons requires much more activation energy than removing the single electron in alkali metals.
- Oxidation State: Group 1 metals change their oxidation state from $0$ to $+1$, whereas Group 2 metals change their oxidation state from $0$ to $+2$ during the reaction.
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