Calcium (Ca) - Chemical Reactions & Properties

Chemical Properties Overview

Calcium (Ca), an alkaline earth metal (Group 2), is a moderately reactive element. Its chemical behaviour is primarily determined by its tendency to readily lose its two valence electrons to attain a stable noble gas configuration.

Position in Reactivity Series

Calcium is more reactive than hydrogen and metals like iron or zinc, but less reactive than alkali metals (Group 1) and magnesium in some contexts. It can displace hydrogen from acids and water.

Electronegativity

Calcium possesses a low electronegativity value (Pauling scale: ~1.00), signifying its strong metallic character and electropositive nature. This property facilitates the formation of ionic compounds where calcium exists as a Ca²⁺ cation.

General Reactivity

• Electronic Configuration: [Ar] 4s²
• Common Oxidation State: +2
• Reducing Agent: Due to its low ionization energy and high electropositivity, calcium acts as a strong reducing agent.
• Bonding: Primarily forms ionic compounds by donating two electrons.

Action of Air and Oxygen

Calcium reacts with components of air, forming both its oxide and nitride. When ignited in pure oxygen, it primarily forms the oxide.

Reaction with Oxygen

When heated in oxygen or air, calcium burns with a characteristic brick-red flame to yield solid calcium oxide.

Equation:

2Ca(s) + O₂(g) → 2CaO(s)

Reaction with Nitrogen (from air)

At elevated temperatures, calcium reacts with nitrogen, a major component of air, to produce solid calcium nitride.

Equation:

3Ca(s) + N₂(g) → Ca₃N₂(s)

Action of Water and Steam

Calcium reacts with water, though the vigour of the reaction is less than that of alkali metals. The reactivity is enhanced at higher temperatures.

Reaction with Cold Water

Calcium reacts steadily with cold water, producing calcium hydroxide and liberating hydrogen gas. The reaction is less violent compared to Group 1 metals because the sparingly soluble calcium hydroxide formed partially coats the metal surface, inhibiting further rapid reaction.

Equation:

Ca(s) + 2H₂O(l) → Ca(OH)₂(aq) + H₂(g)

Reaction with Steam

When red-hot calcium is exposed to steam, a more vigorous reaction occurs, yielding calcium oxide and hydrogen gas.

Equation:

Ca(s) + H₂O(g) → CaO(s) + H₂(g)

Action of Acids and Bases

Reaction with Acids

Calcium reacts readily with dilute acids, producing the corresponding calcium salt and evolving hydrogen gas.

Reaction with Dilute Hydrochloric Acid

Equation:

Ca(s) + 2HCl(aq) → CaCl₂(aq) + H₂(g)

Reaction with Dilute Sulfuric Acid

Equation:

Ca(s) + H₂SO₄(aq) → CaSO₄(s/aq) + H₂(g)

Note: Calcium sulfate (CaSO₄) is sparingly soluble in water. Its formation can lead to passivation of the calcium surface, particularly with concentrated sulfuric acid, thus preventing further reaction.

Reaction with Dilute Nitric Acid

Equation:

Ca(s) + 2HNO₃(aq) → Ca(NO₃)₂(aq) + H₂(g)

Note: With concentrated or hot nitric acid, the reaction can be more complex, potentially yielding oxides of nitrogen rather than hydrogen gas due to the oxidizing nature of nitric acid.

Reaction with Bases (Alkalis)

Calcium, being an electropositive metal, does not react with aqueous solutions of strong bases (e.g., NaOH, KOH).

Key Laboratory Test/Identification Reactions

Calcium ions (Ca²⁺) are identified in qualitative inorganic analysis through characteristic flame colour and specific precipitation reactions.

Flame Test

When a small amount of a calcium salt solution is introduced into a non-luminous Bunsen flame via a platinum wire, it imparts a distinctive brick-red colour to the flame. This phenomenon results from the excitation and subsequent de-excitation of calcium’s valence electrons.

Reaction with Ammonium Oxalate Solution

In a neutral or alkaline medium, calcium ions precipitate as white calcium oxalate (CaC₂O₄). This precipitate is insoluble in acetic acid but readily dissolves in dilute mineral acids (e.g., HCl, HNO₃).

Equation:

Ca²⁺(aq) + (NH₄)₂C₂O₄(aq) → CaC₂O₄(s) + 2NH₄⁺(aq)

Observed as a white precipitate.

Reaction with Ammonium Carbonate Solution

Calcium ions precipitate as white calcium carbonate (CaCO₃) upon addition of ammonium carbonate solution. Ammonium chloride is typically added to prevent the precipitation of magnesium carbonate, if magnesium ions are also present. The precipitate is soluble in acids.

Equation:

Ca²⁺(aq) + (NH₄)₂CO₃(aq) → CaCO₃(s) + 2NH₄⁺(aq)

Observed as a white precipitate.

Reaction with Disodium Hydrogen Phosphate Solution

In an ammoniacal solution, calcium ions precipitate as white calcium hydrogen phosphate (CaHPO₄). If excess ammonia is present, tricalcium phosphate (Ca₃(PO₄)₂) may form.

Equation (simplified):

Ca²⁺(aq) + HPO₄²⁻(aq) → CaHPO₄(s)

Observed as a white precipitate. Note: This test is often performed after separating other interfering cations.