Sblock elements

Lecture 3: S-Block Elements - Group 1

Introduction to Group 1 Elements

• Hydrogen (H) is the first element, usually placed in Group 1, but Lithium (Li) is the actual first member.
• Reason for Hydrogen's Placement:
  • Its electronic configuration is 1s¹.
  • Can behave like an alkali metal by losing its electron to form H⁺.
  • Can behave like a halogen by gaining an electron to form H⁻ (hydride ion).

Group 1 Elements and Their Electronic Configurations

The table below summarizes key properties:

• Group 1 elements are highly reactive.
• Their pure forms are extracted/prepared only by electrolysis.

General Properties of Group 1 Elements

• All are metals and form ionic compounds.
• They are good reducing agents.
• They are highly electropositive.
• They are very reactive, and reactivity increases down the group.
• They do not occur in the free state; found as chlorides, nitrates, sulphates, and carbonates.
• Lithium is not completely typical due to its small ionic size and large hydration energy.

Anomalous Behavior of Lithium

Lithium's anomalous behavior is due to its small ionic size and high charge.

• The polarizing power of the lithium ion is the highest among all alkali metal ions.
• Due to its small size, it forms more covalent compounds.
• Lithium is hard, has high electronegativity, high ionization potential, high melting, and boiling points compared to other Group 1 elements.
• Lithium hydrides are stable.
• It is the only alkali metal that can react with nitrogen to form nitride:
  6Li + N₂ → 2Li₃N

Reactions of Group 1 Elements

Reaction with Oxygen

• Group 1 metals form ionic oxides.
• Normal oxide:
  4Na(s) + O₂(g) → 2Na₂O(s)
• Peroxide (with excess oxygen):
  2Na(s) + O₂(g) → Na₂O₂(s)
• Oxides dissolve in water to give an alkaline solution:
  Na₂O(s) + H₂O(l) → 2NaOH(aq)
• Lithium oxidizes less rapidly than other alkali metals.

Formation of Oxides, Peroxides, and Superoxides

• Lithium and Sodium: Form simple oxides (O²⁻ ion).
  • 4Li + O₂ → 2Li₂O
  • 4Na + O₂ → 2Na₂O
• Sodium and Potassium: Form peroxides (O₂²⁻ ion).
  • 2Na + O₂ → Na₂O₂
  • 2K + O₂ → K₂O₂
• Potassium, Rubidium, and Caesium: Form superoxides (O₂⁻ ion).
  • K + O₂ → KO₂
  • Rb + O₂ → RbO₂
  • Cs + O₂ → CsO₂
• Explanation: The formation of more complicated oxides (peroxides and superoxides) releases more energy and makes the system more energetically stable, especially for larger metal ions with low charge density (lower half of the group). Smaller ions at the top of the group with higher charge density polarize the complicated oxide ions to destruction, thus forming only simple oxides.

Reaction with Water

• Most Group 1 metals react violently with water to give hydroxide and hydrogen gas. Lithium reacts slowly.
  • 2Na(s) + 2H₂O(l) → 2NaOH(aq) + H₂(g)
• Hydroxides of Group 1 metals are strong bases and are very soluble in water.
• Lithium hydroxide (LiOH) is slightly soluble in water.

Hydrides of Group 1 Elements

• They are ionic.
• Metals form positive ions, while hydrogen forms negative (hydride) ions.
• Hydrides are very reactive; they give off hydrogen in water.
  • LiH(s) + H₂O(l) → LiOH(aq) + H₂(g)
• Lithium tetrahydridoaluminate(iii) (LiAlH₄) is a common reducing agent in organic chemistry, e.g., reducing amides (RCONH₂) to amines (RCH₂NH₂).

Halides of Group 1 Metals

• Halides (fluorides, chlorides, bromides, iodides) are generally soluble in water.
• Halides of lithium dissolve with difficulty.
• Hydrohalic acids can be prepared by reacting a metal halide with a strong acid:
  • NaCl(s) + H₂SO₄(l) → NaHSO₄(s) + HCl(g)

Formation of Oxosalts

• Group 1 metals are highly electropositive and form very strong bases, leading to very stable oxosalts.
• Oxosalts formed include:
  • Carbonates
  • Bicarbonates
  • Nitrates
  • Nitrites
  • Sulphates
  • Sulphites

Group 1 Carbonates

• Alkali metals form carbonates of the general formula M₂CO₃.
• They are white solids at room temperature.
• Their carbonates are ionic.
• Down the group, size increases, polarizing power decreases, and ionic nature increases.

Carbonates and Hydrogencarbonates

• Group 1 metals are the only metals that form solid bicarbonates (hydrogencarbonates), except LiHCO₃ which exists only in solution.
• Hydrogencarbonates decompose to give off carbon dioxide and water vapour:
  • 2NaHCO₃(s) → Na₂CO₃(s) + H₂O(g) + CO₂(g)
• Except for lithium carbonate, carbonates of Group 1 elements do not decompose on heating (except above 1000 °C).
  • Na₂CO₃(s) → Na₂O(s) + CO₂(g) (at very high temperatures)
• This decomposition is one of the tests for carbonates in qualitative analysis.
• Hydrated sodium carbonate (Na₂CO₃·10H₂O) is used as washing soda.
• The ease with which hydrogencarbonates (NaHCO₃) give off carbon dioxide is used in fire extinguishers and baking powders.

Thermal Stability of Carbonates

• Group 1 carbonates are generally stable up to 1000 °C.
• Carbonates of K, Rb, Cs are deliquescent.
• Thermal stability increases down the group as the size of the metal ion increases.
• Small and highly charged metal ions possess high polarizing power, which destabilizes the carbonate ion. Large and small charged metal ions possess low polarizing power.

Instability of Li₂CO₃

• Li₂CO₃ is unstable and decomposes on heating into Li₂O and CO₂.
  • Li₂CO₃(s) → Li₂O(s) + CO₂(g)
• The instability is due to:
  • The stable lattice of Li₂O compared to Li₂CO₃.
  • Strong polarizing power of the small Li⁺ ion.

Solubility of Carbonates in Water

• Group 1 carbonates are soluble in water, except lithium carbonate.
• Solubility in water increases from Li to Cs because:
  • High lattice energy in Li₂CO₃ due to the small size of Li⁺.
  • Increase in ionic character down the group, so solubility increases down the group.

Nitrates and Nitrites of Group 1 Elements

• The most common are sodium nitrate (NaNO₃) and sodium nitrite (NaNO₂).
• NaNO₃ can be made by neutralizing nitric acid with sodium carbonate:
  • 2HNO₃(aq) + Na₂CO₃(aq) → 2NaNO₃(aq) + H₂O(l) + CO₂(g)
• NaNO₃ is soluble in water and used as fertilizer.
• Thermal stability increases with increasing atomic number (down the group).

Decomposition of Nitrates

• When heated, nitrates of Group 1 elements (except lithium) are converted to nitrite and oxygen:
  • 2KNO₃(s) → 2KNO₂(s) + O₂(g)
• Nitrites like NaNO₂ are used in dye manufacture and as anti-oxidants in foodstuffs.
• However, lithium nitrate (LiNO₃) decomposes anomalously to its corresponding oxide, nitrogen dioxide, and oxygen:
  • 4LiNO₃(s) → 2Li₂O(s) + 4NO₂(g) + O₂(g)

Sulphates, Hydrogen Sulphates, and Sulphites of Group 1 Elements

• Sulphates (SO₄²⁻) and hydrogen sulphates (HSO₄⁻) are all soluble in water.
• Sulphites (SO₃²⁻) are more reactive than sulphates and hydrogen sulphates.
• On heating, sulphites like sodium sulphite (Na₂SO₃) react with an acid (e.g., HCl) to liberate sulphur dioxide gas:
  • Na₂SO₃(aq) + 2HCl(aq) → 2NaCl(aq) + SO₂(g) + H₂O(l)
• Na₂SO₃ reacts with powder sulfur (S) to yield sodium thiosulphate (Na₂S₂O₃):
  • Na₂SO₃(aq) + S(s) → Na₂S₂O₃(aq)
• Sodium thiosulphate is useful in photography and in iodine titrations.

Uses of Some Group 1 Elements

• Lithium:
  • Used in small, long-lifetime batteries.
  • Hydrides (LiH and LiAlH₄) are popular reducing agents.
• Sodium:
  • Known as a reducing agent.
  • Combines with other metals to form alloys.
• Potassium:
  • Used in the production of potassium oxide (KO₂) for oxygen generators.