CHEM1910 Inorganic Lecture 5 s block

CHEM1910: BLOCK 1 - CHEMISTRY OF THE MAIN GROUP ELEMENTS

The s-Block Part 2

• Date: February 11th 2025

• Presenter: N. Stephenson

CHEMISTRY OF GROUP 1 ELEMENTS

The Alkali Metals

• Focus on the distinct properties and reactions of alkali metals (Group 1 elements).

Recall Reactions Discussed Last Week

Balanced Chemical Equations

• (a) Sodium and Oxygen:4 Na(s) + O2(g) → 2 Na2O(s)

• (b) Lithium and Nitrogen:6 Li(s) + N2(g) → 2 Li3N(s)

• (c) Sodium and Water:2 Na(s) + 2 H2O(l) → 2 NaOH(aq) + H2(g)

• (d) Potassium Superoxide and Water:4 KO2(s) + 2 H2O(g) → 4 KOH(s) + 3 O2(g)

• (e) Potassium and Nitrogen:No Reaction

Stability of Group 1 Compounds

Key Factors Influencing Stability

• Lattice Enthalpy, Enthalpy of Formation, and Solubility.

Lattice Enthalpy (ΔlattHo)

• Defined as the energy released when gaseous ions combine to form solid ionic compounds.

• More negative lattice enthalpy indicates stronger ionic bonding and higher stability.

  • Related Equations: Born-Haber Cycle, Born-Lande Equation, Kapustinskii Equation.

Enthalpy of Formation (ΔfHo)

• Energy change when one mole of a compound forms from stand-alone elements.

• More negative values indicate higher thermodynamic stability.

Relationship to Stability

• Compounds with highly negative lattice enthalpy are more stable and resistant to decomposition.

• A highly negative enthalpy of formation correlates with stability and resistance to breakdown.

Key Relationship

• Lattice enthalpy is always negative (indicating energy release).

• A decrease in lattice enthalpy (becoming less negative) implies a less exothermic enthalpy of formation, therefore decreasing stability.

Group 1 Compounds: Halides

Estimating Lattice Energy

• Kapustinskii Equation:

  • k = 107,900 pm kJ mol-1

  • V = number of ions in formula unit.

Trends in Lattice Energy

• Lattice energy is influenced by the sum of ionic radii; increasing size reduces lattice energy.

• Trends observed in calculated experimental values:

  • NaF: 918 kJ/mol

  • NaCl: 763 kJ/mol

  • NaBr: 724 kJ/mol

  • NaI: 670 kJ/mol

Enthalpy of Formation Trends for Halides

• All Group 1 halides have a significant negative ΔfHo.

• Larger halide ions (such as I-) reduce ΔfHo values due to larger polarizable electron clouds that stabilize the ionic lattice.

Oxides of Group 1 Metals

Types of Oxides Formed

• Transition from lithium oxide to potassium superoxide as cation size increases.

• Stabilization of larger anions by larger cations—a general rule of thumb.

  • Lithium Oxide (Li2O): -3000 kJ/mol

  • Sodium Oxide (Na2O): -2680 kJ/mol

  • Potassium Oxide (K2O): -2454 kJ/mol

Stability of Peroxides

• Lithium peroxide is the least stable, cesium peroxide is the most stable.

• Stability increases down the group due to decreasing ionic radii leading to less negative enthalpy of formation for peroxides.

Group 1 Metal Carbonates and Nitrates

Stability and Decomposition

• Most Group 1 metal nitrates decompose upon heating, with lithium nitrate decomposing to Li2O.

• Group 1 carbonates are generally stable, except lithium carbonate, which decomposes dramatically on heating.

Solubility of Group 1 Compounds

Factors Influencing Solubility

• Most Group 1 salts are soluble in water; however, solubility decreases with larger anions (Cl-, Br-, I-).

• Lithium salts with larger anions are highly soluble, while salts with smaller anions like F- exhibit increasing solubility down the group.

Solubility Data

• Solubility values of various Group 1 compounds in g/100 g H2O:

  • LiF: 0.1 (0.27)

  • NaCl: 1.0 (4.22)

  • KCl: 15.9 (92.3)

  • CsI: 24.2 (367)

Influence of Ion Size on Solubility

Key Relationships

• Lattice Enthalpy (Δlatt G°): High lattice enthalpy reduces solubility.

• Hydration Enthalpy (Δhyd G°): High hydration enthalpy increases solubility.

• Solubility Free Energy (Δsol G°): Determines if a compound dissolves (negative means dissolves).

Summary of Group 1 Metal Behavior

• Stability decreases for hydrides and nitrides down the group with larger cation size.

• Stability trends for peroxides, superoxides, and carbonates generally increase down the group.

Lithium: Anomalies

• Lithium demonstrates unique properties such as reactivity with nitrogen and low solubility in compounds compared to other Group 1 metals.

• Hydration Free Energy (ΔhydG): Influences solubility; high ΔhydG increases solubility of salts in water.

• Solubility Free Energy (ΔsolG): Determines whether a compound dissolves; a negative ΔsolG indicates that a compound will dissolve in solution.

• Lattice Free Energy (ΔlattG): Higher lattice free energy (more negative) corresponds to lower solubility due to stronger ionic interactions.

• Enthalpy of Formation (ΔfG): Reflects energy change when one mole of a compound forms from its constituents; more negative values suggest higher stability of the compound.

• Gibbs Free Energy of Reaction (ΔrG): Determines spontaneity of the reaction; a negative ΔrG indicates spontaneous reactions.

• Ionization Energy (IE): Energy needed to remove an electron from an atom; affects reactivity of the element.

• Hydration Free Energy (of Anion, ΔhydG(anion)): Similar to cations, impacts the solubility of ionic compounds in water but accounts for the anions involved.

• Hydration Free Energy (ΔhydG): Influences solubility; high ΔhydG increases solubility of salts in water.

• Solubility Free Energy (ΔsolG): Determines whether a compound dissolves; a negative ΔsolG indicates that a compound will dissolve in solution.

• Lattice Free Energy (ΔlattG): Higher lattice free energy (more negative) corresponds to lower solubility due to stronger ionic interactions.

• Enthalpy of Formation (ΔfG): Reflects energy change when one mole of a compound forms from its constituents; more negative values suggest higher stability of the compound.

• Gibbs Free Energy of Reaction (ΔrG): Determines spontaneity of the reaction; a negative ΔrG indicates spontaneous reactions.

• Ionization Energy (IE): Energy needed to remove an electron from an atom; affects reactivity of the element.

• Hydration Free Energy (of Anion, ΔhydG(anion)): Similar to cations, impacts the solubility of ionic compounds in water but accounts for the anions involved.