Chemical Bonding, Solid Structures, and Thermochemistry Principles
Classification of Solids and Interparticle Forces
Sulfur Dioxide, SO₂ (Molecular Solid)
- Type of Solid: Simple molecular (molecular solid)
- Type of Particle: Discrete SO₂ molecules
- Attractive Forces: Weak intermolecular forces (van der Waals and dipole–dipole)
Sodium Sulfide, Na₂S(s) (Ionic Solid)
- Type of Solid: Ionic solid (crystalline ionic lattice)
- Type of Particle: Ions (Na⁺ and S²⁻)
- Attractive Forces: Strong electrostatic (ionic) attraction between Na⁺ and S²⁻
Sodium, Na(s) (Metallic Solid)
- Type of Solid: Metallic solid
- Type of Particle: Metal atoms / positive ions in a lattice with delocalised electrons
- Attractive Forces: Metallic bonding (electrostatic attraction between positive ions and delocalised electrons)
Properties of Solid Sodium, Na(s)
Solid sodium, Na(s), is malleable and conducts electricity.
- Malleability: Metal ions are in layers that can slide past each other without breaking the metallic bond because the delocalised electrons continue to hold the ions together.
- Electrical Conductivity: Delocalised (mobile) electrons move through the metal when a potential is applied, carrying charge.
Thermochemical Reactions
Exothermic Reaction: Magnesium and Hydrochloric Acid
Reaction: Magnesium + Hydrochloric Acid → Exothermic
Reason: The reaction releases heat energy as chemical bonds form in the products (MgCl₂ and H₂). The temperature of the solution increases, showing that heat is released to the surroundings.
Endothermic Process: Dissolving Ammonium Nitrate
Process: Dissolving NH₄NO₃(s) → Endothermic
Reason: The process absorbs heat energy from the surroundings to break the ionic lattice and separate ions in solution. The positive ΔH value (+25.7 kJ mol⁻¹) confirms that energy is taken in.
Properties of Solid Sodium Sulfide, Na₂S(s)
Solid sodium sulfide, Na₂S(s), is brittle and does not conduct electricity.
Sodium sulfide has a giant ionic lattice made of Na⁺ and S²⁻ ions held together by strong electrostatic forces.
Why Na₂S is Brittle:
When the solid is hit, layers of ions shift so that like charges come close together (e.g., Na⁺ near Na⁺ or S²⁻ near S²⁻), causing strong repulsion and shattering the lattice.
Why Na₂S Does Not Conduct Electricity (Solid State):
In the solid state, the ions are fixed in place and cannot move, so there are no mobile charge carriers. (Note: Na₂S will conduct when molten or dissolved because the ions are free to move.)
State Comparison: Silicon Dioxide (SiO₂) vs Sulfur Dioxide (SO₂)
Silicon dioxide, SiO₂, is a solid at room temperature, whereas sulfur dioxide, SO₂, is a gas.
Silicon Dioxide (SiO₂): Giant Covalent Network
SiO₂ forms a giant covalent network in which each Si atom is covalently bonded to four O atoms, and each O atom bridges between two Si atoms. Many strong covalent bonds must be broken to melt or boil it, resulting in a very high melting/boiling point, thus making it solid at room temperature.
Sulfur Dioxide (SO₂): Simple Molecular Structure
SO₂ exists as small, discrete molecules held together by weak intermolecular forces (van der Waals and dipole–dipole). These forces are easily overcome at room temperature, so SO₂ is a gas.
Thermochemistry Calculation: Thermite Reaction
The reaction is the reduction of Iron(III) Oxide (Thermite Reaction):
Fe₂O₃(s) + 2Al(s) → Al₂O₃(s) + 2Fe(l) ΔH = -852 kJ mol⁻¹
Calculation 1: Energy Released to Produce 672 g of Fe
- Calculate moles of Fe (Molar Mass M ≈ 55.9 g/mol):
n = m / M = 672 g / 55.9 g mol⁻¹ ≈ 12.02 moles
- Calculate Energy Released:
The reaction stoichiometry shows that 2 moles of Fe are produced when 852 kJ of energy are released.
Energy Released = 12.02 mol Fe × (-852 kJ / 2 mol Fe) ≈ -5120.52 kJ
- Result:
Approximately 5121 kJ of energy are released to create 672 g of Fe.
Calculation 2: Energy Released When 32.0 g of Fe₂O₃ Reacts
- Calculate moles of Fe₂O₃ (Molar Mass M ≈ 159.8 g/mol):
n = m / M = 32.0 g / 159.8 g mol⁻¹ ≈ 0.200 moles
- Calculate Energy Released:
The reaction stoichiometry shows that 1 mole of Fe₂O₃ is consumed when 852 kJ of energy are released.
Energy Released = 0.200 mol Fe₂O₃ × (-852 kJ / 1 mol Fe₂O₃) ≈ -170.4 kJ
- Result:
Approximately 170 kJ of energy are released when 32.0 g of Fe₂O₃ reacts.
Properties of Solid Sodium Iodide, NaI(s)
Solid sodium iodide, NaI(s), is brittle and does not conduct electricity.
Sodium iodide has a giant ionic lattice made of Na⁺ and I⁻ ions held together by strong electrostatic forces (ionic bonds).
Why is NaI Brittle?
When force is applied, layers of ions shift, causing like charges (e.g., Na⁺ near Na⁺ or I⁻ near I⁻) to come close together. The resulting strong repulsion splits the lattice, making it shatter easily.
Why Doesn’t NaI Conduct Electricity as a Solid?
In the solid state, the ions are fixed in place and cannot move, so there are no mobile charge carriers.
When Does NaI Conduct Electricity?
When molten (liquid) or dissolved in water, the ions become free to move, allowing them to carry charge and conduct electricity.