Edexcel International A Level (IAL) Chemistry (YCH11) - Unit 4 - 12.18 Ionic charge and radius effects-Study Notes - New Syllabus
Edexcel International A Level (IAL) Chemistry (YCH11) -Unit 4 – 12.18 Ionic charge and radius effects- Study Notes- New syllabus
Edexcel International A Level (IAL) Chemistry (YCH11) -Unit 4 – 12.18 Ionic charge and radius effects- Study Notes -International A Level (IAL) Chemistry (YCH11) – per latest Syllabus.
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Edexcel International A Level (IAL) Chemistry (YCH11) -Concise Summary Notes- All Topics
12.18 Effect of Ionic Charge and Radius on Hydration Enthalpy and Lattice Energy
The magnitude of both lattice energy and enthalpy change of hydration depends strongly on the charge and size (radius) of ions. These factors determine the strength of electrostatic attractions between ions and between ions and water molecules.
Key Principle
The strength of electrostatic attraction increases with higher charge and smaller ionic radius.
(i) Effect on Lattice Energy
Lattice energy depends on the attraction between oppositely charged ions in a solid lattice.
Effect of Ionic Charge
- Higher ionic charge → stronger electrostatic attraction.
- Leads to more exothermic (more negative) lattice energy.
- Example: \( \mathrm{MgO} \) has a more exothermic lattice energy than \( \mathrm{NaCl} \) due to \( \mathrm{Mg^{2+}} \) and \( \mathrm{O^{2-}} \).
Effect of Ionic Radius
- Smaller ions → shorter distance between charges.
- Stronger attraction → more exothermic lattice energy.
- Larger ions → weaker attraction → less exothermic lattice energy.
(ii) Effect on Enthalpy Change of Hydration
Hydration enthalpy depends on the attraction between ions and polar water molecules.
Effect of Ionic Charge
- Higher charge → stronger ion–dipole attraction with water.
- More energy released → more exothermic hydration enthalpy.
Effect of Ionic Radius
- Smaller ions → higher charge density.
- Stronger attraction to water molecules → more exothermic hydration enthalpy.
- Larger ions → weaker interactions → less exothermic hydration enthalpy.
Summary of Trends
- ↑ Charge → ↑ attraction → more negative (more exothermic) values.
- ↓ Radius → ↑ charge density → more negative values.
Comparison Between Lattice Energy and Hydration Enthalpy
- Both depend on charge and radius in the same way.
- Lattice energy involves ion–ion attraction.
- Hydration enthalpy involves ion–dipole attraction.
Key Features
- Higher charge → stronger attractions → more exothermic values.
- Smaller ions → stronger interactions → more exothermic values.
- Trends are similar for both lattice energy and hydration enthalpy.
- These factors help explain solubility trends.
Example 1:
Explain why \( \mathrm{MgO} \) has a much more exothermic lattice energy than \( \mathrm{NaCl} \).
▶️ Answer/Explanation
\( \mathrm{MgO} \) contains \( \mathrm{Mg^{2+}} \) and \( \mathrm{O^{2-}} \), while \( \mathrm{NaCl} \) contains \( \mathrm{Na^+} \) and \( \mathrm{Cl^-} \).
The higher charges in \( \mathrm{MgO} \) lead to much stronger electrostatic attractions.
Additionally, the ions are smaller, increasing charge density.
Therefore, the lattice energy of \( \mathrm{MgO} \) is much more exothermic.
Example 2:
Explain why \( \mathrm{Al^{3+}} \) has a more exothermic hydration enthalpy than \( \mathrm{Na^+} \).
▶️ Answer/Explanation
\( \mathrm{Al^{3+}} \) has a higher charge than \( \mathrm{Na^+} \), resulting in stronger attraction to water molecules.
It also has a smaller ionic radius, giving a higher charge density.
This leads to stronger ion–dipole interactions with water.
Therefore, more energy is released and hydration enthalpy is more exothermic.