Answer: High hydration energy makes E° very negative, but a high melting point slows the reaction.
- A Lithium has a far lower ionisation enthalpy and so reacts much more gently than sodium
- B Sodium is more electropositive than lithium
- C High hydration energy makes E° very negative, but a high melting point slows the reaction
- D Lithium does not react with water at all
Correct answer: C. High hydration energy makes E° very negative, but a high melting point slows the reaction
Explanation: The very negative E° of Li is due to its exceptionally high hydration enthalpy (a thermodynamic quantity). Kinetically, Li's high melting point keeps it solid and its small size slows the reaction, so it reacts more gently than Na.
Each s-block metal ion emits a characteristic flame colour because heating excites electrons to higher energy levels; light is emitted as they fall back, with the colour determined by the specific energy gap.
Concept context
Group 1 (alkali metals) and Group 2 (alkaline earth metals). Learn their reactions with water and air, important compounds like NaOH, Na₂CO₃, and CaCO₃, and anomalous behaviour of lithium and beryllium.