Which of the following reactions represent the lattice energy of Li2O (ΔHlatt)? 1. 2 Li(s) + 1/2 O2(g) → Li2O(s) 2. 2 Li(g) + 1/2 O2(g) → Li2O(s) 3. Li2O(s) → 2 Li(aq) + 1/2 O2(aq) 4. 2 Li(aq) + 1/2 O2(aq) → Li2O(s) 5. Li2O(s) → 2 Li(g) + 1/2 O2(g)
The Correct Answer and Explanation is:
The lattice energy of a compound represents the energy released when the ions in the gas phase come together to form the ionic solid. In this case, for Li₂O, the lattice energy corresponds to the energy released when the lithium (Li⁺) and oxide (O²⁻) ions in the gas phase combine to form the solid Li₂O.
Let’s evaluate the given reactions to determine which one corresponds to the lattice energy of Li₂O:
1. 2 Li(s) + 1/2 O₂(g) → Li₂O(s)
- This is the formation reaction of solid Li₂O from its elements in their standard states. This reaction gives the enthalpy of formation (ΔHf) of Li₂O, not the lattice energy.
2. 2 Li(g) + 1/2 O₂(g) → Li₂O(s)
- In this reaction, lithium atoms are in the gas phase, and oxygen is in the molecular form. This represents the combination of Li⁺ and O²⁻ ions in the gas phase to form solid Li₂O, which is exactly what the lattice energy represents: the energy released when the gaseous ions come together to form the ionic solid. This represents the lattice energy of Li₂O.
3. Li₂O(s) → 2 Li(aq) + 1/2 O₂(aq)
- This is the dissolution process where Li₂O dissolves in water to form aqueous lithium (Li⁺) and oxygen ions. This is related to the hydration energy and not the lattice energy.
4. 2 Li(aq) + 1/2 O₂(aq) → Li₂O(s)
- This reaction represents the formation of Li₂O from aqueous ions. It corresponds to the hydration energy in reverse, not the lattice energy.
5. Li₂O(s) → 2 Li(g) + 1/2 O₂(g)
- This reaction represents the process of sublimation of Li₂O, where it is broken down into its gaseous components. This is the reverse of the lattice formation, which would require energy input, but it’s not the lattice energy itself.
Conclusion:
The correct reaction that represents the lattice energy of Li₂O is reaction 2:
2 Li(g) + 1/2 O₂(g) → Li₂O(s). This reaction shows the formation of Li₂O from its ions in the gas phase, which corresponds to the lattice energy.
