This question tests your understanding of thermal stability trends in compounds, which depends on factors like bond strength, lattice energy, and intermolecular forces.

Concept Explanation: Thermal Stability

Thermal stability refers to a compound's resistance to decomposition upon heating. A more thermally stable compound requires more energy (higher temperature) to break down. The key factors influencing thermal stability are:

• Bond Strength: Stronger bonds (higher bond energy) generally lead to greater thermal stability.
• Lattice Energy (for ionic compounds): Higher lattice energy (due to smaller ion size and/or higher charge) increases stability.
• Intermolecular Forces (for molecular compounds): Stronger forces (e.g., hydrogen bonding) can enhance stability.

Analysis of Each Pair:

1. LiH vs NaH

These are ionic hydrides. Thermal stability of ionic hydrides increases down the group in the periodic table. This is because as the size of the cation increases (Li⁺ to Na⁺), the lattice energy decreases, but the decrease in hydration energy is more significant. The larger anion (H^-) stabilizes better with the larger cation (Na⁺).

Conclusion: NaH is more stable than LiH. Therefore, the statement "LiH > NaH" is false.

2. LiOH vs NaOH

These are ionic hydroxides. For hydroxides of group 1 metals, thermal stability increases down the group. LiOH decomposes at a lower temperature than NaOH. This is again due to the smaller size of Li⁺ which polarizes the OH^- ion more strongly (Fajans' rules), making the O-H bond weaker and easier to break.

Conclusion: NaOH is more stable than LiOH. Therefore, the statement "LiOH > NaOH" is false.

3. NH₃ vs PH₃

These are covalent hydrides (pnicogens). NH₃ is significantly more thermally stable than PH₃. This is due to the much stronger N-H bonds (bond energy ~391 kJ/mol) compared to P-H bonds (bond energy ~322 kJ/mol). Additionally, NH₃ molecules are held together by strong intermolecular hydrogen bonding.

Conclusion: NH₃ is more stable than PH₃. Therefore, the statement "NH₃ > PH₃" is true.

4. Graphite vs Diamond

These are allotropes of carbon. Diamond has a stronger, three-dimensional covalent network structure where each carbon is ${\mathrm{sp}}^3$ hybridized. Graphite has a layered structure with ${\mathrm{sp}}^2$ hybridized carbons and weaker van der Waals forces between layers. However, graphite is actually more thermodynamically stable than diamond at room temperature and pressure. Diamond will eventually transform into graphite, though the reaction kinetics are extremely slow.

Conclusion: Graphite is more stable than Diamond. Therefore, the statement "Graphite > Diamond" is true.

Final Answer

Two pairs have a true statement where the first compound is more stable than the second: NH₃, PH₃ and Graphite, Diamond. The question likely expects the molecular compound pair, NH₃, PH₃, as the primary answer based on bond strength trends.

Related Topics & Formulae

Key Theory: Thermal stability is governed by the strength of the bonds that must be broken for decomposition. For ionic compounds, lattice energy (U) is a major factor, calculated approximately by the Born-Lande equation: $U=\frac{N{A}_0{Z}^{+}{Z}^{-}{e}^2}{4\pi {ϵ}_0{r}_0}\left(1,-,\frac{1}{n}\right)$ Higher lattice energy (smaller r₀) favors stability. For covalent compounds, average bond energy is the primary factor.