chemistry : class12 Thermal Stability of Ionic Solids : Ionic solids : Solid State MCQ Questions and Answers
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Thermal Stability of Ionic Solids
MCQ questions and answers.
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MCQ Questions
1. Among the following alkaline-earth metal carbonates, which is expected to decompose at the highest temperature?
A.
MgCO3
B.
CaCO3
C.
SrCO3
D.
BaCO3
Correct Answer:
BaCO3
Explanation:
The thermal stability of alkaline-earth metal carbonates increases down Group 2 because the larger cation has lower polarizing power and distorts the carbonate ion less. Therefore, BaCO3 requires the highest temperature for decomposition.
2. Which factor most directly explains why Li2CO3 decomposes more readily on heating than Na2CO3?
A.
Li+ has a greater polarizing power and destabilizes CO3^2−
B.
Li2CO3 has a lower lattice enthalpy than Na2CO3
C.
Li+ has a larger ionic radius than Na+
D.
CO3^2− is smaller in Li2CO3 than in Na2CO3
Correct Answer:
Li+ has a greater polarizing power and destabilizes CO3^2−
Explanation:
The small Li+ ion has high charge density and strongly polarizes the carbonate ion. This weakens the carbonate structure and makes Li2CO3 thermally less stable than Na2CO3.
3. The thermal decomposition of a metal carbonate is represented as MCO3(s) → MO(s) + CO2(g). Which thermodynamic change generally favors decomposition as temperature increases?
A.
Decrease in entropy because a solid is formed
B.
Increase in entropy because a gaseous product is generated
C.
Decrease in enthalpy caused by absorption of heat
D.
Increase in lattice enthalpy of the carbonate only
Correct Answer:
Increase in entropy because a gaseous product is generated
Explanation:
Formation of gaseous CO2 from a solid reactant produces a positive entropy change. The TΔS term becomes increasingly favorable at higher temperature, promoting thermal decomposition.
4. Which nitrate is expected to show the greatest thermal stability among the following, considering cation polarization of the nitrate ion?
A.
LiNO3
B.
NaNO3
C.
KNO3
D.
RbNO3
Correct Answer:
RbNO3
Explanation:
For alkali-metal nitrates, thermal stability generally increases down the group. The larger Rb+ ion has lower polarizing power, so it distorts NO3− less and stabilizes the nitrate ion against decomposition.
5. On heating, which decomposition product is characteristic of most group 2 metal nitrates, excluding the exceptional behavior of some very heavy members under particular conditions?
A.
Metal oxide, nitrogen dioxide, and oxygen
B.
Metal, nitrogen, and oxygen
C.
Metal nitrite and oxygen only
D.
Metal oxide, ammonia, and oxygen
Correct Answer:
Metal oxide, nitrogen dioxide, and oxygen
Explanation:
Most group 2 metal nitrates decompose on heating according to 2M(NO3)2 → 2MO + 4NO2 + O2. The formation of gaseous products drives the thermal decomposition.
6. Which sequence correctly represents the increasing thermal stability of the alkaline-earth metal hydroxides?
Explanation:
Thermal stability of group 2 hydroxides increases down the group. Increasing cation size reduces polarizing power, so the hydroxide ion is less distorted and the solid decomposes less readily.
7. A cation with the same charge but a smaller radius generally lowers the thermal stability of an ionic solid containing a large polyatomic anion because it
A.
decreases the charge density of the cation
B.
increases the polarizing power of the cation
C.
reduces the lattice enthalpy of the solid in every case
D.
increases the size of the polyatomic anion
Correct Answer:
increases the polarizing power of the cation
Explanation:
A smaller cation with the same charge has higher charge density and stronger polarizing power. It distorts the electron cloud of the polyatomic anion, weakening the anion and reducing the thermal stability of the ionic solid.
8. Why is MgCO3 thermally less stable than BaCO3, despite Mg2+ forming a strong electrostatic interaction with CO3^2−?
A.
Mg2+ is larger and less polarizing than Ba2+
B.
Mg2+ strongly polarizes and destabilizes CO3^2−
C.
BaCO3 contains no ionic interactions
D.
Ba2+ converts CO3^2− into a monatomic ion
Correct Answer:
Mg2+ strongly polarizes and destabilizes CO3^2−
Explanation:
Although Mg2+ can contribute strongly to lattice interactions, its small radius gives it high polarizing power. It distorts the carbonate ion significantly, making MgCO3 decompose more readily than BaCO3.
9. For two ionic carbonates containing cations of equal charge, which combination would generally indicate the greater thermal stability of the carbonate?
A.
Smaller cation radius and higher polarizing power
B.
Larger cation radius and lower polarizing power
C.
Smaller cation radius and lower lattice energy
D.
Higher cation charge density and stronger anion distortion
Correct Answer:
Larger cation radius and lower polarizing power
Explanation:
A larger cation has lower charge density and polarizes the carbonate ion less. Reduced distortion of CO3^2− makes the carbonate thermally more stable.
10. Which statement best compares the thermal stability of Na2CO3 and NaHCO3?
A.
Na2CO3 decomposes at a lower temperature because CO3^2− is more stable than HCO3−
B.
NaHCO3 decomposes readily on heating, whereas Na2CO3 is comparatively thermally stable
C.
Both decompose at the same temperature because they contain the same cation
D.
NaHCO3 is more stable because HCO3− has a lower charge than CO3^2−
Correct Answer:
NaHCO3 decomposes readily on heating, whereas Na2CO3 is comparatively thermally stable
Explanation:
Sodium hydrogen carbonate decomposes as 2NaHCO3 → Na2CO3 + CO2 + H2O. Sodium carbonate is the more thermally stable final carbonate, while bicarbonate releases gaseous products on heating.
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