chemistry : class11 Isotopic Substitution : Melting point : States of Matter: Gases and Liquids MCQ Questions and Answers
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Isotopic Substitution
MCQ questions and answers.
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MCQ Questions
1. Why does replacing H by D generally alter the melting point of a molecular solid, even though the intermolecular forces are nearly unchanged?
A.
It changes the nuclear mass and therefore the vibrational zero-point energy and lattice stability
B.
It changes the atomic number of hydrogen and creates a new chemical element
C.
It always increases the molecular charge and strengthens ionic bonding
D.
It eliminates all rotational motion in the solid state
Correct Answer:
It changes the nuclear mass and therefore the vibrational zero-point energy and lattice stability
Explanation:
Isotopic substitution changes mass but not electronic configuration. The altered vibrational frequencies and zero-point energies can change the relative stability of the solid and liquid phases, producing a melting-point isotope effect.
2. For a harmonic vibrational mode, the zero-point energy is proportional to (1/mu)^(1/2), where mu is the reduced mass. What is the main consequence of replacing H by D in an X-H bond?
A.
The vibrational frequency and zero-point energy decrease
B.
The vibrational frequency increases while zero-point energy decreases
C.
The vibrational frequency remains unchanged because isotopes have identical chemistry
D.
The zero-point energy becomes exactly zero at ordinary temperature
Correct Answer:
The vibrational frequency and zero-point energy decrease
Explanation:
Deuterium has greater mass than protium. Since vibrational frequency varies inversely with the square root of reduced mass, X-D vibrations have lower frequency and lower zero-point energy than corresponding X-H vibrations.
3. The melting points of H2O and D2O differ substantially despite their identical molecular formulas apart from isotopic labeling. Which explanation is most appropriate?
A.
Different isotope-dependent vibrational energies modify hydrogen-bond network stability in the two phases
B.
D2O contains twice as many oxygen atoms as H2O
C.
D2O is ionic whereas H2O is covalent
D.
The deuterium nucleus has a different positive charge from the proton
Correct Answer:
Different isotope-dependent vibrational energies modify hydrogen-bond network stability in the two phases
Explanation:
Both substances have essentially the same electronic bonding pattern, but replacing H with D changes vibrational amplitudes and zero-point energies. These changes affect hydrogen-bonded structures differently in solid and liquid water, shifting the melting point.
4. Consider two isotopologues of a molecular crystal that have nearly identical intermolecular potential-energy curves. Which factor can still produce different melting points?
A.
Their different vibrational zero-point energies and vibrational entropy changes upon melting
B.
A change in the number of protons in every molecule without isotope substitution
C.
A compulsory change from molecular to metallic bonding
D.
The disappearance of translational motion in the liquid
Correct Answer:
Their different vibrational zero-point energies and vibrational entropy changes upon melting
Explanation:
The melting temperature is determined by the free-energy equality of solid and liquid phases. Isotopic substitution changes vibrational energy and entropy, so the free-energy difference can change even when the intermolecular potential is almost the same.
5. A student claims that the heavier isotopologue must always have the higher melting point because heavier particles move more slowly. What is the best evaluation of this claim?
A.
It is false because the melting-point isotope effect depends on competing solid- and liquid-phase free-energy changes
B.
It is true because kinetic energy is the only factor determining melting
C.
It is true because heavier isotopes always form stronger chemical bonds
D.
It is false only because isotopes have different numbers of electrons
Correct Answer:
It is false because the melting-point isotope effect depends on competing solid- and liquid-phase free-energy changes
Explanation:
Heavier isotopes usually lower vibrational frequencies, but the resulting changes in zero-point energy and entropy need not stabilize the solid more than the liquid. Therefore, the direction and size of the melting-point shift are molecule- and structure-dependent.
6. For a centrosymmetric molecular crystal, replacing one H atom in each molecule by D can alter the melting point primarily by changing which property without changing the molecular composition pattern?
A.
The vibrational spectrum and isotope-dependent lattice dynamics
B.
The formal oxidation state of the substituted atom
C.
The number of valence electrons in the molecule
D.
The stoichiometric ratio of carbon to hydrogen nuclei
Correct Answer:
The vibrational spectrum and isotope-dependent lattice dynamics
Explanation:
Isotopic substitution preserves the electronic structure and formal composition type, but changes masses. This changes normal-mode frequencies, zero-point motion, and phonon behavior in the crystal, all of which can influence melting.
7. Why can isotopic substitution produce a measurable melting-point shift even when the isotope effect on ordinary intermolecular potential energy is small?
A.
Thermodynamic phase transitions depend on free-energy differences, which include vibrational contributions
B.
Small potential-energy changes are automatically amplified into nuclear-charge changes
C.
The isotope changes the periodic table position of the element
D.
Melting points are determined only by atomic mass and not by molecular interactions
Correct Answer:
Thermodynamic phase transitions depend on free-energy differences, which include vibrational contributions
Explanation:
The melting point occurs when the Gibbs free energies of solid and liquid become equal. Vibrational zero-point energies and temperature-dependent vibrational entropies contribute to these free energies, allowing a mass change to shift the transition temperature.
8. In comparing CH4 and CD4 crystals, which prediction is most defensible regarding their melting points?
A.
They may differ because isotopic substitution changes molecular vibrational frequencies and phase free energies
B.
They must be identical because both molecules contain one carbon atom
C.
They must differ only because CD4 has a different carbon isotope
D.
They cannot form molecular crystals because deuterium is radioactive
Correct Answer:
They may differ because isotopic substitution changes molecular vibrational frequencies and phase free energies
Explanation:
CH4 and CD4 have the same electronic structure and carbon isotope, but their hydrogen masses differ. The resulting changes in intramolecular vibrations and crystal lattice dynamics can alter the free-energy balance at melting.
9. Suppose isotope substitution lowers the solid's zero-point energy more than it lowers the liquid's zero-point energy. What qualitative effect would this tend to have on the melting point?
A.
It would tend to stabilize the solid relative to the liquid and raise the melting point
B.
It would necessarily destabilize the solid and lower the melting point
C.
It would have no effect because zero-point energy is absent in condensed phases
D.
It would convert the solid directly into a gas at the same temperature
Correct Answer:
It would tend to stabilize the solid relative to the liquid and raise the melting point
Explanation:
If substitution lowers the solid free energy by a greater amount than the liquid free energy, the solid becomes relatively more stable. A higher temperature is then generally required to make the liquid equally stable, tending to raise the melting point.
10. Which experimental comparison best isolates a melting-point isotope effect caused by replacing H with D?
A.
Compare chemically identical, structurally corresponding samples differing only in H/D isotopic composition
B.
Compare water with sodium chloride because both can melt
C.
Compare a hydrogen compound with a compound containing a different element and different structure
D.
Compare a solid sample with a gas sample at unrelated pressures
Correct Answer:
Compare chemically identical, structurally corresponding samples differing only in H/D isotopic composition
Explanation:
A valid isotope-effect comparison keeps molecular structure, purity, pressure, and crystal form as similar as possible while changing only isotopic composition. This allows the melting-point difference to be attributed primarily to isotope-dependent vibrational and thermodynamic effects.
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