Question

unit 3 college board questions
20.
which of the following best helps to explain why the boiling point of hcn is so much higher than that of n₂, as shown by the data in the table above?
(a) hcn contains three atoms, whereas n₂ contains two atoms.
(b) hcn is polar, whereas n₂ is nonpolar.
(c) hcn has a molar entropy greater than that of n₂.
(d) the c≡n bond is stronger than the n≡n bond.
21.
based on the diagram above, which of the following best helps to explain why mgo(s) is not able to conduct electricity, but mgo(l) is a good conductor of electricity?
(a) mgo(s) does not contain free electrons, but mgo(l) contains free electrons that can flow.
(b) mgo(s) contains no water, but mgo(l) contains water that can conduct electricity.
(c) mgo(s) consists of separate mg²⁺ ions and o²⁻ ions, but mgo(l) contains mgo molecules that can conduct electricity.
(d) mgo(s) consists of separate mg²⁺ ions and o²⁻ ions held in a fixed lattice, but in mgo(l) the ions are free to move and conduct electricity.

Explanation:

• For question 20:
• Intermolecular forces play a crucial role in determining boiling points. Polar molecules (\(HCN\)) have dipole - dipole forces in addition to London dispersion forces. Non - polar molecules (\(N_2\)) only have London dispersion forces. Dipole - dipole forces are stronger than London dispersion forces for molecules of similar size, so \(HCN\) (polar) has a higher boiling point than \(N_2\) (non - polar).
• Option A: The number of atoms in a molecule does not directly determine the strength of intermolecular forces and boiling point.
• Option C: Molar entropy is related to the disorder of a system and not directly to boiling point differences in this context.
• Option D: The strength of the covalent bonds (\(C\equiv N\) and \(N\equiv N\)) within the molecules does not affect the intermolecular forces which are responsible for boiling point.
• For question 21:
• In a solid ionic compound like \(MgO(s)\), the \(Mg^{2 + }\) and \(O^{2-}\) ions are held in a fixed lattice structure. Ions need to be free to move to conduct electricity. In the liquid state \(MgO(l)\), the ions are free to move.
• Option A: \(MgO\) is an ionic compound. The conduction of electricity in \(MgO\) is due to the movement of ions, not free electrons (which are more relevant in metals).
• Option B: \(MgO\) does not contain water in either the solid or liquid state when considering its pure form for electrical conduction.
• Option C: \(MgO\) does not contain water in either the solid or liquid state when considering its pure form for electrical conduction.

Answer:

1. B. \(HCN\) is polar, whereas \(N_2\) is nonpolar.
2. D. \(MgO(s)\) consists of separate \(Mg^{2 + }\) ions and \(O^{2-}\) ions held in a fixed lattice, but in \(MgO(l)\) the ions are free to move and conduct electricity.