Question

unit 3 college board questions
3.
the diagram above is a molecular model of a gaseous diatomic element that is just above its boiling point. intermolecular
forces between the gas molecules will cause them to condense into the liquid phase if the temperature is lowered. which of the
following best describes how the model is limited in its depiction of the phenomenon?
(a) it does not show how hydrogen bonds are constantly forming, breaking, and reforming, which results in a net force of
attraction between the molecules.
(b) it does not show how the interactions between ions and the induced molecular dipoles result in a net force of attraction
between the molecules.
(c) it does not show how the interacting permanent dipoles of the molecules result in a net force of attraction between the
molecules.
(d) it does not show how the temporary fluctuating dipoles of the molecular electron clouds result in a net force of
attraction between the molecules.
sodium hydroxide can be produced commercially from the electrolysis of sodium chloride solutions, as indicated by the following
balanced equation.

$$ 2 mathrm { nacl } ( a q ) + 2 mathrm { h } _ { 2 } mathrm { o } ( l ) ightarrow 2 mathrm { naoh } ( a q ) + mathrm { cl } _ { 2 } ( g ) + mathrm { h } _ { 2 } ( g ) quad delta g ^ { circ } = 400 mathrm { kj } / mathrm { mol } _ { mathrm { rrn } } $$

4.
the gases produced from the electrolysis reaction are condensed in order to prevent their escape into the atmosphere. which of
the following indicates the gas that condenses at the lower temperature and gives a particle - level explanation for why?
(a) $ mathrm { h } _ { 2 } $, because the hydrogen bonding in $ mathrm { h } _ { 2 } ( l ) $ is stronger than the dispersion forces in $ mathrm { cl } _ { 2 } ( l ) $.
(b) $ mathrm { h } _ { 2 } $, because the dispersion forces in $ mathrm { h } _ { 2 } ( l ) $ are weaker than the dispersion forces in $ mathrm { cl } _ { 2 } ( l ) $.
(c) $ mathrm { cl } _ { 2 } $, because the $ mathrm { cl } - mathrm { cl } $ bond in $ mathrm { cl } _ { 2 } $ is stronger than the $ mathrm { h } - mathrm { h } $ bond in $ mathrm { h } _ { 2 } $.
(d) $ mathrm { cl } _ { 2 } $, because $ mathrm { cl } $ atoms are more electronegative than $ mathrm { h } $ atoms.

Explanation:

• Question 3:
• For a diatomic element gas (non - polar), the main intermolecular force is London dispersion forces (temporary fluctuating dipoles).
• Option A: Hydrogen bonds occur in molecules with \(N - H\), \(O - H\), or \(F - H\) bonds. Since it's a diatomic element (not \(H_2O\), \(NH_3\), etc.), there are no hydrogen bonds.
• Option B: There are no ions in a diatomic element gas (it's a covalent substance), so no ion - induced dipole interactions.
• Option C: The diatomic element is non - polar (e.g., \(N_2\), \(O_2\)), so no permanent dipoles.
• Option D: London dispersion forces (temporary fluctuating dipoles of molecular electron clouds) are the intermolecular forces for non - polar diatomic elements, and the model doesn't show this.
• Question 4:
• The boiling point (and thus condensation temperature) of a substance is related to its intermolecular forces.
• Both \(H_2\) and \(Cl_2\) are non - polar molecules, and the main intermolecular force is London dispersion forces.
• The strength of London dispersion forces is related to the molar mass. \(M(Cl_2)=71\space g/mol\) and \(M(H_2) = 2\space g/mol\). Larger molar mass means more electrons and stronger London dispersion forces.
• Option A: \(H_2\) has no hydrogen bonding (needs \(N - H\), \(O - H\), or \(F - H\) bonds).
• Option B: The comparison should be between intermolecular forces (\(H_2\) and \(Cl_2\) intermolecular forces), not between \(H_2\) intermolecular (dispersion) and intramolecular (\(H - H\) bond).
• Option C: The question is about intermolecular forces (for condensation, a physical change), not intramolecular (\(Cl - Cl\) and \(H - H\) bonds, which are about chemical changes).
• Option D: Electronegativity difference is related to bond polarity (for polar bonds). But \(H_2\) and \(Cl_2\) are non - polar, and condensation is about intermolecular forces. The larger molar mass of \(Cl_2\) gives it stronger London dispersion forces (intermolecular), so it condenses at a higher temperature (lower temperature for \(H_2\) to condense).

Answer:

• Question 3: D. It does not show how the temporary fluctuating dipoles of the molecular electron clouds result in a net force of attraction between the molecules.
• Question 4: B. \(H_2\), because the dispersion forces in \(H_2(l)\) are weaker than the dispersion forces in \(Cl_2(l)\).