Questions and Answers
What type of intermolecular attractions causes gas phase molecules to condense into liquids and solids?
If a liquid has a high boiling point, it has strong intermolecular attractions.
True
How does molecular dipole moment correlate with boiling point?
Compounds with larger dipoles have a greater boiling point due to stronger intermolecular attractions.
What are London dispersion forces?
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Do molecular weights correlate with freezing points for monatomic substances?
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If a liquid has a low freezing point, it has strong intermolecular attractions.
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Based on polarity, would CH3F have a higher or lower boiling point than CH3Cl?
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Based on dispersion forces alone, would CH3F have a higher or lower boiling point than CH3Cl?
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Which interactions are more important for determining relative boiling points: dispersion or dipole interactions?
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What is hydrogen bonding?
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Rank H2O, H2S, and H2Se in order of increasing dispersion interactions.
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Rank NH3, PH3, and AsH3 in order of increasing dispersion interactions.
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For CH4, the polar/nonpolar nature is ______.
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For CH3Cl, the polar/nonpolar nature is ______.
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For CH3OH, the polar/nonpolar nature is ______.
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Predict the order of freezing and boiling points for CH4, CH3Cl, and CH3OH from lowest to highest.
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What does a phase diagram show?
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Identify where water is a gas, liquid, and solid in a phase diagram.
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If starting at point A and increasing temperature, what phases will you observe?
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Study Notes
Intermolecular Forces
- Intermolecular attractions lead to condensation from gas to liquid/solid states.
- Key attractions involve dipole moments in polar covalent molecules.
- Bond dipole examples: H → F and S → H.
Electronegativity Values
- Electronegativity scale: F (4.0), O (3.5), Cl (3.0), C and S (2.5), H (2.1), Si (1.8).
Boiling Points of Molecules
- Comparative boiling points (°C): H2O (100), HF (20), CH3Cl (-24), H2S (-60), HCl (-85), CH4 (-161).
- High boiling points indicate strong intermolecular attractions; low points indicate weak attractions.
Molecular Dipole Moments and Boiling Points
- Dipole strength influences boiling points: CH4 < HCl < HF / H2S < H2O.
- Higher dipole strength correlates with greater boiling points among nonpolar molecular compounds.
London Dispersion Forces
- Significant for nonpolar molecules; arise from temporary dipole-induced dipoles.
- Relevant data: Ne (-249°C, MW 20.1797 g/mol), He (-270°C, MW 4.003 g/mol), CO2 (-56°C, MW 44.01 g/mol), CH4 (-183°C, MW 16.04 g/mol), H2 (-259°C, MW 2.016).
Correlation of Molecular Properties
- Monatomic substances show correlation between freezing point, molecular weight, and electron count.
- Nonpolar polyatomic species display similar correlations.
Freezing and Intermolecular Strength
- Low freezing points indicate weak intermolecular attractions.
Comparing Boiling Points of CH3F and CH3Cl
- CH3F is expected to have a higher boiling point than CH3Cl due to greater polarity (higher EN for F).
- Ultimately determined by stronger dipole-dipole attractions in CH3F versus larger dispersion forces in CH3Cl.
Relative Importance of Interactions in CH3F and CH3Cl
- Actual boiling points: CH3F (-78°C) vs. CH3Cl (-24°C); CH3Cl has stronger dispersion interactions, leading to a higher boiling point.
Hydrogen Bonding
- Hydrogen bonds occur with H atoms bonded to F, O, or N; they are stronger than typical dipole-dipole and dispersion interactions but weaker than covalent bonds.
- Notable boiling points include: H2O (100°C), H2S (-60°C), NH3 (-33°C).
Ranking Dihydrides and Trihydrides
- Dihydrides: Predicted order based on dispersion interactions is H2O < H2S < H2Se; H2O ranks higher due to hydrogen bonding.
- Trihydrides: Order should be NH3 < PH3 < AsH3; NH3 is also elevated due to hydrogen bonding.
Intermolecular Interactions by Molecule
- CH4: Nonpolar, exhibits only dispersion forces.
- CH3Cl: Polar, exhibits both dispersion and dipole-dipole forces.
- CH3OH: Polar, exhibits dipole-dipole, dispersion, and hydrogen bonding interactions.
Freezing and Boiling Point Order
- Expected order: CH4 < CH3Cl < CH3OH, driven by the presence of hydrogen bonding in CH3OH.
Phase Diagrams
- Phase diagrams illustrate conditions (temperature and pressure) where substances exist as solid, liquid, or gas.
- Curves delineate boundaries between stable phases.
Water’s Phase Regions
- Solid phase: low temperature, high pressure.
- Liquid phase: moderate temperature and pressure.
- Gas phase: high temperature and low pressure.
Phase Transitions of Water
- As temperature increases from a solid, water first transitions to liquid, then to gas.
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Description
This quiz focuses on the concepts related to intermolecular forces, including bond dipoles and electronegativity of various elements. It also examines the boiling points of specific molecules as a result of these forces. Test your understanding and grasp these foundational chemistry principles.
