9701 · 3.6
Intermolecular forces, electronegativity and bond properties flashcards
Revision flashcards for Cambridge 9701 Intermolecular forces, electronegativity and bond properties (syllabus 3.6). Flip, recall, then mark a real past-paper question.
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What is electronegativity?
Electronegativity is the ability of an atom to attract the bonding electrons in a covalent bond towards itself.
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What is a polar covalent bond?
A covalent bond between two atoms with different electronegativity values, resulting in an unequal sharing of electrons and the formation of bond dipoles ($\delta+$ and $\delta-$).
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What are the three conditions for a hydrogen bond to form?
1. A hydrogen atom is covalently bonded to a highly electronegative atom (Nitrogen, Oxygen, or Fluorine). 2. A lone pair of electrons is present on a neighbouring N, O, or F atom. 3. The lone pair attracts the $\delta+$ hydrogen atom.
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What are van der Waals forces (instantaneous dipole-induced dipole forces)?
Weak intermolecular forces caused by the random movement of electrons, creating temporary dipoles. These dipoles induce dipoles in neighbouring molecules, leading to a weak attraction. They exist between all molecules.
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Why does the strength of van der Waals forces increase with molecular size?
Larger molecules have more electrons. This increases the probability of forming larger temporary dipoles, resulting in stronger instantaneous dipole-induced dipole attractions.
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What are permanent dipole-dipole forces?
Electrostatic forces of attraction between the permanent partial positive charge ($\delta+$) on one polar molecule and the permanent partial negative charge ($\delta-$) on a neighbouring polar molecule.
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Why is $CO_2$ a non-polar molecule, even though it contains polar C=O bonds?
Carbon dioxide has a linear shape. The two C=O bond dipoles are equal in magnitude and point in opposite directions, so they cancel each other out, resulting in no overall molecular dipole.
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Why is the boiling point of water ($H_2O$) much higher than that of hydrogen sulfide ($H_2S$)?
Water molecules can form strong hydrogen bonds with each other. Hydrogen sulfide molecules are also polar but can only form weaker permanent dipole-dipole forces. More energy is needed to overcome the stronger hydrogen bonds in water.
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What is a common exam mistake regarding boiling and bonding?
Confusing intermolecular forces with intramolecular covalent bonds. Boiling a simple molecular substance involves overcoming the weak IMFs *between* molecules, not breaking the strong covalent bonds *within* them.
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Why is ice less dense than liquid water?
In ice, hydrogen bonds hold water molecules in a fixed, open lattice structure, with large empty spaces. In liquid water, the molecules are closer together as the rigid hydrogen-bonded structure has partially collapsed.
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Rank the three main types of intermolecular forces from weakest to strongest.
1. Van der Waals forces (weakest). 2. Permanent dipole-dipole forces. 3. Hydrogen bonds (strongest).