Molecules: Shapes & Forces (AQA A Level Chemistry): Flashcards

Fluorine has the highest electronegativity, with a value of 4.0 on the Pauling scale.

A primary alcohol is an alcohol in which the carbon bearing the –OH group is bonded to only one other carbon (or to no other carbon in the case of methanol), giving the structure RCH₂OH.

Electronegativity increases with increasing nuclear charge and decreasing atomic radius, but decreases with increased shielding.

True.

Moving across a period increases nuclear charge (constant shielding), and moving up a group decreases shielding and atomic radius — both effects increase electronegativity, so it is greatest at the top-right (fluorine).

Induced dipole-dipole forces are weak intermolecular forces that exist between all atoms and molecules, caused by temporary dipoles arising from the uneven distribution of electron density, which induce dipoles in neighbouring molecules.

False.

Intramolecular forces (e.g. covalent bonds) are stronger than intermolecular forces. For example, a hydrogen bond is approximately one-tenth the strength of a covalent bond.

For hydrogen bonding to occur, a molecule must contain a hydrogen atom covalently bonded to a highly electronegative atom — specifically O, N or F.

Permanent dipole-dipole forces are attractive intermolecular forces between neighbouring polar molecules that each have a permanent dipole; the δ+ end of one molecule is attracted to the δ− end of another.

One water molecule can form up to four hydrogen bonds: two as a donor (via its two O–H bonds, each donating a hydrogen to a lone pair on a neighbouring oxygen) and two as an acceptor (via its two lone pairs on oxygen, each accepting a hydrogen from a neighbouring O–H bond).

Propanone has a higher boiling point than butane despite having the same number of electrons because propanone is polar and has permanent dipole-dipole forces, which are stronger than the induced dipole-dipole forces in butane.

True.

Hydrogen bonding is a special, particularly strong type of permanent dipole-permanent dipole interaction. It occurs when H is bonded to O, N or F, making the H sufficiently δ+ to attract a lone pair on O, N or F in a neighbouring molecule.

A reduction reaction is a reaction in which a species gains electrons (or hydrogen, or loses oxygen). In organic chemistry, reduction typically converts a carbonyl compound to an alcohol.

False.

Ice is less dense than liquid water by about 9%. In ice, molecules form an open lattice held by hydrogen bonds with relatively long bond lengths, so they are slightly further apart than in liquid water.

Water has an anomalously high boiling point for a molecule of its size because of hydrogen bonding between molecules, which requires a lot of energy to overcome.

H₂O has hydrogen bonds in addition to induced dipole-dipole forces. The other Group 16 hydrides lack hydrogen bonds, so their enthalpies of vaporisation are determined only by induced dipole-dipole forces, which increase gradually down the group. The hydrogen bonds in water make its enthalpy of vaporisation almost three times larger than the extrapolated trend predicts.

Ice floats on liquid water because ice has a lower density than liquid water, due to its open lattice structure held together by hydrogen bonds.

Anomalous properties are physical properties of water that are unexpectedly different from those predicted by general trends, such as its unusually high melting point, boiling point, surface tension and lower density as a solid — all caused by hydrogen bonding.

True.

Hydrogen bonds are the strongest type of intermolecular force. A large amount of energy must be supplied to overcome them, which is why water has unusually high melting and boiling points relative to its molecular mass.