Intermolecular Forces, Electronegativity & Bond Properties (Cambridge (CIE) A Level Chemistry): Flashcards
Exam code: 9701
1/360Still learning
Know0
- FrontHydrogen Bonding
Define hydrogen bond.
Was this flashcard helpful?
Cards in this collection (36)
Define hydrogen bond.
A hydrogen bond is the strongest type of intermolecular force, formed between a highly polarised O-H, N-H or F-H bond and a lone pair on an O, N or F atom in an adjacent molecule.
State the two conditions required for hydrogen bonding to occur between molecules.
A molecule must contain a highly electronegative atom (O, N or F) with a lone pair of electrons.
A hydrogen atom must be covalently bonded directly to O, N or F, making the bond highly polar.
True or False?
Ice is denser than liquid water because the molecules in ice are more tightly packed.
False.
Ice is less dense than liquid water. In ice, water molecules form a rigid open 3D hydrogen-bonded lattice with relatively long bond lengths, so the molecules are slightly further apart than in liquid water.
Each water molecule can form a maximum of .......... hydrogen bonds, because oxygen has .......... lone pairs and .......... polar O-H bonds.
Each water molecule can form a maximum of four hydrogen bonds, because oxygen has two lone pairs and two polar O-H bonds.
Why does water have a much higher enthalpy of vaporisation than expected from the trend in Group 16 hydrides?
The trend from H2S to H2Te suggests H2O should have an enthalpy of vaporisation of about 17 kJ mol-1. The actual value is almost three times larger because of the additional energy needed to break hydrogen bonds in water, which are absent in the other hydrides.
True or False?
NH3 can form more hydrogen bonds per molecule than H2O.
False.
NH3 can form up to four hydrogen bonds per molecule (one as acceptor via its lone pair on nitrogen, three as donors via N–H bonds) — the same maximum as H2O, not more. The statement is false because NH3 cannot form more than H2O.
Hydrogen bonds are strongest when the angle between the covalent bond and the hydrogen bond is .......... because this .......... the electrostatic attraction.
Hydrogen bonds are strongest when the angle between the covalent bond and the hydrogen bond is 180° because this maximises the electrostatic attraction.
Why does water have a high surface tension?
Surface molecules are pulled downwards by hydrogen bonds with interior molecules. This compresses and tightens the surface layer, resisting external forces. Interior molecules are pulled in all directions so the net force is zero.
True or False?
Hydrogen bonding is the strongest type of intermolecular force.
True.
Hydrogen bonding is the strongest intermolecular force. It is a special, stronger case of permanent dipole-permanent dipole interaction, arising when H is bonded directly to O, N or F.
Define electronegativity.
Electronegativity is the ability of an atom to draw a pair of electrons towards itself in a covalent bond.
How does electronegativity change across a Period and down a Group?
Electronegativity increases across a Period and decreases going down a Group.
True or False?
A covalent bond between two atoms of the same element is polar.
False. When two atoms have the same electronegativity, the bonding electrons are shared equally and the bond is nonpolar.
In a polar covalent bond, the more electronegative atom gains a partial charge of .......... and the less electronegative atom gains a partial charge of ...........
In a polar covalent bond, the more electronegative atom gains a partial charge of δ− and the less electronegative atom gains a partial charge of δ+.
Define dipole moment.
A dipole moment is a measure of how polar a bond is. The arrow representing it points towards the δ− end of the dipole.
What two factors must be considered when deciding whether a molecule with more than two atoms is polar overall?
The polarity of each bond in the molecule.
The arrangement of those bonds — if individual dipole moments cancel each other out, the molecule is nonpolar overall.
True or False?
CCl4 is a polar molecule because it contains four polar C–Cl bonds.
False. Although CCl4 contains four polar C–Cl bonds, the individual dipole moments cancel each other out due to the symmetrical tetrahedral arrangement, making CCl4 a nonpolar molecule.
The greater the difference in .......... between two bonded atoms, the more .......... the bond becomes.
The greater the difference in electronegativity between two bonded atoms, the more polar the bond becomes.
Explain why CH3Cl is a polar molecule but CCl4 is not, even though both contain polar C–Cl bonds.
In CH3Cl the four polar bonds (three C–H and one C–Cl) do not cancel each other out, giving a net dipole towards the electronegative Cl atom. In CCl4 the four identical C–Cl bond dipoles are arranged symmetrically and cancel each other out, so there is no overall dipole.
Define van der Waals' forces.
Van der Waals' forces are weak intermolecular forces between molecules. They include instantaneous dipole – induced dipole forces (London dispersion forces) and permanent dipole – permanent dipole forces.
What causes an instantaneous dipole to form in a non-polar molecule?
The electron charge cloud is constantly moving. At any moment it can be more concentrated on one side of the molecule than the other, creating a temporary uneven distribution of charge — an instantaneous dipole.
True or False?
Instantaneous dipole – induced dipole forces exist only between polar molecules.
False. London dispersion forces exist between all atoms and molecules, including non-polar ones.
Id–id forces increase with an increasing number of .......... in the molecule and with increasing .......... where molecules come close together.
Id–id forces increase with an increasing number of electrons in the molecule and with increasing contact points where molecules come close together.
Define permanent dipole – permanent dipole forces.
Permanent dipole – permanent dipole forces are the attractive intermolecular forces between the δ+ end of one polar molecule and the δ− end of a neighbouring polar molecule.
Propanone has a higher boiling point than butane, even though both molecules have the same number of electrons. Explain why.
Propanone is polar so it has permanent dipole – permanent dipole forces between its molecules. Butane is non-polar and only has weaker id–id forces. For molecules with the same number of electrons, pd–pd forces are stronger than id–id forces, so more energy is needed to separate propanone molecules.
True or False?
Pentane has a higher boiling point than 2,2-dimethylpropane because it has more electrons.
False. Both isomers have the same number of electrons. Pentane has a higher boiling point because its straight-chain shape allows more contact points between molecules, increasing id–id forces.
Define hydrogen bonding.
Hydrogen bonding is a strong type of permanent dipole – permanent dipole intermolecular force. It forms between a δ+ H atom bonded to an O or N atom in one molecule and the lone pair on an O or N atom in a neighbouring molecule.
Why is the δ+ charge on H particularly large in molecules that form hydrogen bonds?
H is bonded to a highly electronegative O or N atom. The O or N atom draws nearly all the electron density from the O–H or N–H bond towards itself, leaving the H with a large δ+ and the O/N with a large δ−.
Define intramolecular forces.
Intramolecular forces are forces within a molecule. The three types are ionic bonding, covalent bonding and metallic bonding.
What are the three types of intramolecular force and how does each form?
Ionic bonding — electrostatic attraction between cations and anions formed by electron transfer.
Covalent bonding — formed when outer electrons of two atoms are shared.
Metallic bonding — electrostatic attraction between positive metal ions and delocalised electrons in a metal lattice.
True or False?
Intermolecular forces are generally stronger than intramolecular forces.
False. Intramolecular forces are generally stronger than intermolecular forces.
Intermolecular forces are forces .......... molecules. They are also called .......... forces.
Intermolecular forces are forces between molecules. They are also called van der Waals' forces.
Define intermolecular forces.
Intermolecular forces are forces between molecules. They include permanent dipole – permanent dipole forces, hydrogen bonds and instantaneous dipole – induced dipole (London dispersion) forces.
Place the following in order from strongest to weakest: covalent bonding, instantaneous dipole – induced dipole forces, hydrogen bonding, permanent dipole – permanent dipole forces.
Strongest to weakest:
Covalent bonding
Hydrogen bonding
Permanent dipole – permanent dipole forces
Instantaneous dipole – induced dipole forces
True or False?
Hydrogen bonding is a special type of permanent dipole – permanent dipole force.
True. Hydrogen bonds are a stronger form of permanent dipole – permanent dipole interaction, arising from the large δ+ on H bonded to a highly electronegative O or N atom.
Instantaneous dipole – induced dipole forces arise between a .......... dipole and a neighbouring molecule with an .......... dipole.
Instantaneous dipole – induced dipole forces arise between a temporary dipole and a neighbouring molecule with an induced dipole.
Metallic bonding is described as an electrostatic attraction. What two species are attracted to each other?
In metallic bonding, positively charged metal ions (cations) are electrostatically attracted to the delocalised electrons that surround them in the metal lattice.
Sign up to unlock flashcards
or