Section A 2

Question 1

Covalent bonds:

Answer 1

• Link atoms within a molecule
• Strong bonds (~100s kJ/mol or ~100kBT)
• Short ranged
• Highly directional / geometrically well
defined
• Generally long lasting

Question 2

Non-Covalent Interactions:

Answer 2

• Between neighbouring molecules
(intermolecular), or non-bonded parts of the
same molecule (intramolecular)
• Weak interactions (~kBT)
• Longer range
• Less directional / geometrically well defined.
(depending on the type of n-c interaction)
• Generally short lived or fluctuating (though
not always…)

Question 3

Boltzmann constant

Answer 3

kB = 1.38 x 10-23 J K-1

Question 4

Znając:
Interaction: London dispersion

Podaj:
Acts between:
Examples:
Typical energy kJ/mol:

Answer 4

All types of molecules

H2, CH4, CL2, HCL

-5 (in gas phase)

The London dispersion, dipole-dipole and dipole-induced dipole interactions
are generally all classed as Van der Waals interactions and are present between
all molecules.

Question 5

Znając:
Interaction: Dipole-dipole

Podaj:
Acts between:
Examples:
Typical energy kJ/mol:

Answer 5

Polar molecules

HCL

2 (in gas phase)

The London dispersion, dipole-dipole and dipole-induced dipole interactions
are generally all classed as Van der Waals interactions and are present between
all molecules.

Question 6

Znając:
Interaction: Dipole-induced dipole

Podaj:
Acts between:
Examples:
Typical energy kJ/mol:

Answer 6

A polar molecule and a molecule that may or may not be polar

Between HCL and CL2 or between HCL and CH4

Question 7

Znając:
Interaction: Ion-dipole

Podaj:
Acts between:
Examples:
Typical energy kJ/mol:

Answer 7

An ion and a polar molecule

A metal ion dissolved in water

15

Question 8

Znając:
Interaction: Hydrogen bond

Podaj:
Acts between:
Examples:
Typical energy kJ/mol:

Answer 8

A molecule containing the electromagnetive atoms O, N or F bound to H and a molecule containing O, N or F. The H atom forms a link between the two electromagnetive atoms

H20, NH3, HF

10-40*

• The hydrogen bond in the ion, [HF2]-, is particularly strong, 165 kJ/mol

Question 9

Znając:
Interaction: Ion- ion

Podaj:
Acts between:
Examples:
Typical energy kJ/mol:

Answer 9

Ionic groups

Between -NH3+ and -CO2- groups in proteins

250

Question 10

The overall average interaction energy between freely rotating dipolar molecules is dependent on ….

It is also inversely dependent on …., so the interaction becomes ….

Dipoles can arise as a result of fluctuating electron density. This instantaneous dipole in a non-polar molecule can then induce a dipole in the neighbouring non-polar molecule. The induced-dipoles attract. This is called …. and also…

Answer 10

the distance between them, r, to the sixth power:
E~= - 1 / r^6

the temperature
weaker as T increases.

the Dispersion Interaction, and also varies as 1/r6.

Question 11

Przypomnienie:

Answer 11

Acetone
Dipole moment, μ: 2.88 D
Boiling point (at 1 Atm): 56.2°C

Diethyl ether
Dipole moment, μ: 1.15 D
Boiling point (at 1 Atm): 34.5°C

Butane
Dipole moment, μ: 0 D
Boiling point (at 1 Atm): -0.5°C

Boiling point (at 1 Atm), in Kelvin:
He Ne Ar Kr Xe
4.22 27.1 87.3 121 166 K

Question 12

Cl2 is a gas at room temperature and pressure.Br2 is a liquid under the same
conditions. WHY?

Answer 12

Consider the electronic structure of Cl and Br:
Chlorine: 1s2 2s2 2p6 3s2 3p5
Bromine: 1s2 2s2 2p6 3s2 3p6 4s2 3d10 4p5

More electron-rich Br2 is more polarisable and therefore has much stronger dispersion interactions with neighbouring Br2 molecules

Neither Cl2 or Br2 are polar, so there cannot be a difference in permanent dipole interactions.

Question 13

When an ion is surrounded by polar molecules, many of
their dipoles align in the field of the ion and this lowers
the self-energy of the ion.

If the solvent is water, this is call ….. .
…. energies can be very large: e.g. 200 – 2000 kJ/mol at
room temperature.

Answer 13

hydration

Question 14

The polarity of a solvent is often discussed with respect to its dielectric constant also called …

+ description

Answer 14

the relative permittivity, εr.

Dielectric constants are measured experimental values that determine how well a material stores electrical energy when exposed to an electric field. Generally more
polar molecules have higher dielectric constants as they move in response to a field
so that their negative and positive ends are correctly aligned.

Question 15

solubility in water:

Hexane
Toluene
Tetradecanoic acid
Dichloromethane
Butan-1-ol
Ethanol
Glucose
Urea

Answer 15

Hexane 		                 insoluble
Toluene		                 insoluble
Tetradecanoic acid 	insoluble
Dichloromethane 	        slightly soluble
Butan-1-ol 		        soluble
Ethanol			        very soluble
Glucose 	           	        very soluble
Urea 			        very soluble

Question 16

solubility in hexane:

Water
Tetradecanoic acid
Toluene

Answer 16

Water insoluble
Tetradecanoic acid very soluble
Toluene very soluble

Question 17

The polarity of a solvent is often discussed with respect to its….also called….

….. are measured experimental values that determine ….

. Generally more polar molecules have …. as they ….

Answer 17

dielectric constant also called the relative permittivity, εr.

dielectric constants
how well a material stores electrical energy when exposed to an electric field

higher dielectric constants
move in response to a field so that their negative and positive ends are correctly aligned.

Question 18

But what happens when we try to dissolve non-polar species in water?

Answer 18

This reduces the entropy* of the system, increasing the
free energy.

The result: it is unfavourable for non-polar molecules to be surrounded by water, so they will tend to separate from water to minimise the contact area.

Question 19

…. is used to identify and separate components of a mixture using chromatography, e.g. thin layer chromatography (TLC).

Chromatography involves two phases ……
In TLC, ….. phase is….
… phase is is ….

Answer 19

The polarity of different solvents

the mobile phase and the stationary phase

the stationary phase is the solid SiO2 layer on the surface of the plate.
The mobile phase is a liquid solvent which will pass up the plate driven by capillary forces.

Question 20

Explain the Grotthuss mechanism:

Answer 20

Hydrogen bonds can become covalent bonds, and covalent bonds become hydrogen bonds, thus causing the transfer of charge from one location to another in aqueous solution without diffusion of any atoms through the water.