Week 2

Question 1

What does it mean if an ion is aqueated/hydrated?

Answer 1

An ion dissolved in water.

Aquated ions are a different species from anhydrous ions.

(Each ion surrounded by rapidly changing shell of water.)

Question 2

What do ionic solids dissociate into when dissolved in water?

Answer 2

Cations and anions.

Question 3

What does the chemical activity of an element depend on?

Answer 3

The species in which it exists.

Question 4

What are the basic features of the kinetic-molecular model of matter?

Answer 4

That all matter is made of small particles that are in random motion and that have space between them.

This also applies to liquids.

Question 5

What are the properties of water (H20)?

Answer 5

“The change of density of water with temperature is different from that of other common substances. Among substances swith similarly sized molecules, water has remarkably high specific heat capacity, molar enthalpy change of vaporisation, boiling point, and surface tension; and its equilibrium vapour pressure is relatively low.”

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Water’s density changes near freezing point.

1. When ice melts at 0°c the liquid water formed is 10% more dense than the ice, thus the ice floats in water. In contrast the solid phase of nearly every other substance sinks in its liquid.
2. 4°c is when water reaches it’s maximum density

The properties of water suggest that the molecules of water are rather strongly attracted to one another. (Strong intermolecular forces.)

S͟P͟E͟C͟I͟F͟I͟C͟ ͟H͟E͟A͟T͟ ͟C͟A͟P͟A͟C͟I͟T͟Y͟

The specific heat capacity of water is high when compared to other liquids (4.18).

E͟N͟T͟H͟A͟L͟P͟Y͟ ͟C͟H͟A͟N͟G͟E͟ ͟O͟F͟ ͟V͟A͟P͟O͟R͟I͟Z͟A͟T͟I͟O͟N͟

Water has a large enthalpy change accompanying vaporisation of water; one mole of water requires much more energy to be vaporised than 1 mol of other common substances with small molecule.

E͟Q͟U͟I͟L͟I͟B͟R͟I͟U͟M͟ ͟V͟A͟P͟O͟U͟R͟ ͟P͟R͟E͟S͟S͟U͟R͟E͟

At a given temperature, water has a lower equilibrium vapour pressure than other substances whose molecules are of a similar size.

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Water has a higher boiling point (100°c), this is due to it having a lower equilibrium vapour pressure.

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Water has a very high surface tension compared to that of other liquids.

Question 6

What is the ‘specific heat capacity’ of a substance?

Answer 6

The amount of heat required to raise the temperature of 1g of it by 1K.

This value varies from liquid to liquid.

Question 7

What is vaporization/evaporation?

Answer 7

The process in which a substance in the liquid state changes to the gaseous state.

At a molecular level, molecules escape from the liquid surface into a gaseous state, molecules RETAIN their identity; the O-H bonds are not broken.

Question 8

What are enthalpy changes?

Answer 8

An energy change under constant pressure conditions.

The symbol for enthalpy is H, and for enthalpy changes ∆H.

Question 9

What is the molar enthalpy change of vaporisation (ΔᵥₐₚH)?

Answer 9

The amount of energy required to bring about vaporization of one mole of a substance.

This varies from substance to substance.

Question 10

What is equilibrium vapour pressure (also known as just vapour pressure)?

Answer 10

When the pressure of vapour above a liquid comes to a constant value in a closed flask.

This situation is an example of dynamic equilibrium.

Question 11

What is the ‘volatility’ of a compound?

Answer 11

The tendency of a compound’s molecules to escape from the liquid phase and enter the vapour phase.

The higher the equilibrium vapour pressure at a given temperature, the more volatile the compound.

The higher the temperature is, the higher the equilibrium vapour pressure.

Question 12

What is the boiling point of a substance?

Answer 12

‘The temperature at which the vapour pressure of a liquid is the same as the pressure of the atmosphere acting down on the liquid’s surface.’ At this point bubbles of vapour will form in the liquid and rise to the surface.

Boiling point usually refers to the normal boiling point.

Question 13

What is the normal boiling point?

Answer 13

The particular boiling point when the external pressure is exactly 1 atm.

Question 14

What is the surface tension of a liquid?

Answer 14

The amount of energy required to increase its surface area, per square metre.

It’s units are J m⁻² “joules per square metre”

Liquids (especially water) behave as though they have a skin stretched across their surface that minimises the area of surface: energy is required to increase the surface area of a sample of any liquid.

Question 15

What are intermolecular forces?

Answer 15

Refers to the attraction between molecules.

Physical properties are determined by intermolecular forces.

Question 16

What is a dipole moment (µ)?

Answer 16

A force that aligns molecules toward a nearby electric charge, or towards the poles of an electrostatic field.

Question 17

What is an electric dipole?

Answer 17

A molecule behaving in the same way as a linear object with a positive charge at one end and a negative charge at the other.

Question 18

What happens to a dipole that is free to rotate and is placed in an electric field?

Answer 18

It will align itself with the field, the end with the positive charge pointing toward the negative plate of the field.

If the dipole is near a POSITIVE charge, it will rotate so that its negative end is pointing toward it.

If the dipole is near a NEGATIVE charge, it will rotate so that the positive end is pointing to it.

Question 19

What does electronegativity (χ) refer to?

Answer 19

“The ability of an atom to attract bonding electrons” that bind it to another atom in a molecule.

The greater the difference in electronegativity of two atoms bound to each other, the greater the charge separation is, and the more polar the bond.

Net negative charge resides on the atom with greater electronegativity.

Question 20

What is a polar bond?

Answer 20

When atoms bound to each other have different charges.

When atoms bound by a covalent bound are not identical the bonding of electrons are unequally distributed, the bond becomes polar.

Question 21

What does bond polarity refer to?

Answer 21

A measure of the magnitude of the charges in a polar bond.

Question 22

How do you determine if a substance whose molecules have more than one polar bond, is a dipole?

Answer 22

Determined by its three-dimensional shape.

Symmetrical = non-polar
Asymmetric = polar

Question 23

What is a non-polar substance?

Answer 23

A molecule that has polar bonds that are symmetrical, so the forces experienced in an electric field cancel out.

The molecules have zero dipole moment: non-polar molecules.

Question 24

What is a polar substance?

Answer 24

A molecule with polar bonds that are not symmetrically distributed.

Question 25

How is dipole moment (µ) measured?

Answer 25

The debye, “D” is the unit of measurement

Question 26

What is an electrostatic potential map?

Answer 26

A space-filling model of the molecule with colour shading: RED indicates regions of the molecule where the negative charge of electrons exceeds the positive charge on the nuclei, BLUE in regions where there is a deficiency of electrons and net positive charge.

The more red the colour the greater is the net negative charge, the more blue the colour the greater the net charge closer to zero.

Intermediate colours (orange, yellow, and green) indicate net charge closer to. zero.

Question 27

What is a dipole-dipole force?

Answer 27

“Forces of attraction between polar molecules.”

As polar molecules move past each other, they align themselves so that the positive end of one polar molecule is oriented toward the negative end of the adjacent molecule, the attraction between unlike charges result in stronger intermolecular forces than if the molecules were non-polar.

This type of intermolecular force is present in any substance whose molecules are dipoles.

Question 28

What is a hydrogen bond?

Answer 28

A particularly strong form of intermolecular dipole-dipole interactions, between a H atom on one molecule and an O, N or F atom on nearby molecules.

Hydrogen bonds are forces between atoms in different molecules, not the O-H covalent bonds within molecules.

The electronegativities of F, O and N are the highest of all elements, and the large difference in electronegativity in N-H, O-H and F-H bonds means that they are very polar, with partial positive charge on the H atom in each case.

Question 29

Why is the hydrogen bond in water molecules so strong?

Answer 29

There is a rapidly changing network of hydrogen bonding between the O atom on each molecule and one or both of the H atoms on nearby molecules.

• The difference between the electronegativity of O and H gives rise to polarity of the O-H bond.
• A hydrogen atom only has on electron, and when H is covalently bound to another atom, the electron is largely localised in the bond. Consequently, the positively charged hydrogen nucleus is rather exposed to adjacent molecules.
• The H atom is the smallest of all atoms. As a result, the H atom in a molecule can get very close to the O atom on other molecules. The force of attraction between opposite charges is inversely proportional to the square of the distance between them.
• A simple picture of the electron distribution in a water molecule is that some of the O atom’s electrons are in the covalent bonds to H atoms, and two pairs of electrons not used in bonding (non-bonding electrons, or lone pairs) are localised on the O atom. These provide two localised centres of negative charge on the O atom, each of which can engage in hydrogen bonding to H atoms in nearby water molecules.

Question 30

How much energy is required to break a hydrogen bond?

Answer 30

In the range of 5-30 kJ mol⁻¹

Question 31

How much energy is required to break a covalent bond?

Answer 31

100-500 kJ mol⁻¹

Question 32

What is the importance of hydrogen bonds?

Answer 32

Determining the properties of water and other liquids, play important roles in the structures of natural polymers such as proteins and DNA, as well as synthetic polymers like nylon.

Question 33

Are the forces in polar substances or non-polar stronger?

Answer 33

Polar forces are stronger.

There is a correlation between the magnitude of intermolecular force, and molar mass.

Question 34

What are “dispersion forces” / Landon forces / van der Waals force?

Answer 34

Intermolecular forces of attraction in non-polar substances.

Averaged over time, the electrons in a molecule (other than those in a covalent bonds) are symmetrically distributed. The electron cloud is rather mobile and may fluctuate, so that at any time the distribution over space may become distorted.

These are the only forces in non-polar substances but also found in polar substances along with dipole-dipole forces.

Question 35

What is an electron cloud?

Answer 35

The electrons around a molecule.

The electron cloud can be pulled and pushed around by electrical charges in their immediate environment.

Question 36

What is an instantaneous dipole?

Answer 36

The electron cloud is not exactly symmetrically distributed, and there is a region with a slight excess of electron matter (and a transitionary negative charge) and another region with a slight deficiency of electron matter (and a transitionary positive charge).

Question 37

What are instantaneous dipole-induced dipole forces?

Answer 37

The dispersion forces in non-polar substances.

As two non-polar molecules approach each other, an instantaneous dipole in one of them can lead to distortions in the electron cloud of the other.

Instantaneous dipoles can induce dipoles momentarily in neighbouring molecules, giving rise to attraction between them.

Question 38

What is polarisation?

Answer 38

The distortion of the electron cloud of an atom or molecule

Question 39

What is polarisability?

Answer 39

The ease at which the electron cloud of another atom or molecule can be distorted.

Molecules with large electron clouds can be polarised more readily than the electron cloud in a much smaller atom or molecule; this is due to the electrons being further from the nucleus and less tightly held.

Question 40

What can indicate the strength of disperion forces?

Answer 40

The higher the molar mass, melting and boiling point indicates stronger dispersion foces.

Within a family of compounds, the more electrons that are in the electron cloud of a molecule, generally, the greater is the polarisability of the electron cloud.