3.3

Question 1

What is surface tension?

Answer 1

Surface tension is the energy required to expand the surface of a liquid

Question 2

Why is surface tension?

Answer 2

Molecules at the surface behave differently to molecules in the interior or bulk because they experience a net inward force of attraction

Question 3

What shape has minimal surface tension?

Answer 3

The shape of a droplet minimises surface tension

Question 4

Why does water on a waxy plant form droplets?

Answer 4

Water forms droplets on a waxy leaf because there is a stronger attraction between water molecules due to the H bond than the dipole-induced dipole of water and the hydrophobic leaf.

Question 5

What is the relationship between IMF and surface tension?

Answer 5

The stronger the IMF, the greater the surface tension required to break through the surface.

Question 6

How do surfactants reduce surface tension?

Answer 6

Surfactants assemble at the air-surface interface and reduce the amount of energy required to expand the surface

Question 7

What causes capillary action?

Answer 7

Capillary action is due to a difference in cohesive vs adhesive forces.

Question 8

What are cohesive forces?

Answer 8

Cohesive forces are attractive forces between molecules in a liquid towards EACH OTHER (same substance)

Question 9

What are adhesive forces?

Answer 9

Adhesive forces are attractive forces of a liquid and the container wall (DIFFERENT SUBSTANCES)

Question 10

What causes capillary rise?

Answer 10

capillary rise occurs when adhesive forces are stronger than cohesive forces.
- Results in concave meniscus + water rising up tube

Question 11

What causes capillary depression?

Answer 11

Capillary depression occurs when the cohesive force is larger than the adhesive force.
- Results in convex meniscus, e.g. mercury depressed in glass tube.

Question 12

What is viscosity?

Answer 12

Viscosity is the resistance to flow

Question 13

What affects viscosity?

Answer 13

• IMF
• Chain length: liquids composed of long, flexible molecules can entangle, resulting in inc viscosity.
• Temp (dec. with temp: molecules have more kinetic energy so IMF more easily overcome)

Question 14

What are the units of viscosity?

Answer 14

Viscosity is measured in newton-seconds per square meter (Nsm-2)

Question 15

What is diffusion?

Answer 15

Diffusion is the spontaneous mixing of materials resulting from random motion of molecules.
Occurs in liquids and gases.

Question 16

What is the thermodynamic driving force for diffusion?

Answer 16

Diffusion is driven by entropy

Question 17

What is vapour pressure?

Answer 17

Vapour pressure is the pressure exerted by molecules in gaseous phase of a liquid at equilibrium between its liquid and gaseous phase.

aka the pressure of its vapour in eq with the liquid phase in a closed system

Vapour pressure occurs because some molecules will have enough energy to overcome intermolecular forces and become a vapour.

Question 18

What is vapour pressure a function of?

Answer 18

Vapour pressure is a function only of temperature
(Increases as temp inc because molecules have more kinetic energy so more are able to overcome liquid forces and enter gaseous phase)
- non-linear relationship: related to the molar enthalpy of vapourisation .

Question 19

What is going on with vapour pressure in a system at equilibrium?

Answer 19

In a system at eq vapour pressure is of constant value because rate of evaporation = rate of condensation.

Question 20

what causes difference in vapour pressures?

Answer 20

Differences in vapour pressure are due to differences in IMF between different compounds (e.g. H bonds vs VDWs etc)

Question 21

What is a volatile material?

Answer 21

A material with a high vapour pressure is described as volatile. This is because it is held together by weak IMF so more molecules can escape into the gaseous phase (i.e. why you can smell petrol).

Question 22

What’s the BP of a liquid?

Answer 22

The BP of a liquid is the temperature at which the vapour pressure = atmospheric pressure.

• Changes with altitudal pressure

Question 23

What’s up with the can thing?

Answer 23

Idk man ¯_(ツ)_/¯

Question 24

A 5.00% K3PO4 aqueous solution has a density of 1.043 g×mL-1.

Determine the molality of the potassium phosphate solution.

Answer 24

Find the mass ratio (50g:950g) then calculate the no. of moles based off this: 50g/212gmol-1. Then find molality by dividing by mass of solvent: /950g. Ignore density if given.

Question 25

Way to get molarity by density?

Answer 25

Remember that vol = mass/density.

use total vol (of solution) not of solvent.

Question 26

Molality vs molarity?

Answer 26

Molarity: relative to volume of solUTION
- Dependent on T
Molality: relative to mass of solVENT
- Independent of T

Question 27

Molality (mol/kg) is a measure of what quantity?

Answer 27

Concentration

Question 28

Osmotic pressure is described by the equation Π= iMRT. Osmotic pressure is directly related to the [1] of a solute (in mol/L). If we can measure the [2] required to stop osmosis (the osmotic pressure) we can determine the [3] of an unknown substance.

Answer 28

__3__ molecular weight
__1__ concentration
__2__ pressure

Question 29

How is solubility (calculated by Henry’s Law, in molL-1) useful for finding molarity?

Answer 29

Literally the same value: both the no. mols per litre