3.3 Flashcards
What is surface tension?
Surface tension is the energy required to expand the surface of a liquid
Why is surface tension?
Molecules at the surface behave differently to molecules in the interior or bulk because they experience a net inward force of attraction
What shape has minimal surface tension?
The shape of a droplet minimises surface tension
Why does water on a waxy plant form droplets?
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.
What is the relationship between IMF and surface tension?
The stronger the IMF, the greater the surface tension required to break through the surface.
How do surfactants reduce surface tension?
Surfactants assemble at the air-surface interface and reduce the amount of energy required to expand the surface
What causes capillary action?
Capillary action is due to a difference in cohesive vs adhesive forces.
What are cohesive forces?
Cohesive forces are attractive forces between molecules in a liquid towards EACH OTHER (same substance)
What are adhesive forces?
Adhesive forces are attractive forces of a liquid and the container wall (DIFFERENT SUBSTANCES)
What causes capillary rise?
capillary rise occurs when adhesive forces are stronger than cohesive forces.
- Results in concave meniscus + water rising up tube
What causes capillary depression?
Capillary depression occurs when the cohesive force is larger than the adhesive force.
- Results in convex meniscus, e.g. mercury depressed in glass tube.
What is viscosity?
Viscosity is the resistance to flow
What affects viscosity?
- 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)
What are the units of viscosity?
Viscosity is measured in newton-seconds per square meter (Nsm-2)
What is diffusion?
Diffusion is the spontaneous mixing of materials resulting from random motion of molecules.
Occurs in liquids and gases.
What is the thermodynamic driving force for diffusion?
Diffusion is driven by entropy
What is vapour pressure?
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.
What is vapour pressure a function of?
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 .
What is going on with vapour pressure in a system at equilibrium?
In a system at eq vapour pressure is of constant value because rate of evaporation = rate of condensation.
what causes difference in vapour pressures?
Differences in vapour pressure are due to differences in IMF between different compounds (e.g. H bonds vs VDWs etc)
What is a volatile material?
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).
What’s the BP of a liquid?
The BP of a liquid is the temperature at which the vapour pressure = atmospheric pressure.
- Changes with altitudal pressure
What’s up with the can thing?
Idk man ¯_(ツ)_/¯
A 5.00% K3PO4 aqueous solution has a density of 1.043 g×mL-1.
Determine the molality of the potassium phosphate solution.
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.
Way to get molarity by density?
Remember that vol = mass/density.
use total vol (of solution) not of solvent.
Molality vs molarity?
Molarity: relative to volume of solUTION
- Dependent on T
Molality: relative to mass of solVENT
- Independent of T
Molality (mol/kg) is a measure of what quantity?
Concentration
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.
__3__ molecular weight
__1__ concentration
__2__ pressure
How is solubility (calculated by Henry’s Law, in molL-1) useful for finding molarity?
Literally the same value: both the no. mols per litre
