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Physical States of Matter BIOB111 Session 5 (Intermolecular Forces…
Physical States
of Matter
BIOB111 Session 5
Intermolecular
Forces (IMF)
attractive force between two molecules
weak compared to intramolecular forces
high in solids and liquids but negligible in gases
Van Der Waals
Interactions
occurs in all molecules - only type of intermolecular force present between non-polar molecules
results from momentary/short-lived uneven electron distributions in molecules
weak temporary intermolecular force that occurs between an atom/molecule (polar or non-polar) and another atom/molecule
dispersion forces,
London Forces
partially -ve - more electrons distributed close to it
partially -ve - less electrons close to it
interaction breaks as soon as both atoms/molecules have the same amount of electrons near them
Dipole-Dipole
Interactions
the greater the polarity (related to electronegativity difference) between the partial +ve and partial -ve charge, the greater the strength of the dipole-dipole interaction
electrostatic attraction between +ve and -ve. The +ve and -ve ends are close to each other
lots of different molecules form the electrostatic attractions, therefore lots of intermolecular bonds
atoms have a difference on partial charge (from a difference in electronegativity)
molecules orient themselves to maximise the +ve ‒ -ve interactions and minimise +ve ‒ +ve and -ve ‒ -ve interactions
occurs between
polar covalent molecules
What is required:
must have 2/more polar covalent compounds/molecules
partial charge difference
Hydrogen
Bonds
anything with H and O, N, S, F, Cl and a spare pair of electrons
H is the least electronegative atom, therefore can have the greatest different in partial charge between two atoms
δ- is due to a lone pair of electrons
H has the greater partial +ve than the other atoms that can be involved
occur between the H δ+ (partial +ve) and the δ- (partial -ve) on another atom of the other molecule
observed among
H containing polar covalent molecule
s where
H is covalently bonded
to a
highly electronegative small elemental atom
(O, N, S, F, Cl) (δ-)
dipoles can approach close to each other due to the small size of the H atom
unusually
strong dipole-dipole interactions
Intramolecular
Forces
true chemical bonds
strong forces within molecules
Solutions
a homogeneous mixture of 2/more substances in which each substance retains its own chemical identity
Components
Solvent
it is the medium in which other present substances dissolve
commonly H2O
the component present in the largest/greatest quantity in the solution
Solute
substance that dissolves in the solution
more than 1 solute can be present in one solution
the component present in a lesser amount (lower concentration) relative to the solvent
Solubility
#
the maximum amount of solute that will dissolve in a given amount of solvent under a specific condition
solubility depends on: temperature, pressure (increase in pressure aids gas dissolving into the solution), concentration, polarity (dissolve best in other polar solvents)
Factors
affecting
solubility
Temperature
solubility of solids increases with increasing temperature
gas solubility decreases with increasing temperature (particles move faster, less like to come into contact and mix with other molecules)
Pressure
has little effect on the solubility of solids and liquids in water
plays a major role in the solubility of gases in water (gases are more soluble at higher pressures)
Henry's Law
#
high pressure = higher solubility
low pressure = lower solubility
at a given temperature, the solubility of a gas in a liquid is directly proportional to the partial pressure of the gas above the liquid
at high partial pressure, more gas molecules dissolve in the liquid (gas particles are more condensed, increased contact between the gas and liquid)
Concentration
if any more solute is added to this saturated solution it will not dissolve. It will precipitate.
solubility can be increased in increasing temperature (saturation point becomes higher)
saturated solution contains the maximum amount of solute that can be dissolved in the solvent under given conditions (
saturation point
)
Polarity
polar solvents can dissolve polar solutes (e.g. H2O)
non-polar solvents can dissolve non-polar solutes
Concentration
of solutions
Percent
concentration
% of component in total volume of 100%
%v/v or % w/v
the amount of solute in a specified amount of solution
Molar
concentration
(Molarity)
moles/litre
1M solution of NaCl - how much NaCl in 1 mole. Weigh out molar mass in grams. 1M = 58.44g of NaCl dissolved in 1L of water
moles of solute in a solution divided by the litres of the solution
Parts per
million
represents 1 part of solute (weight or volume) in 1 million parts of solution (weight or volume)
1ppm = 1g/m3 = 1mg/L = 1μg/mL
Dilution
does not alter the number of moles of solute present
C1 x V1 = C2 x V2
the process of adding more solvent (usually water) to a solution in order to lower its concentration
Osmosis
the movement of a solute through a semi-permeable membrane from an area of higher solvent concentration to an area of lower solvent concentration
