Intermolecular forces Flashcards
describe van der waals forces
A type of intermolecular forces which occurs between all molecules, caused by instantaneous (temporary) dipoles
What is a temporary dipole
Temporary dipole is created when the electron distribution in a neutral atom/molecule shifts -
formed temporarily due to random electron movement
What is an induced dipole
When the temporary dipole of one molecule affects the electron distribution in nearby molecules.
It induced dipoles in the surrounding molecules
electrons are randomly moving from place to place
atom 1 atom2 and atom3
electrons are randomly moving from place to place
and for a fraction of a second, more electrons are on the right hand side of the atom - how does this affect the other atoms
for a fraction of a second, more electrons are on the right hand side of the atom 1
for this fraction of a second - atom 1 has a dipole
left side of atom 1 has slight positive charge
right side of atom 1 has slight negative charge
CALLED AN INSTANTANEOUS/TEMPORARY DIPLE
Because right side of atom 1 has a slight negative charge - this repels electrons in atom 2
causes e- in atom 2 to move towards right side of atom 2
this means that atom 2 now has a dipole
called induced dipole - dipole in atom 2 did not happen randomly caused by dipole on atom 1
negative charge on right side atom 2 - repels electrons on atom 3 - causes e- in atom 3 to move to right side of atom
atom 3 now has a dipole - induced
caused by atom 2
ALL THESE DIPOOLWA NOW EXPERIENCE A FORCE OF ATTRACTION
This attraction is called van der waals forces
explain how temporary dipoles are temporary and not permanent
Temporary as it has just formed instantaneously due to random electro movement
explain why all atoms/molecules experience van de waals forces
Van der waals forces are caused by random electron movement
therefore every single atom/molecule will experience van der vwaals forces even if they experience other intermolecular forces as well
what do strength of van der waals forces depend on
Strength of van der waals forces depend on number of electrons
Explain why helium has a lower boiling point than xenon
Xenon has more electrons
Therefore have stronger van der waals forces between molecules/experience stronger van der Waals forces
therefore stronger intermolecular forces
therefore higher BP
as no. of electrons increases, BP increases
since the strength of van der waals forces increases as no. of electrons increases
describe permanent dipole-dipole interactions/forces
Intermolecular force resulting from the attraction between molecules with permanent dipoles
permanent dipoles - there is a significant difference in electronegativity between dipoles - ends of molecule
describe how permamnent dipole-dipole interactions form
When two molecules with permanent dipoles/polar molecules get near enough, then their permanent dipoles can lead to an atttraction
called permanent dipole- dipole interaction
H - Cl ——— H - Cl
——— = permanent dipole-dipole interaction
attractive forces
H - delta positive
Cl -delta negative
Which types of molecules can experience permanent dipole-dipole forces/interactions
Only molecules with a permanent dipole can experience permanent dipole-dipole force interaction
permanent overall dipole = dipole moment
explain why CHCL3 can have permanent dipole-dipole forces but CCL4 cannot
CHCl3 has a permanent overall dipole ( a dipole moment)
So can experience /haspermanent dipole-dipole forces
However, in CCL4, the molecule is completely symmetrical
Because of this, the bond polarities cancel out so CCL4 has no overall permanent dipole
thereforce CCL4 does not have permanent dipole-dipole interactions
Explain why CCL4 has a higher BP than CHCL3
AND WHY THIS IS STRANGE
Strange because CHCL3 - experiences dipole-dipole forces (which are stronger than van der waals) while CCL4 does not
However permanent dipole dipole forces are not the only forces interacting
All molecules and atoms (Both ccl4 and chcl3 experience van der waals forces)
size/strength of van der waals forces depend on the number of electrons present
CHCL3 has a total of 58 e- while CCL4 has a total of 74
THEREFORE VAN DER WAAL FORCES ARE STRONGER IN CHCL3 - WHICH EXPLAINS THE HIGHER BOILING POINT
Explain why HCL < HBR < HIodide in terms of BP
All molecules have a permanent dipole
So permanent dipole-dipole interactions are acting in all three cases
The strength of permanent dipole decreases going down the table from HBr to HI
Due to decreasing electronegativity of the halogens as we move down group 7
However BP increases as we move from HCl to HI
because HI has more electrons than HBr which has more electrons than HCL
meaning it experiences/has stronger van der waals forces between its molecules
stronger intermolecular forces between its molecules
higher BP
When looking at permanent dipole dipole interactions remember, that Van der Waal forces will also be acting
When looking at permanent dipole dipole interactions remember, what other force will be acting
When looking at permanent dipole dipole interactions remember, that Van der Waal forces will also be acting
what is meant by hydrogen bonding
Hydrogen bonding is a type of intermolecular force in which a hydrogen atom interacts with a significantly more electronegative atom (Fluorine, Oxygen or Nitrongen)
H-F polar? + WHAT ELSE DOES IT CONTAIN
Fluorine - most electronegative element so it strongly attracts the pair of electrons in the covalent bond therefore molecule is polar with the hydrogen atom having a positive charge
Has 3 lone pairs of electrons
Describe how Hydrogen bonding works
Two molecules of H-F
H - F —— H - F
—— = HYDROGEN BOND
The positive hydrogen atom on the right hand molecule is attracted to the lone pair of electrons on the fluorine atom of the left hand molecule
tHIS ATTRACTION IS CALLED A HYRDROGEN BOND
When drawing the hydrogen bond, it must run from the hydrogen atom directly to the lone pair of electrons
strongest type of intermolecular force
rank intermolecular forces in terms of strength
Strongest
Hydrogen bonding
Permanent dipole-dipole interactions
Van der Waals forces
Conditions for hydrogen bonding to take place
A hydrogen atom needs to be bonded to a strongly electronegative element
The electronegative atom must have at least one lone pair of electrons
What this means, is that we find hydrogen bonding with fluorine, oxygen or nitrogen
Explain why hydrogen bonding is usually with fluorine, oxygen or nitrogen
Explain why we find hydrogen bonding with fluorine, oxygen and nitrogen
All three of these elements are strongly electronegative
All three have at least one lone pair of electrons
show hydrogen bonding is H-F
H - F —— H -F
Fluorine has 3 lone pairs
Delta positive and negative
straight line
show hydrogen bonding in water
H - O ——- H - O
/ \
H H
Oxygen here has 2 lone pairs of electrons
Hydrogen bonding is from hydrogen atom directly to the lone pair of electrons
Straight line
draw hydrogen bonding in ammonia molecule
h
\
H - N : ——- H - n :
/
h
Explain how hydrogen bonding affects the properties of water
Describe how hydrogen bonding gibes water its usual properties
H2O and H2S
Both compounds have two hydrogen atoms covalently bonded to a group 6 element
In water - hydrogen atoms are bonded to oxygen (second most electronegative element)
Means water molecules form hydrogen bonds
Because of this, when we boil water, a lot of energy is required to break hydrogen bonds - meaning water has a relatively high BP and MP
In hydrogen sulfide - hydrogen atoms are bonded to sulfur
Electronegativity of sulfur is very similar to hydrogen
Meaning h2s cannot form hydrogen bonds
therefore has a much lower BP of -60 degrees celcius (has other intermolecular forces like van der vaals forces)
Water is very unusual
The Solid form of water, Ice, is less dense than the liquid form, water.
Because of this, ice floats on the surface of water
Very few examples of substances where the solid is less dense than the liquid
This unusual property of water can be explained by hydrogen bonding
In liquid water, the water molecules are moving randomly
Sometimes, the water molecules are close together and sometimes they are further apart
Hydrogen bonds are constantly being formed and broken
As we cool the water down, the molecules move more slowly
As we reach the freezing point of water which is 0 degrees Celsius, the water molecules arrange themselves into an ordered structure.
This ordered structure is ice. This is stabilised by the network of hydrogen bonds
In ice, the water molecules are further apart than in liquid water
This makes ice less dense than water and because of this, ice floats on the surface of water
explain why the fact that ice is less dense than water is important for organisms
Means ice floats on surface of water
The ice insulates the water below, preventing the water from freezing completely
