Intermolecular forces Flashcards
Ionic and covalent bonds are examples of …
… intramolecular chemical bonds.
What are intermolecular forces?
- Interactions between atoms from separate adjacent MOLECULES
- Much weaker than intramolecular forces, but…
- Responsible for the physical properties of substances (boiling points, melting points)
- The stronger the attractive forces the higher the temperature of melting pointsand boiling points
Difference between intra and inter molecular forces?
‘Intra’ means within, so intramolecular forces occur within a
molecule. ‘Inter’ means between, so intermolecular forces occur
between molecules
What does the state of a substance depend on and how does state change?
The state of a substance depends largely on the balance between the kinetic energies of the particles and the interparticle energies of attraction.
By changing the temperature, we change the average kinetic energies of the particles.
This leads to changes in the state of the substance
Solid have…
…ordered arrangement
Liquid have…
…disordered arrangement
Gas have…
…arrangement in total disorder
Increasing the temperature breaks…
… the forces that keep molecules together
How many different types of intermolecular attractive forces are known to exist between neutral molecules?
Three types of intermolecular attractive forces are known to exist between neutral molecules.
Three types of intermolecular attractive forces are known to exist between…
…neutral molecules.
What are the three types of intermolecular forces?
Dipole – dipole forces
London dispersion forces
Hydrogen bonding
These three are a type of van der Waals forces.
Ion-dipole force exists between an…
… ion and the partial charge on the end of a polar molecule.
Ion-dipole force is an example of…
…polar molecule interactions
What is ion-dipole force especially important for?
Especially important for solutions of ionic substances - e.g. NaCl in water
When do dipole-dipole forces occur?
When neutral polar molecules (e.g water) attract each other when the positive end of one molecule is near the negative end of another and are weaker than ion – dipole forces
How do dipole dipole forces orient themselves?
Using attractive forces and repulsive forces.
Where do attractive forces occur in Dipole - Dipole forces?
Between opposite charges (+ and -)
Where do repulsive forces occur in Dipole - Dipole forces?
Between like charges (+ and + or - and -)
Two molecules that are attracting each other will spend…
…more time near each other than those repelling each other
In Dipole - Dipole Forces, overall there is a…
…net attraction
What happens in molecules of approx equal size in Dipole - Dipole Forces?
strength of intermolecular attraction
increases with increasing polarity
When do London Dispersion Forces occur?
In Non-polar atoms / molecules
Why is there no dipole in London Dispersion Forces?
no polarity so no dipole
What is the average distribution of electrons in Londom Dispersion Forces about a nucleus?
Average distribution of
electrons about each nucleus is symmetrical
What occurs with instantaneous distribution of electrons?
instantaneous dipole moment
In London Dispersion Forces, because electrons repel one another, the motions on one electron influence…
…the motions of the electrons on
its neighbours
What can London Dispersion forces cause?
An induced dipole
Strength of london dispersion force depends on …
…the ease of which the charge distribution can be distorted to induce the dipole Polarisability.
Dispersion forces increase in strength with…
…increasing molecular mass
Shapes of molecules can also influence…
…the magnitudes of dispersion forces. Allows molecules to have the same molecular formula but different boiling temps.
Dispersion forces operate in…
…all molecules whether polar or non-polar.
Polar molecules experience both…
…dispersion forces and dipole - dipole forces.
In hydrogen bonding, in general, the boiling points increase with …
…increasing molecular mass due to increasing dispersion forces.
In hydrogen bonding, the intermolecular attractions between these molecules are…
…abnormally strong and result from hydrogen bonding
What is a hydrogen bond?
A hydrogen bond is a special type of intermolecular attraction between the
hydrogen atom in a polar bond and an unshared pair of electrons on a nearby
small electronegative ion or atom.
Why is a bond between F, O and N and hydrogen quite polar?
Because F, O and N are so electronegative
Because F, O and N are so electronegative…
…a bond between any of these and
hydrogen is quite polar
Hydrogen has no inner electrons, thus the…
…positive end of the dipole has the proton of the H nucleus partially exposed
The electron ‘poor’
hydrogen is…
…very small, so it
can approach an
electronegative atom very
closely
Strength of Hydrogen Bonds:
much weaker than covalent bonds
Hydrogen bonds are much weaker than covalent bonds but are…
…much stronger than dipole – dipole or dispersion forces and as a result play important roles in many chemical systems.
Why are hydrogen bonds bologically important and crucial to life?
- DNA Hydrogen bonding between complementary base pairs – keeps the
two DNA strands together.
—–H-bonds are strong enough to
keep DNA stable, but weak
enough that they can be broken
and two DNA strands separated –
essential for “reading” the DNA - Proteins: Hydrogen bonding helps stabilize the three dimensional structure of the
folded polypeptide chain
—-The correct or native structure of a protein is essential to its activity, but again the H-bonds are weak enough to allow the protein to have internal motion.
London dispersion forces have their greatest effect on what structure of molecules?
Lengthy, straight molecules
How can you tell if a molecule is non polar?
- Its a monoatomic atom or diatomic atom (made up of only one element) EG: N2, O2, Cl2, F2, H2. These are all non polar
- Noble gases (group 8 elements - eg: He, Ne, Ar, Xe) are all non polar
- The molecule contains only carbon and hydrogen. Those molecules are non polar. EG methane and the alkanes. Carbon and hydrogen bonds are non polar.
- If the molecule has symmetry. If all the outer molecules are all the same. EG: CF4, SBr6, CO2, PCl5, BH3
- If electronegativity difference between the is less than 0.5
How can you tell if a molecule is polar?
- If the molecule has hydrogen bonding. If hydrogen is directly attached to nitrogen, oxygen or fluorine, its polar. EG: Water (H2O), NH3, HF, CH3OH (presence of alcohol (OH group) makes it polar due to presence of hydrogen bonding), CH3NH2 (Because of NH2), CH3COOH (because of OH group + presence of hydrogen bonding).
- If the molecule lack symmetry.
- Electronegativity difference of greater than 0.5 between molecules.
What higly electronegative elements are commonly bound to hydrogen and are commonly found in hydrogen bonding?
Fluorine, oxygen, nitrogen.
How to identify a hydrogen bond?
- Look for the presence of hydrogen atoms bonded to highly electronegative atoms (nitrogen, oxygen, or fluorine) in the molecule. These are the hydrogen bond donors.
- Identify nearby electronegative atoms (nitrogen, oxygen, or fluorine) with lone pairs of electrons. These are the hydrogen bond acceptors
