Chapter 12.3 - Types Of Intermolecular Forces Flashcards
What are the three main types of intermolecular forces?
- Dipole - Dipole forces
- Hydrogen Bonding
- Dispersion forces
Explain Hydrogen Bonding?
Hydrogen bonding is a special form of Dipole - Dipole force.
Occurs when between molecules in which a hydrogen atom is covalently bonded to an OXYGEN, NITROGEN OR A FLUORINE ATOM.
There is a great electronegative difference between H and O,N,F atoms
Therefore they strongly attract the electrons in a covalent bond - this creates a significant partial (+) charge of H which attracted to lone pairs on N,O,F
This intermolecular bonds is 10x stronger than dipole-dipole bond, 1/10th strength of ionic or covalent bond
What are the 2 key requirements for Hydrogen bonding?
- A hydrogen atom covalently bonded to an Oxygen, nitrogen or fluorine atom
- A lone pair of electrons on the nitrogen, oxygen or fluorine atoms of neighbouring molecules.
It is both the very high electronegative and small atomic radii of O, F and N combined with a single electron of the Hydrogen atom = hydrogen bonding
Why is ice less dense than liquid water?
Ice is less dense than liquid water and can float in water due to HYDROGEN BONDING.
The Bent/‘V’ shape of a water molecule allows the water molecule to make 4 hydrogen bonds (H attracted to neighbouring O) with surrounding molecules.
They hydrogen bonding holds the water molecules in ice in a regular crystal lattice.
In this lattice, the molecules are held further apart in liquid water.
When ice melts into liquid water, the density of the water increases rapidly Because the OPEN CRYSTAL COLLAPSES and water molecules pack together more tightly.
Why are intermolecular forces important?
Without intermolecular forces there would be nothing to hold molecules together in forms of liquids and solids - only will exist as gases.
What are dispersion forces?
The forces of attraction between NON-POLAR substances are dispersion forces.
**It is a very weak and temporary force of attraction.
They are a result of TEMPORARY dipoles - resulting due to random movements of electron surrounding the molecule.
Theses temporary dipoles are also known as INSTANTANEOUS DIPOLES
They are always present between polar and non polar molecules as electrons are constantly in motion within atoms.
Explain how dispersion forces form within non-polar substances
- In a molecule electrons are constantly moving around atoms
- Occasionally the electrons gather more closely at one end of the molecule; causing that end to be more negative and the other more positive
* This known as a temporary dipole - Temporary dipoles induce/create dipoles in neighbouring molecules
- The neighbouring molecules then induce dipoles in their neighbours and so on…
The temporary dipoles attract each other to create the intermolecular forces known as DISPERSION FORCES.
What is INSTANTANEOUS DIPOLE?
When non polar molecules come into close contact the electrons on one molecule are repelled creating an INSTANTANEOUS DIPOLE
How does the size of a molecule affect dispersion forces?
Larger molecules have a larger number of electrons.
It is easier to produce temporary dipoles with molecules with large number of electrons.
Larger molecules = STRONGER DISPERSION FORCES
- has higher melting and boiling point
How does the shape of molecule also influence the STRENGTH of dispersion forces?
Molecules that are long and thin; forming long chains will tend to have stronger dispersion forces.
- Long and thin means it allows more space for contact area to interact with its neighbouring molecules to from stronger dispersion forces.
Being short, fat and compact decreases surface area - LESS STRONG DISPERSION FORCES
Compare dispersion forces with Hydrogen Bonding and Dipole- dipole forces
Dispersion forces in substances with small polar molecules do not have a significant influence on the properties of the substance.
- Weaker than dipole-dipole
- greatly weaker than H bonds
However in larger molecules, the dispersion forces can dominate over the dipole-dipole forces and H bonds
- because there are more electrons
What are DIPOLE - DIPOLE FORCES ?
Dipole-dipole forces are forces of attraction with ONLY occur in POLAR molecules.
They result from the attraction between the (+) and (-) ends of polar molecules.
What affects the strength of Dipole-dipole forces?
Dipole-Dipole forces are relatively weak since the partial charges on the molecules are small.
However the MORE POLAR a molecule is, the STRONGER the dipole dipole forces are.
*Polarity will be larger when there is a large difference in the electronegativities of the atoms
Or high degree difference with the ASymmetry in the shape of the molecule causing a large imbalance in bond dipole bonds.
How does the strength of Dipole-Dipole forced directly related to the melting and going points of the substance?
Stronger dipole-dipole forces = higher melting and boiling points
*This is because dipole - dipole forces bond the molecules together in the solid of liquid
*stronger dipole-dipole forces require more energy (higher temperature) to break these bonds ad change the solid to a liquid or liquid to a gas.
Dipole - dipole interactions
Solubility is affected by this force of attraction - where like dissolves like
Polar substances are soluble in Polar solvents
