3.1.3.7 Forces between molecules Flashcards
What is an intramolecular force?
Forces within a molecule, that are usually covalent bonds
Eg. Single, double, triple and co-ordinate bonds
What is an intermolecular force?
Forces between molecules
There are 3 types of intramolecular force, permanent dipole-dipole forces, induced dipole-dipole forces and hydrogen bonding.
What is a permanent dipole-dipole force?
Forces found between polar molecules
-if a compound is made of polar molecules, it will have permanent dipole-dipole forces between the + end of one molecule, and the - end of another.
-it is a weak intermolecular force of attraction
What is an induced dipole-dipole force?
Forces that are present between all atoms and molecules that exist
-known as van der waals forces, London forces and dispersion forces
How are van der waals forces formed?
-in a non-polar atom or molecule, the electron clouds are constantly moving, so at any time it can have an instantaneous dipole due to uneven distribution of electrons
-this causes the bond to have a temporary dipole
-this then induces temporary dipoles in neighbouring atoms and molecules, in a domino effect
-the two dipoles are attracted together by induces dipole-dipole forces of attraction
How are non-polar molecules held together?
-by induced dipole-dipole forces of attraction
-dipoles are temporary as they are constantly being created or destroyed
-a dipole is always there at any given time, so the atoms in a molecule remain attracted to each other
-the only forces present in non-polar molecules are induced dipole-dipole forces
What is hydrogen bonding?
Only formed when hydrogen forms a covalent bond with nitrogen, oxygen or fluorine
How are hydrogen bonds formed?
-nitrogen, oxygen and fluorine are more electronegative than hydrogen, so attract the electron pair in the covalent bond towards themselves
-this forms a polar bond
-the hydrogen atoms have a high charge density as they only have one shell of electrons
-the +H atom can be attracted to the lone pair of electrons of nearby N, O or F atoms in nearby molecules
-this is known as hydrogen bonding
Which two groups indicate hydrogen bonding in a structure?
Amine group
Hydroxyl group
What are the 3 factors that influence the strength of intermolecular forces?
-molecular shape
-molecular size
-molecular distance
Describe why ice floats on water
-ice has a lower density than water
-in water, molecules are closer together as hydrogen bonds constantly break and reform, so it does not have a lattice structure
-in ice, more stable hydrogen bond hold molecules further apart in a regular lattice structure
-the molecules in solid ice are therefore further apart than in liquid water, so ice has a lower density than water
Describe how molecular shape impacts the m.p and b.p of molecules
-unbranded and straight chain structures means molecules are packed more closely
-this increases the strength of intermolecular forces between them
-this requires lots of energy to overcome
-this means they have a higher m.p and b.p than branched chain molecules.
Give the relative strength of each bond
Ionic and covalent- 1000
Hydrogen- 50
Permanent dipole-dipole- 10
Induced dipole-dipole- 1
Describe how molecular size impacts the m.p and b.p of molecules
- a larger molecule contains more electrons so consists of a larger electron cloud
-this leads to stronger van der waals forces, which require lots of energy to overcome
-this means larger molecules have higher m.p and b.p.
Describe how distance between molecules impacts m.p and b.p
-increased distance between molecules decreases the strength of the intermolecular forces between them
-this decreases the m.p and b.p of the molecules
