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
How do hydrogen bonds affect the m.p and b.p of molecules?
Hydrogen bonds increase the m.p and b.p as they are the strongest intermolecular force.
