3.5- FORCES ACTING BETWEEN MOLECULES Flashcards
How are atoms in molecules and in giant structures held together?
by strong covalent, ionic, or metallic bonds
How are molecules and separate atoms attracted to one another?
by weaker forces called intermolecular forces
What does inter mean?
between
What happens if the intermolecular forces are strong enough?
then molecules are held closely enough together to be liquids or even solids
What are the three types of intermolecular forces and order from weakest to strongest?
van der Waals forces
dipole-dipole forces
hydrogen bonding
What is polarity the property of?
property of a particular bond
What may molecules with polar bonds have?
dipole moment
What may happen in molecules with more than one polar bond? (2)
effects of each bond may cancel, leaving molecule with no dipole moment
effects may add up so reinforce each other
What is the dipole moment?
sums up the polarity of all the bonds in the molecule
What determines what happens in a molecule with more than one polar bond?
shape of the molecule
What shape does carbon dioxide have?
linear molecule
What happens to the dipoles in carbon dioxide as it’s a linear molecule?
dipoles cancel
What shape does tetrachloromethane have?
tetrahedral
What happens to the dipoles in tetrachloromethane as it has a tetrahedral shape?
dipoles cancel
Where does dipole-dipole forces act between?
molecules that have permanent dipoles q
What will two molecules which both have dipoles do?
attract one another
Whatever the starting positions of the two molecules which have dipoles, what will they do?
molecules with dipoles will ‘flip’ to give an arrangement where the two molecules attract
What are all atoms and molecules made up of although they’re neutral overall?
positive and negative charges
What do the charges in all atoms and molecules produce?
very weak electrostatic attractions between all atoms and molecules
What is the very weak electrostatic attractions between all atoms and molecules called?
van der Waals forces
How long does any particular dipole last for?
just an instant- temporary dipole
What does the dipole do to nearby atoms?
affects electron distribution in nearby atoms
What happens as the dipole affects the distribution of electrons in nearby atoms?
nearby atoms attracted to atom with dipole
What happens when the electron distribution of the original atom changes?
induce new dipoles in the atoms around it, which will be attracted to original one
What can van der Waals forces also be called?
instantaneous dipole-induced dipole forces
Where and when do van der Waals forces act?
between all atoms or molecules at all times
Are VDW forces an addition to any other intermolecular forces?
yes
What is the dipole caused by?
changing position of the electron cloud
What causes a larger instantaneous dipole?
the more electrons there are
What increases the size of VDW forces?
size of VDW forces increases with number of electrons present
What types of atoms/molecules will produce stronger VDW forces?
atoms/molecules with larger atomic or molecular masses
What does atoms/molecules with larger atomic or molecular masses having stronger VDW forces explain? (2)
boiling points of noble gases increase as atomic numbers of noble gases increase
boiling points of hydrocarbons increase with increased chain length
What characteristics does hydrogen bonding have?
some characteristics of dipole-dipole attraction and some of a covalent bond
What does a hydrogen bond consist of?
hydrogen atom ‘sandwiched’ between two very electronegative atoms
What condition is needed for hydrogen bonding?
need a very electronegative atom with lone pair of electrons covalently bonded to hydrogen atom
Example of molecule than can exhibit hydrogen bonding?
water molecules
Is water polar?
yes
Why is water polar?
oxygen is much more electronegative than hydrogen
Why is the intermolecular bonding in water much stronger than expected? (2)
oxygen atoms in water molecules have lone pairs of electrons
in water hydrogen atoms highly electron deficient
because oxygen very electronegative and attracts shared electrons in bond towards it
hydrogen atoms in water positively charged and very small
these exposed protons have very strong electric field because of their small size
Between where is the hydrogen bonding formed in water molecules?
lone pair of electrons on oxygen atom of another water molecule strongly attracted to electron deficient hydrogen atoms
How are hydrogen bonds usually represented?
by dashes
What must there be for a hydrogen bond to form?
hydrogen atom that’s bonded to very electronegative atom. this will produce strong partial positive charge on hydrogen atom
very electronegative atom with lone pair of electrons
these will be attracted to partially charged hydrogen atom in another molecule and form the bond
What are the only atoms that are electronegative enough to form hydrogen bonds?
oxygen, nitrogen and fluorine
How can the effects of hydrogen bonding between molecules be seen?
looking at boiling points of hybrids of elements of groups 4, 5, 6, and 7 against period number
What do the noble gases show against period number?
gradual increase in boiling point as only forces acting between atoms are VDW + these increase with number of electrons present
Why is boiling point of water and ammonia higher than those of hybrids of other elements in their group?
hydrogen bonding present between molecules in each of these intermolecular forces of attraction make molecules more difficult to separate
What happens to hydrogen bonding in water in its liquid state/?
hydrogen bonds break and reform easily as molecules moving about
What happens to hydrogen bonding when water freezes?
water molecules no longer free to move about and hydrogen bonds hold molecules in fixed positions
What happens to the structure when water is frozen?
results in three-dimensional structure
What happens in order for water molecules to ft the three-dimensional structure when water is frozen?
molecules are slightly less closely packed than in liquid water
What does it mean as the water molecules are slightly less closely packed when water is frozen?
ice is less dense than water
As ice is less dense than water how do they form on ponds?
forms on top of ponds rather than at the bottom
Why is ice forming on top of ponds good?
insulates ponds and enables fish to survive through the winter
