mr allen test mod2- dipoles Flashcards
shapes of molecules: how many bonds doe a double bond count for?(odd)
1 (c=c)= 1 bond
if a molecule has 2 bond electron pairs, state its shape and bond angle
linear, 180
if a molecule has 3 bond electron pairs, state its shape and bond angle
trigonal planar, 120
if a molecule has 4 bond electron pairs, state its shape and bond angle
tetrahedral, 109.5
if a molecule has 6 bond electron pairs, state its shape and bond angle
octahedral, 90
shapes of molecules, why is O2 an exception, why isn’t it linear??
there are two lone pairs, as a result of extra repulsion, bond angles tend to be slightly less and the bonds are squeezed together
why does NH3 not have a trigonal planar shape
- it has 3 bond pairs and 1 lone pair so the shape is based on a tetrahedral ( bond angles go from 109.5 to 107)
what is oxygens shape based on? ( it isn’t linear)
-4 total bonds (2 lone, 2 bonding pairs) so tetrahedral
how much do you minus for every lone pair?
2.5
why are intermolecular forces weak
as it takes not a lot of energy for them to break unlike strong covalent bonds- however if there are a lot of covalent bonds, a lot of energy is required.
what are physical forces?
the forces between different molecules within a substance (intermolecular forces), there are 3 different types of physical forces.
what are the three types of physical forces (weakest to strongest)
induced dipole-dipole interactions(london forces)
permanent dipole dipole interactions
hydrogen bonds
describe induced dipole-dipole interactions( london forces)
- at any instant in time, it is possible that more electrons will lie to one side of the atom/molecule than the other An instantaneous dipole is produced. this induces weak dipole in nearby atoms/molecules. this creates weak forces between atoms/molecules. (helium example)
describe what happens when size of a molecule/ atom increases and there are more electrons (london)
boiling point increases as the size of the molecule/atom increases, this is because as the sizes of the atoms/ molecules increase and there are more electrons, the size of the induced dipole dipole interaction forces increase, thus more energy required to break stronger forces.
what is electronegativity?
the ability of an atom to attract the pair of electrons in a covalent bond itself
describe trend of electronegativity on periodic table…
as you go up and across, electronegativity increases.
talk about permanent dipole-dipole interactions(why do they occur in terms of electrons)- how do polar covalent bonds FORM
electrons in a covalent bond on average spend their time nearer to one of the 2 bonded atoms. That atom gains a small negative charge. the other atom gains a small positive charge, this produces a polar covalent bond
what is the most electronegative atom
Fluorine
simmilar atoms have simmilar electronegativity and so molecules will be non polar, give an example of a non polar molecule
carbon and hydrogen
when atoms have a difference in electronegativity, what is formed?
a polar molecule ( difference in polarity in a dipole)
polar molecules???
often occur within molecules, we must consider the remaining bonds and shape of the molecule to determine if the molecule itself will be polar.
is co2 polar or non polar
non-polar but made up of polar bonds
what are the rules if a molecule is polar or non polar?
- molecules are polar id they contain bonda and are asymmetric
- if a molecule contains polar bonds but is symmetrical, its non polar.
what is the movement of symmetrical
no net movement
why is water polar
not a symmetrical molecule, net upwards movement
why is co2 non polar
overall, dipoles cancel (tug of war). non-polar but made of polar bonds.
what are polar molecules held by
by dipole-dipole interactions (2nd type)
talk about occurrence of dipole-dipole interactions “2”
between molecules containing polar bonds in addition to the basic induced dipole-dipole forces.
the extra attraction between dipoles mean that more energy must be used to separate molecules.
get higher melting/boiling points that expected given mass
what is the 3rd imf ( hydrogen bonding) a version of
permanent dipole-dipole
what is hydrogen bonding exclusive to
F,O,N
Talk about hydrogen bonding
strongest imf
water is highly polar due to large difference in electronegativity between o and h
this results in a very strong dipole-dipole attraction called a hydrogen bond between water molecules.
can occur between molecules containing H,F,O,N atoms as N,O,F have high electronegativity and H has a low electronegativity,
what are the features of drawing hydrogen bonding?
- lone pairs on O,N,F
- partial charges on atoms
- dashed line for hydrogen bonds and solid bond for covalent bonds.
water has a high melting/ boiling point, explain why?
the strong H bonds need to be broken which requires more energy tan other imfs
ice floats on water, explain…
the long H bonds hold water molecules apart in lattice structure giving it a larger volume in liquid water
water has a high surface tension and viscosity, explain
the string H bonds need to be broken
what is the shape and angle of a molecule with 3 bond pairs and one lone pair
pyramidal
107 degrees
Give an example of a simple covalent molecule which has all bond angles equal to 90°
SF6
Give an example of a simple covalent molecule which has all bond angles equal to 180 degrees
CO2
