chp 12 and 15

Question 1

importance of shape of molecule

Answer 1

affects vapour pressure, melting point, boiling point and solubility. shape determines how it interacts with other molecules

Question 2

VSEPR

Answer 2

uses knowledge of valence shell to predict shape
based on principle that negatively charged electron pairs in valence shell repel each other. meaning that these electrons are arranged as far away from each other as possible
electron pairs in the single covalent bonds repel each other
lone pairs are treated the same as electron pairs in covalent bonds because they still take up space int he atom
double and triple bonds are treated as a single bond

Question 3

tetrahedral (methane)

Answer 3

electron pairs in the single covalent bonds repel each other
repulsion forces the bonds as far apart from each other as possible, so hydrogens are arranged in a tetrahedral shape at an angle of 109.5º

Question 4

pyramidal (ammonia)

Answer 4

lone pairs are treated the same as electron pairs in covalent bonds because they still take up space int he atom
3 hydrogen atoms for a pyramidal shape with the nitrogen. the ,one pair occupies more space because it isn’t covalently bonded to a and is therefore closer to the nitrogen. this means that the 3 covalent bonds are pushed closer together making the angle less than 109.5º

Question 5

trigonal planar (water)

Answer 5

2 lone pairs and 2 covalent bonds
4 electron pairs repel each other and 2 hydrogen atoms for a v shape with the oxygen. because of the two lone pairs the 2 covalent bonds are pushed closer together
angle around central atom less than 109.5º
120º

Question 6

linear (HF)

Answer 6

fluorine has 3 lone pairs and 1 covalent bond
four pairs form a tetrahedral arrangement due to mutual repulsion
results in a linear model
180º

Question 7

BH3

BeH2

Answer 7

3 electron pairs, no lone pairs
trigonal planar

2 electron pairs, no lone pairs
linear

Question 8

strength of intermolecular forces

Answer 8

covalent molecular forces have lower melting and boiling points because forces BETWEEN molecules are weaker. these forces are disturbed when covalent substance change state

Question 9

vapour pressure

Answer 9

pressure that gaseous molecules exert on the closed container walls when the rates of evaporation and condensation become equal

liquids with stronger intermolecular forces have lower vapour pressure molecules are held together tightly making it harder for them to escape from the surface of the liquid

increasing temp increase vapour pressure. molecules have a higher average kinetic energy that enables them to overcome intermolecular forces. as temperature increases, more particles have enough energy to escape from liquid to gas. increasing pressure

Question 10

boiling point of a liquid

Answer 10

temperature at which the liquids vapour pressure reaches the atmospheric pressure of the surroundings

Question 11

electronegativity and polarity

Answer 11

in intermolecular forces, the electrostatic attraction is between positive and negative charges in the molecules
charges result because of uneven electron distribution within molecules
electronegativity is the tendency of an atom in a covalent bond to attract electrons
in a covalent bond atoms are competing for electrons being shared between them

Question 12

non polar diatomic molecules

Answer 12

when atoms have identical electronegativities, then electrons are shared equally between them. non polar because there is no charges on either end
even distribution of valence electrons between atoms
if electronegativities between atoms are very similar, then it can be considered non-polar

Question 13

electron density

Answer 13

measure of the probability of an electron being present at a particular location within an atom

Question 14

polar diatomic molecules

Answer 14

2 atoms with different electronegativities, electrons stay closer to the most electronegative atom because they are more strongly attracted to that atom
polar: imbalanced electron distribution. has two oppositely charged poles
all diatomic with different atoms are polar to some extent, greater difference in electronegativities the greater the polarity

Question 15

delta

Answer 15

partial charge, adding up partial charges = 0 because charge are due to uneven sharing of electrons, so molecules are still neutral

negative: has larger share of electrons, excess of electrons
positive: lost some of its share of electrons

Question 16

dipole

Answer 16

separation of positive and negative charges

Question 17

polyatomic molecules polarity

Answer 17

symmetrical balanced dipoles= non-polar
methane: carbon is more electronegative but individual dipoles cancel each other out. results in a molecule with no overall dipole

in non-symmetrical molecules, individual dipoles don’t cancel each other, creating a net dipole making the molecule polar

Question 18

dipole dipole forces

Answer 18

only in polar molecules
results from attraction between positive and negative ends of polar molecules
weak since partial charge are small
more polar the molecule, the stronger the force

stronger force the higher the melting and boiling points because force bond molecules together in solid or liquid. higher energy is required to to break stronger bonds

Question 19

hydrogen bonding

Answer 19

only occurs in molecules in which a hydrogen atom is covalently bonded to a N,O or F
NOF are small and very electronegative so they strongly attract electrons in a covalent bond. creates a large partial positive charge on hydrogen atom, which is attracted to lone pairs of electrons in neighbouring NOF atoms

hydrogen in covalent bond with NOF is is exposed due to lack of electrons and is highly attracted to lone pairs of NOF in other molecules

strong intermolecular bond, x10 than dipole dipole, but 1/10 of ionic

Question 20

dispersion forces

Answer 20

forces of attraction in non polar substances (existence of intermolecular forces, without them nothing will hold molecules together and they would be gases)
caused by temporary (instantaneous) dipoles in molecules that result in random movement of the electrons surrounding molecule

strength increases as size of molecules increase. larger molecules have more electrons so it is easier to create temporary dipoles. larger molecules have higher melting and boiling points
molecules that form long chains have higher dispersion forces, more contact area to interact with it neighbouring molecules to form stronger forces

Question 21

how to increase polarity of a molecule

Answer 21

larger difference in electronegativities

high degree of asymmetry in the shape of the molecule causing an imbalance in the bond dipole

Question 22

requirement of hydrogen bonding

Answer 22

1. hydrogen atom covalent bonded to an NOF atom
2. lone pair of electron of NOF atom of neighbouring molecules

H bonds are caused by NOF’s small radius and high electronegativities and single electron of hydrogen atom
larger radii reduce concentration of negative charge around them.

Question 23

hydrogen bonding in water

Answer 23

ice floats because it is less dense
in ice water molecules are held in a crystal lattice. in which molecules are held further part then in liquid water
when ice melts crystal lattice collapses and molecules pack together more tightly
crystal has greater volume, 4 hydrogen bonds to 4 neighbouring molecules

electrostatic attraction
strong in water because of large electronegativity difference meaning larger partial charge

Question 24

water properties

Answer 24

can dissolve many substances easily
high specific heat capacity 
high latent heat of vaporisation 
expands on freezing
react with CO2 to make glucose and oxygen

Question 25

water shape

Answer 25

lone pairs of water have greater repulsion than electron pairs in covalent bonds so they push the hydrogen atoms closer together

Question 26

going down hydrides (except water)

Answer 26

melting and boiling points increase
Intermolecular forces are stronger (dispersion)
dispersion forces increase with strength as mass increases

Question 27

water vs hydrides

Answer 27

hydrogen bonds in water give it a much higher boiling and melting point

Question 28

high surface tension of water

Answer 28

water molecules at the surface are not completely surrounded by other water molecules, so they form hydrogen bonds everywhere BUT above.

molecules in body of water have no net force but surface level molecules have a downwards force. water molecules at surface are so attracted to each other by strong hydrogen bonds

Question 29

surface tension meaning

Answer 29

measure of the resistance of a liquid to increasing its surface area

Question 30

heat capacity of water

Answer 30

measure of a substances capacity to absorb and store heat energy, how much energy a substance absorbs as temp increases

SHC: measure of how much energy is required to increase temp by 1º of 1 gram of substance

SHC of water is relatively high because of hydrogen binds. these bonds can absorbs and store large amount of heat energy before they break

Question 31

latent heat

Answer 31

when a something reaches its boiling/melting point the temp remains constant until all of substance changes state even though further energy is being absorbed

energy still being absorbed while state is changing is latent heat
energy absorbed by a fixed amount of substance as it changes state at its melting (fusion)/boiling (vaporisation)point
depends on strength of intermolecular forces

LHF of water is 6.0KJ/mol. energy required to disrupt ice lattice by breaking some hydrogen bonds
LHV of water is 44KJ/mol.

Question 32

dissolution

Answer 32

particles of solute separate
particles of solvent separate
particles of solute and solvent are attracted to each other

forces:
holding solute, holding solvent (H Bonds), form between solute and solvent

Question 33

like dissolves like

Answer 33

H bonds between water i such stronger that dispersion forces in non polar. attraction between water molecules cannot be overcome and water doesn’t separate to form a solution

Question 34

water dissolving covalent

Answer 34

molecular compounds that form H bonds:
strength of intermolecular forces are similar, so they readily interact with each other
the more polar, the more likely it will dissolve

molecular compounds that ionise:
highly polar molecular compounds form ions when dissolved in water
polar covalent bonds break producing ions, covalent bond forms between ion and water molecule (makes hydronium in HCl), ion dipole attraction between ions and water

Question 35

water dissolving ions

Answer 35

ion dipole: attraction between ion and polar molecule
if ion dipole attraction are strong enough, the water molecules can pull out ions from crystal lattice and into the solution

ionic compound–>hydrated ions (dissociation)
energy required to separate ions from lattice must be less than energy released when ions are hydrated

Question 36

solubility

Answer 36

maximum amount of solute that will dissolve in a given amount of solvent
increasing temp increases solubility, at higher temps solute and solvent have more energy to overcome forces of attraction that hold particles together in the solid

saturated: no more solute can be dissolved at a certain temp
unsaturated: contains less solute needed to make solution saturated
supersaturated: contains more dissolved solute than a saturated solution

Question 37

crystallisation

Answer 37

when an unsaturated solution becomes saturated and crystals form.
1. cooling: may reduce the solubility of a dissolved solute to the point where not all of the substance available is soluble. slow cooling = larger crystals
2. evaporating the solvent from the solution
falser rate of evaporation = smaller crystals

Question 38

gases in water

Answer 38

Solubility decreases with rising temp

deoxygenates water so water must be cooled before released into environment