chemistry exam stuff you dont know

Question 1

what determines the shape of molecules

Answer 1

the arrangement of electrons pairs surrounding the central atom

Question 2

what does VSEPR stand for and what does this theory state

Answer 2

stands for valence shell electron pair repulsion

theory states that electrons pairs in the valence shell of an atom repel each other, and are arranges as far apart as possible

Question 3

what does polarity refer to

Answer 3

related to the sharing of the bonding electrons between atoms

Question 4

what is the difference between a polar and non polar bond

Answer 4

non polar: electrons are shared evenly between atoms, equal charge distribution

polar: electrons are not shared evenly between atoms because different atoms have different electronegativities

Question 5

explain what a polar bond is

Answer 5

occurs between atoms with 2 different electronegativities

electrons spend more time moving around the atom with the greatest electronegativity

this leads to an uneven charge distribution in the bond

the region near the more electronegative atoms has a partial negative bond

the region near the less electronegative atoms has a partial positive bond

this is called a bond dipole

Question 6

what are intermolecular forces

Answer 6

weak attractive forces existing between molecules

Question 7

explain what dispersion forces are

Answer 7

temporary dipoles form between molecules in order for them to change shape

temporary polarity: electrons are constantly moving around atoms and are not always symmetrically distributed across the atom

this temporary asymmetrical distribution produces a temporary/instantaneous dipole

if it is close to another molecule w/ a temporary dipole, it will cause polarity in the neighbouring atom

these two temporary dipoles will be attracted to one another

this force between molecules is dispersion force

Question 8

how can you predict the strength of dispersion forces

Answer 8

increase in strength with increasing number of electrons
- as ayomic radius increases, e’ have greater space to
mover and greater chance to arrange asymmetrically

molecular shape

the larger the surface area over which the electrons can develop an instantaneous dipole and the closer molecules can fit next to each other, the strogner the dispersion force

Question 9

explain what dipole-dipole forces are

Answer 9

weak attractive forces between polar molecules

molecule that has a partial negative charge is attracted to neighbouring molecule with partial positive charge

the more polar the molecules, the stronger the force will be

Question 10

explain what hydrogen bonding is

Answer 10

dipole-dipole forces containing NH, OH or FH as they’re the most electronegative atoms

very polar bond –> stronger dipole dipole forces

more energy required to overcome bonds: higher BP

atomic radii are smaller - high charge density at the end of each molecule

e- are attracted more strongly toward N, O or F

Question 11

what are the requirements for hydrogen bonding

Answer 11

presence in 1 molecules of at least 1 NH OH or FH covalent bond

presence of another polar molecule containing at least 1 N, O or F atoms with a non-bonding pair of electrons

Question 12

define hydrogen bond

Answer 12

the attractive force between the lone pair of electrons on the N, O of F atom on on of the polar molecules and the partially positive charge of the H bonded to N, O of F on the second polar molecule

Question 13

what are the unique properties of water

Answer 13

relatively high MP (O˚) and BP (100˚)

density of water in the solid phase is lower than its density in liquid phase

high surface tension

Question 14

explain water’s property of its density

Answer 14

water has a maximum density of 1.0g ML-1 as a liquid at 4˚

when freezed it expands significantly and density drops by 8%

reduced density is why ice floats on water and freezes top down providing a layer of insulation preventing the rest of the water from freezing

this is because of the crystal lattice formed when water freezed - each water molecule can form 4 hydrogen bonds, maximising hydrogen bonding but takes up more space

Question 15

explain water’s property of its high surface tension

Answer 15

water has strong intermolecular forces

imbalance of these forces at waters surface causing water molecules to be pulled inward toward the bulk of the liquid

as a result the surface tries to contract to minimise surface area

Question 16

what is chromatography

Answer 16

separates compounds in a solution with the same properties

paper chromatography: separates a mixture of solutes with difference solubilities of absorption

Question 17

explain the process of chromatography

Answer 17

uses paper and a solvent that can move over the medium

samples are placed along chromatography paper and paper is dipped in a solvent and left to dry

solvent will run up the paper, dissolving the samples

different solubilities cause some samples to run further up the paper than others

liquid with the lowers BP is first to evaporate and be distilled out of the paper and collected

Question 18

what are the physical properties of intermolecular forces

Answer 18

changing phase

solubility

equilibrium vapour pressure

Question 19

explain the physical property of changing phase for intermolecular forces

Answer 19

MP and BP of substances increases as intermolecular force strength increases

as molecular mass increases (number of e’), MP and BP increases

substances that have dipole-dipole forces have a slightly higher MP and BP

substances that have hydrogen bonding have a slightly higher MP and BP

Question 20

explain the physical property of solubility for intermolecular forces

Answer 20

a solute and solvent can only form a solution if the attractive forces between molecules are comparable in strength

e,g, non polar molecules (dispersion forces) can’t be soluble in polar substances (dipole-dipole)

Question 21

explain the physical property of equilibrium vapour pressure for intermolecular forces

Answer 21

vapour pressure: tendency of a substance to evaporate

substances that easily evaporate have weak intermolecular forces

vapour pressure increases as temperature increases