chemistry exam stuff you dont know Flashcards
what determines the shape of molecules
the arrangement of electrons pairs surrounding the central atom
what does VSEPR stand for and what does this theory state
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
what does polarity refer to
related to the sharing of the bonding electrons between atoms
what is the difference between a polar and non polar bond
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
explain what a polar bond is
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
what are intermolecular forces
weak attractive forces existing between molecules
explain what dispersion forces are
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
how can you predict the strength of dispersion forces
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
explain what dipole-dipole forces are
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
explain what hydrogen bonding is
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
what are the requirements for hydrogen bonding
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
define hydrogen bond
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
what are the unique properties of water
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
explain water’s property of its density
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
explain water’s property of its high surface tension
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
what is chromatography
separates compounds in a solution with the same properties
paper chromatography: separates a mixture of solutes with difference solubilities of absorption
explain the process of chromatography
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
what are the physical properties of intermolecular forces
changing phase
solubility
equilibrium vapour pressure
explain the physical property of changing phase for intermolecular forces
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
explain the physical property of solubility for intermolecular forces
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)
explain the physical property of equilibrium vapour pressure for intermolecular forces
vapour pressure: tendency of a substance to evaporate
substances that easily evaporate have weak intermolecular forces
vapour pressure increases as temperature increases
