shapes and forces Flashcards
intermolecular forces
attractive (repulsive) forces between molecules
intramolecular forces
attractive (repulsive) forces within a molecule
Van der Waals forces
weak attractive forces between molecules resulting from the formation of temporary dipoles
Dipole-Dipole
Dipole-Dipole forces are forces of attraction between the negative pole of one molecule and the positive pole of another
hydrogen bonds
- particular types of dipole-dipole attractions between molecules in which hydrogen atoms are bonded to nitrogen, oxygen or fluorine
- the hydrogen atom carries a partial positive charge and is attracted to the electronegative atom in another molecule. Thus, H acts as a bridge between two electronegative atoms
the Law of Conservation of Mass
the total mass of the products of a chemical reaction is the same as the total mass of the reactants
the Law of Conservation of Matter
that in any chemical reaction, matter is neither created nor destroyed but merely changes from one form into another
types of shapes
linear v-shaped trigonal planar trigonal pyramidal tetrahedral
only bonding pairs around the central atom
electrons in the bonding pairs repel each other and want to be as far apart as possible
no. of bonding pairs around central atom - 2
shape of molecule - lienar
example - BeH₂ , BeCl₂
bond angle - 180°
no. of bonding pairs around central atom - 3
shape of molecule - trigonal planar
example - BF₃ BCl₃
bond angle - 120°
no. of bonding pairs around central atom - 4
shape of molecule - tetrahedral
example - CH₄
bond angle - 109.5°
lone pairs and bonding pairs around central atom examples
in hback
shapes of molecules
- molecules formed when atoms are joined together by covalent bonds
- arrangement of atoms dictates shape of molecule
- figure out arrangement using Electron Pair Repulsion Theory
- Look at the bonding pairs or lone pairs around central atoms
lone pairs and bonding pairs around central atom
- lone pairs of electrons closer to nucleus of atom (nuclear charge sucks them in)
- bc of this they are closer together (create strong negative charge)
- their collective nuclear charge pushes bonding pairs further apart
- decreases the bond angle + distorts shape of molecule
polarity
- atoms in covalent bonds share electrons
- we figure it out by looking at electronegativity
non-polar
when atoms in a covalent bond share electrons equally, bond is said to be non-polar
Eg. H₂ , N₂ , O₂
polar
when electrons are shared unequally, bond is polar
eg. HCl, NH₄
electronegativity
- the greater the electronegativity, greater the pull on electrons
- to work it out use electronegativity table
- atom w/ greater pull on electrons becomes slightly negative while other atom in bond becomes slightly positive
use of electronegativity
can be used to predict nature of a bond
everyday life: polar
water (good solvent for ionic and polar substances)
ethanol (used in alcoholic beverages)
everyday life: non-polar
tetrachloroethene (used in dry cleaning)
propanone (also called acetone) (used in nail varnish remover)
polar molecule can be non-polar overall if it has these shapes
linear
trigonal planar
tetrahedral
ie shapes with no lone pairs
eg of polar molecules that is non-polar overall
BeH₂
BF₃
CH₄
molecules that are not symmetrical (has one lone pair) are one of these shapes
trigonal pyramidal
v-shaped
eg of non-symmetrical molecules
NH₃
H₂
polar bonds and shapes
some molecules, even tho they have polar bonds, doesn’t mean they are polar molecules due to their shapes
(shape is symmetrical as there are no lone pairs of electrons)
eg. carbon dioxide, chloroform, boron trichloride
intermolecular bonding
between molecules
eg. between water molecules
intramolecular bonding
within the molecule
eg. an oxygen atom and a hydrogen atom in a molecule
3 types of intermolecular bonding
Van der Waals
Dipole-Dipole
Hydrogen Bonding
intermolecular bonding: Van der Waals forces
- temporary dipoles can be set up in non-polar molecules (eg. H₂, O₂)
- weak forces of attractions can be set up between oppositely charged poles in the atom
- do not last v long
- weakest of 3 types
- low boiling points
intermolecular bonding: Dipole-DIpole forces
- polar molecules have positive + negative poles eg. HCl
- positive pole of one molecule can become attracted to negative pole of another
- much weaker than ionic bonds
intermolecular bonding: Hydrogen Bonding
- strongest of 3
- N, O, and F are v. electronegative (when bonded with hydrogen, the electron spends more time on the N, O, or F –> creates strong poles)
- High boiling points
exception to octet rule
BF₃ (boron trifluoride)
has trigonal planar shape
does not satisfy octet rule (no lone pair present to make 8 in outer shell) as the 3 covalent bonds are stable enough
electron pair repulsion theory
The shape of a molecule or ion is governed by the arrangement of the electron pairs around the central atom.
to do with repulsion between bond pairs and (if any) lone pairs
why h2o has v planar shape
repulsion between two bond pairs and two lone pairs
intermolecular vs intramolecular bonds (not definitions)
intramolecular: forces which keeps molecule together, ie. bonds
intermolecular: attractive forces between molecules. Responsible for keeping matter in solid or liquid phase
bond angles
know the bond angles
Predict the type of bond formed between carbon and chlorine atoms in CCl4
polar covalent
“why does x have a lower bp than y?”
If electroneg diff is lower in x, then say “smaller degree of intermolecular forces”
