2.2 Flashcards
what is a discrete molecule
an electrically neutral group of two or more atoms held together by chemical bonds
state some exceptions to the octet rule
beryllium chloride
boron trichloride
phosphorus(V) chloride
sulfur hexafluoride
what is a displayed formula
shows each bonding pair as a line drawn between the atoms involved (H-O-H)
what is a dative bond
a dative bond is formed when an empty orbital from one atom overlaps with an orbital containing a lone pair of electrons of another atom
what is the electron pair repulsion theory
the shape of a molecule or ion is caused by repulsion between bonding pairs and lone pairs of electrons that surround the central atom
the electron pair arrange themselves around the central atom so that the repulsion between them is a minimum
what shape is a molecule with 2 bond pairs and its bond angle
linear
180
what shape is a molecule with 3 bond pairs and its bond angle
trigonal planar
120
what shape is a molecule with 4 bond pairs and its bond angle
tetrahedral
109.5
what shape is a molecule with 5 bond pairs and its bond angles
trigonal bipyramidal
90 and 120
what shape is a molecule with 6 bond pairs and its bond angle
octahedral
90
what shape is a molecule with 3 bond pairs and 1 lone pair and its bond angle
trigonal pyramidal
107
what shape is a molecule with 2 bond pairs and 2 lone pairs and its bond angle
V-shaped
104.5
define dipole
exists when two charges of equal magnitude but opposite signs are separated by a small distance
what is a dipole caused by
the more electronegative element attracts the electrons resulting in separation of charge
what molecule shapes are symmetrical (therefor dipoles cancel out)
linear
trigonal planar
tetrahedral (some exceptions such as CHCl3)
explain london forces
electron density fluctuates over time in a molecule
at any instant the electron density can become unsymmetrical generating an instantaneous dipole
which then induces a dipole in a neighboring molecule
these dipoles then attract one another
state 3 features of london forces
attractive force increases with increasing number of electrons in a molecule
the strength of these attractions depends upon the shape and size of the molecules (the more points of contact between molecules the greater the london forces)
they’re always present with any molecules
explain permanent dipoles
if 2 permanent dipoles are aligned correctly they will attract one another
are london forces or permanent dipoles stronger and why
london forces are stronger because in order for permanent dipoles to occur the molecules must be correctly aligned
define hydrogen bond
an intermolecular interaction between a hydrogen atom of a molecule bonded to an atom which is more electronegative than hydrogen and another atom in the same or a different molecule
what are the 3 most common atoms hydrogen bonds form with
oxygen
nitrogen
fluorine
what is the rough bond angle of hydrogen bonds
180
how do hydrogen bonds form between water molecules
partial bond formation occurs between the lone pairs from an oxygen and a hydrogen atoms of another water molecule
what groups can form hydrogen bonds
amines
alcohols
what is the only significant interaction between alkane molecules
london forces
why does boiling temperature increase as relative molecular mass (chain length) of alkanes increases
number of electrons per molecule increases so london forces are increased
number of points of contact increases so london forces increase
how does branching of alkanes effect boiling temperature and why
lowers boiling temperature as there are fewer points of contact for london forces
why do alcohols have higher boiling temperatures than alkanes
because alcohols have an OH group and can therefor form hydrogen bonds on top of london forces which require more energy to overcome than just london forces
why does boiling temperature increase from HCl -> HBr -> HI
due to increased number of electrons per molecule so there is an increase in london forces
why does water have a relatively high mp & bp
water has hydrogen bonds which have strong intermolecular forces of attraction that require a lot of energy to overcome
why does HF have a lower boiling point than water even though they have the same number of electrons
- both form the same london forces (roughly)
- both form hydrogen bonds
however
HF on average only forms on hydrogen bond per molecule whereas water forms 2
not all of the hydrogen bonds are broken in the vaporization of HF whereas they are in water
why is ice less dense than water
ice is arranged in rings of 6 held by hydrogen bonds creating large areas of space inside the rings so when ice melts the ring structure is destroyed and distance between molecules decreases
how do solutes dissolve in solvents
the solute particles must particles must separate from each other and become surrounded by solvent particles
the force of attraction between solute and solvent must be strong enough to overcome solute-solute attractions and solvent-solvent attractions
where does the energy come from when dissolving ionic solids
hydration of ions
explain the hydration of an ionic solid
the dipole negative attracts the anions sufficiently to remove them from the lattice and the anion becomes surrounded by water molecules
the dipole positive attracts the cations and surrounds the cation in water molecules
how is solubility of alcohols effected by chain length
solubility decreases as chain length increases as london forces predominate between the alcohol molecules
why may a substance not dissolve in water
it is a non-polar molecule therefor can’t form hydrogen bonds with water molecules
attractions between the molecule and water is not strong enough to disrupt the hydrogen bond system
