Chapter 6 - Shapes of Molecules and Intermolecular Forces Flashcards
do electron pairs attract and repel?
electron pairs repel because each electron has a negative charge
what determines the shape of the molecule/ion?
the electron pairs surrounding a central atom
how are electron pairs arranged?
they are arranged to minimise repulsion so are arranged as far apart as possible. they are held in a definite shape by this repulsion.
what causes molecules/ions to have different shapes?
different numbers of electron pairs
what is the shape of methane?
a molecule of methane is symetrical with 4 C-H covalent bond
- 4 bonded pairs of electrons surround the central carbon atom
- the 4 electron pairs repel one another as far apart as possible in 3D space
this results in a tetrahedral shape with 4 equal bond angles of 109.5 degrees.
what does a solid line represent when drawing shapes of molecules?
a bond in the plane of the paper
what does a solid wedge represent when drawing shapes of molecules?
a bond coming out of the plane of the paper
what does a dotted wedge represent when drawing shapes of molecules?
a bond coming into the plane of the paper
do lone pairs repel the same as a bonding pair?
lone pairs repel more strongly than bonding pairs as they are slightly closer to the central atom and occupy more space
what do lone pairs do to the bond angle?
they decrease the bond angle by about 2.5 for each lone pair
what is the name of the shape and bond angle of a molecule with 4 bonded pairs and 0 lone pairs?
name: tetrahedral
bond angle: 109.5
what is the name of the shape and bond angle of a molecule with 3 bonded pairs and 1 lone pairs?
name: pyramidal
bond angle: 107
what is the name of the shape and bond angle of a molecule with 2 bonded pairs and 2 lone pairs?
name: non-linear
bond angle: 104.5
what happens to molecular shape when there are multiple bonds?
each multiple bond is treated as a bonding region
what are the principles of electron-pair repulsion theory?
- electron pairs around the central atom repel each other as far apart as possible
- the greater the number of electron pairs, the smaller the bond angle
- lone pairs of electrons repel more strongly than bonded pairs of electrons
what is the name of the shape and bond angle of a molecule with 3 bonded pairs and 0 lone pairs?
name: trigonal planar
angle: 120 degrees
what is the name of the shape and bond angle of a molecule with 2 bonded pairs and 0 lone pairs?
name: linear
angle: 180 degrees
what is the name of the shape and bond angle of a molecule with 6 bonded pairs and 0 lone pairs?
name: octahedral
angle: 90 degrees
what is the name of the shape and bond angle of a molecule with 2 bonded pairs and 1 lone pairs?
name: bent
angle: 118 degrees
what is the name of the shape and bond angle of a molecule with 5 bonded pairs and 0 lone pairs?
name: trigonal bipyramidal
angle: 120 degrees and 90 degrees
what is the name of the shape and bond angle of a molecule with 4 bonded pairs and 1 lone pairs?
name: trigonal pyramidal
angle: 119 degrees and 89 degrees
what is the name of the shape and bond angle of a molecule with 4 bonded pairs and 2 lone pairs?
name: trigonal planar
angle: 120 degrees and 89 degrees
what is the name of the shape and bond angle of a molecule with 5 bonded pairs and 1 lone pairs?
name: square pyramidal
angle: 89 degrees
what is the name of the shape and bond angle of a molecule with 5 bonded pairs and 1 lone pairs?
name: square planar
angle: 90 degrees
what is the name of the shape and bond angle of a molecule with 5 bonded pairs and 1 lone pairs?
name: square pyramidal
angle: 89 degrees
what is the shape of ammonium?
- an ammonium ion has the same number of bonded pairs of electrons around the central atom as a methane molecule
- so the shape and bond angle will be the same (tetrahedral and 109.5)
what are the shapes of carbonate and nitrate ions?
- carbonate and nitrate ions both have 3 bonded pairs of electrons
- so will have the same shape and bond angle (trigonal planar and 120)
what is the shape of a sulfate ion?
- sulfate ions have 4 bonded pairs of electrons
- so will have the same shape and bond angle as a methane and ammonium ion (tetrahedral and 109.5)
what attraction happens in a covalent bond?
the nuclei of the bonded atoms attract the shared pair of electrons
how is the bonded electron pair shared in atoms of the same element?
the bonded electron pair is shared evenly as the atoms are the same element
how is the bonded electron pair shared in atoms of the different elements?
they can be shared unevenly because:
- the nuclear charges are different
- the atoms may be different sizes
- the shared pair of electrons may be closer to one nucleus than the other
so the shared pair of electrons in the covalent bond may now experience more attraction from one of the bonded atoms than the other
what is electronegativity?
the attraction of a bonded atom for the pair of electrons in a covalent bond
which is the most electronegative atom?
flourine
what is the trend in electronegativity?
electronegativity increases up the periodic table and across the periodic table
which group of atoms is the most electronegative?
the non-metals: nitrogen, oxygen, flourine and chlorine
which group of atoms is the least electronegative?
the group 1 metals: lithium, sodium and pottasium
what happens if electronegativity difference is very large?
one bonded atom will have a much greater attraction for the shared pair than the other bonded atom
the more electronegative atom will have gained control of the electrons and the bond will now be ionic rather than covalent
when is a bond non-polar?
- if the bonded atoms are the same
- if the bonded atoms have the same or similar electronegativity
what is a pure covalent bond?
when the electron pair is attracted equally to each bonded atom
which 2 elements form non-polar covalent bonds?
carbon and hydrogen as they have very similar electronegativities
when is a bond polar?
when the bonded atoms are different and have different electronegativity values
what is a polar bond?
the bonded electron pair is shared unequally between the bonded atoms
what is a dipole?
the separation of opposite charges (delta + and delta -)
which atom has the delta - charge?
the atom with the larger electronegativity
which atom has the delta + charge?
the atom with the smaller electronegativity
what type of dipole is in a polar covalent bond?
a permanent dipole as it does not change
what is a polar molecule?
a molecule with a permanent dipole acting in a direction
if the molecule is symmetrical the dipoles will cancel each other out as they act in opposite directions, so the molecule wont be polar
what type of molecule is water? (polar/non-polar)
- the two O-H molecules each have a permanent dipole
- the two dipoles act in different directions but don’t exactly oppose each other (molecule isn’t symmetrical)
- overall the oxygen end of the molecule has a delta - charge and the hydrogen end has a delta + charge
- so it is polar
what type of molecule is carbon dioxide? (polar/non-polar)
- the two C=O bonds each have a permanent dipole
- the two dipoles act in opposite directions and exactly oppose each other
- over the whole molecule, the dipoles cancel and the overall dipole is zero
- so it is non-polar
how do ionic molecules dissolve in polar substances? (example: NaCl in H2O)
- water molecules attract Na+ and Cl- ions
- the ionic lattice breaks down as it dissolves
- in the resulting solution, water molecules surround the Na+ and Cl- ions:
Na+ ions are attracted towards the oxygen of water molecules (delta -)
Cl- ions are attracted towards the hydrogen of water molecules (delta +)
what are the 3 main categories of intermolecular forces?
- induced dipole-dipole interactions (London forces)
- permanent dipole-dipole interactions
- hydrogen bonding
in order of strength
what properties are intermolecular forces largely responsible for?
physical properties such as melting or boiling points
when do London forces occur?
London forces exist between all molecules, polar or non-polar
what causes London forces to occur?
- movement of electrons produce a changing dipole in a molecule
- at any instant, an instantaneous dipole will exist, but its position is constantly shifting
- the instantaneous dipole induces a dipole on a neighbouring molecule
- the induced dipole induces further dipoles on neighbouring molecules, which then attract one another
how long do induced dipoles last?
they are only temporary
when do induced dipoles become stronger?
when there are more electrons in each molecule because:
- more electrons means a larger instantaneous dipole
- which means greater induced dipole-dipole interactions
- meaning a stronger force
why does boiling point increase when molecules have more electrons?
- larger numbers of electrons mean larger induced dipoles
- more energy is then needed to overcome the intermolecular forces, increasing the boiling point
when do permanent dipole-dipole interactions occur?
permanent dipole-dipole interactions act between the permanent dipoles in different polar molecules
why is the boiling point of molecules with permanent dipole-dipole interactions higher?
- molecules with permanent dipole-dipole interactions require extra energy to break the additional permanent dipole-dipole interactions (they also require energy to break their London forces)
- so the boiling point will be higher as more energy is required
what is a simple molecular substance?
a substance made up of simple molecules (small units containing a definite number of atoms with a definite molecular formula)
what structure do simple molecules form?
a simple molecular lattice where:
- molecules are held in place by weak intermolecular forces
- the atoms within each molecule are bonded together by strong covalent bonds
what is the boiling point of simple molecular substances?
- the weak intermolecular forces can be broken even by the energy present at low temperatures
- so they have low melting and boiling points
what happens when a simple molecular lattice is broken apart during melting?
only the weak intermolecular forces break, the strong covalent bonds do not break
what is the solubility of non-polar simple molecular substances?
non-polar solvent:
- intermolecular forces form between the molecules and the solvent
- the interactions weaken the intermolecular forces in the simple molecular lattice, and the intermolecular forces break and the compound dissolves
- so they tend to be soluble
polar solvent:
- there is little interaction between the molecules in the lattice and the solution molecules
- the intermolecular bonding within the polar solvent is too strong to be broken
what is the solubility of polar simple molecular substances?
polar covalent substances may dissolve in polar solvents as the polar solute molecules and the polar solvent molecules can attract each other, similar to dissolving an ionic compound
polar covalent substances do not dissolve in non-polar solvents
what does solubility of a polar substance depend on?
the strength of the dipole
can substances dissolve in both polar and non-polar solvents?
some compounds contain both polar and non-polar parts in their structure and can dissolve in both polar and non-polar solvents
what is the electrical conductivity of simple molecular substances?
- there are no mobile charged particles in simple molecular structures
- with no charged particles that can move, there’s nothing to complete an electrical circuit
- so simple molecular substances dont conduct electricity
when is a hydrogen bond found?
between molecules containing:
- an electronegative atom with a lone pair of electrons e.g. oxygen, nitrogen and fluorine
- a hydrogen atom attached to an electronegative atom e.g H-O, H-N, H-F
what does the hydrogen bond act between?
a lone pair of electrons on an electronegative atom in one molecule and a hydrogen atom in a different molecule
how is a hydrogen bond drawn?
as a straight, dashed line
what are anomalous properties of water?
- the solid is less dense than the liquid
- water has relatively high melting and boiling point
why is ice less dense than water?
- hydrogen bonds hold water molecules apart in an open lattice structure
- the water molecules in ice are further apart than in water
- solid ice is less dense than liquid water and floats
why does water have a relatively high melting and boiling point?
- hydrogen bonds are extra forces, over and above London forces
- an appreciable quantity of energy is needed to break the hydrogen bonds in water, so water has much higher melting and boiling points than would be expected from just London forces
- when the ice lattice breaks, the rigid arrangement of hydrogen bonds in ice is broken. when water boils, the hydrogen bonds break completely
