Chapter 6 - Shapes of Molecules and Intermolecular Forces Flashcards
Shapes of molecules and ions, Electronegativity and polarity, Intermolecular forces and Hydrogen bonding.
What does molecular shape depend on?
Electron pairs around the central atom.
In what way do electron pairs interact?
They repel each other.
Which type of pair repels the most?
Lone pairs
What is the order of pair repulsion, from strongest to weakest?
Lone pair/lone pair Lone pair/bonding pair Bonding pair/bonding pair
What is the name of the way to predict a molecule’s shape?
Electron pair repulsion theory
What shape name is given to a molecule with no lone pairs?
Tetrahedral
What is the bond angle in a tetrahedral molecule?
109.5
What shape name is given to a molecule with two bonding pairs and one lone pair?
non-linear
What is the bond angle in a trigonal pyramidal molecule?
107
What shape name is given to a molecule with two lone pairs?
Non-linear
What is the bond angle in a non-linear molecule with two lone pairs?
104.5
What shape name is given to a molecule with 2 electron pairs around the central atom?
Linear
What is the bond angle in a linear molecule?
180
What shape name is given to a molecule with 3 electron pairs around the central atom?
Trigonal planar
What is the bond angle in a trigonal planar molecule?
120
What shape name is given to a molecule with 5 electron pairs around the central atom?
Trigonal bipyramidal
What is the bond angle in a trigonal bipyramidal molecule?
There are actually two, 120 in one plane and 90 in the other plane.
What shape name is given to a molecule with 6 electron pairs around the central atom?
Octahedral
What is the bond angle in an octahedral molecule?
90
What is electronegativity?
An atom’s ability to attract the electron pair in a covalent bond.
How is electronegativity measured?
On the Pauling scale.
What does a higher Pauling value mean?
A higher electronegativity and thus a greater attraction for an electron pair in a covalent bond.
What makes a bond polar?
Two atoms with electronegativities differing by at least 0.5, the bonding electrons are pulled towards the more electronegative atom, making it polar.
What does a polar bond form?
A permanent dipole
How is a dipole formed?
A difference in charge between the two atoms, caused by a shift in electron density in the bond.
Are diatomic gases polar or non-polar, why?
Non-polar Because the atoms have equal electronegativities and so the electrons are equally attracted to both nuclei.
What happens if the polar bonds are arranged symmetrically in a molecule?
The dipoles cancel each other out, so the molecule has no overall dipole and is non-polar.
What happens if the polar bonds are arranged unsymmetrically in a molecule?
Uneven distribution of charge and the molecule will have an overall dipole.
What can electronegativity be used to predict about bonding?
Whether it will be covalent or ionic bonding.
What is the general strength of intermolecular forces?
Very weak
What is the order of the intermolecular forces from strongest to weakest?
Hydrogen bonding Permanent dipole-dipole interactions Induced dipole-dipole
What are induced dipole-dipole forces?
Electrons in clouds can at one moment be unevenly distributed, creating a temporary dipole. This dipole induces another dipole on a neighbouring atom and the two are attracted to each other. The dipoles can then be brought about in a domino effect. Overall the atoms are attracted to one another.
What is the effect of stronger induced dipole-dipole forces?
Higher boiling points
Why are higher boiling points brought about?
Larger molecules have larger electron clouds so stronger induced dipole-dipole forces. Boiling point is overcoming the intermolecular forces so stronger ones means a higher boiling point.
How are permanent dipole-dipole interactions labelled?
With charges of delta+ and delta- on the atoms.
Why are hydrogen bonds brought about?
Hydrogen has a high charge densiy and F, N and O are very electronegative. The bond is so polarised that a weak bond forms between the hydrogen of one molecule and a lone pair on a neighbouring molecule’s F, N or O.
What effect does hydrogen bonding have on a molecule?
Soluble in water. Higher boiling and freezing points than molecules of a similar size that don’t form hydrogen bonds.
What is an interesting property of ice caused by hydrogen bonding?
hydrogen bonds hold water molecules apart in an open lattice structure. The water molecules in ice are held further apart than in water. As a result, solid ice is less dense than liquid water and therefore floats.
How do intermolecular forces explain simple covalent compounds having low melting and boiling points?
Weak intermolecular forces to overcome.
How do intermolecular forces explain simple covalent compounds sometimes being soluble in water?
Water is a polar molecule, and as we the water molecule can form hydrogen bonds with other hydrogen-based molecules.
How do intermolecular forces explain simple covalent compounds not conducting electricity?
Overall covalent molecules are uncharged, no mobile charge carriers so cannot conduct electricity.
Therefore, it is a non-conductor of electricity
what is a pure covalent bond?
Molecules of elements such as hydrogen and oxygen, there bonded atoms come from the same element and the electron pair is shared equally.
Name and bond angle
Name: Trigonal pyramidal
Angle: 107*
Name and bond angle
Non-linear & 117.5*
Name & Bond angle
trigonal planar & 120*
Name & bond angle
non-linear & 104.5*
Name & bond angle
linear & 180*
name & bond angle
tetrahedral & 109.5*
name & bond angle
trigonal pyramidal & 107*
name & bond angle
non-linear & 117.5*
name & bond angle
non-linear & 104*
name & bond angle
trigonal bipyramidal
what intermolecular forces are involved between water molecules
London forces
permanent dipole-dipole interactions
hydrogen bonding
Describe in terms of the lattice structure of H20 when in solid state (ice) and when it is melting.
When it is ice, the water molecules in the lattice are held further apart in the lattice. When it’s melting, the ice lattice collapses, and the molecules move closer together; thus increasing the density.
state the three anomalous properties of water
ice less dense than water, high melting points, and boiling points
Explain why simple molecular compounds:
a) have low melting and boiling points
b) do not usually dissolve in water
c) have poor electrical conductivity.
a) Weak intermolecular forces are broken by the energy present at low temperatures.
b) There are little interactions between the molecules in the lattice and the polar solvent molecules.
c) There is no mobile charged particle.
Define dipole
Charge separation across a bond with one atom having a delta +ve charge and one atom a delta -ve charge.
Define polar covalent bond
shared pair of electrons where the electron pair is not shared equally between the two bonded atoms.
is water a non-polar or polar molecule?
polar molecule
