Chapter 6 - Shapes of molecules and intermolecular forces Flashcards
What is the electron pair repulsion theory used for?
Determining the shape of a molecule or ion
Give the wedges used to visualise structures in 3 dimensions.
- Solid line represents a bond in the plane of the paper
- solid wedge comes out of the plane of the paper.
- Dotted wedge goes into the plane of paper
Give the relative repulsions between lone pairs and bonding pairs.
Bonding pair/ bonding pair < bonding pair/ lone pair < lone pair/ lone pair
What shape is a molecule with 4 bonding pairs? What is the bonding angle? Draw the shape.
Tetrahedral - 109.5 degrees
What is the shape of a molecule with 3 bonding pairs and 1 lone pair? What is the bonding angle? Draw the shape.
Pyramidal - 107 degrees
What shape is a molecule with 2 bonding pairs and 2 lone pairs? What is the bonding angle? Draw the shape.
Non linear - 104.5 degrees
What shape is a molecule with 2 bonding pairs? What is the bonding angle? Draw the shape.
Linear - 180 degrees
What shape is a molecule with 3 bonding pairs? What is the bonding angle? Draw the shape.
Triagonal planar - 120 degrees
What shape is a molecule with 6 bonding pairs? What is the bonding angle? Draw the shape.
Octohedral - 90 degrees
What is electronegativity?
The ability of an atom to attract shared electron pair in (covalent) bond
- measured on the pauling scale. Larger value = more electronegative
- Increases across a period, but decreases down a group
Why are some bonds non-polar?
When the bonding atoms are the same or bonding atoms have similar electronegativity. This is due to electron pair being shared equally (pure covalent bond).
What causes polar bonds?
- When a bonded electron pair is shared unequally between the bonded atoms (bonding atoms have different electronegativity).
- Small partial charges occur on each atom, with delta - on the atom with a higher electronegativity, and delta + on the atom with lower electronegativity.
What is a dipole?
- separation of opposite charges (delta - and delta+) in a polar molecule
What are intermolecular forces? And what are the 3 main categories.
Weak interactions between dipoles of different molecules.
- Induced dipole-dipole interactions (London forces)
- Permanent dipole-dipole interactions
- Hydrogen bonding
What properties are intermolecular forces responsible for?
- Physical properties - such as melting and boiling point.
- covalent bonds determine identity + chemical reactions of molecules
What are London forces?
Weak intermolecular forces that exist between all molecules, whether polar or non-polar. They act between induced dipoles in different molecules.
How does an induced dipole form?
- Movement of electrons produces a changing dipole in a molecule. At any instant, an instantaneous dipole will exist, but its position is constantly changing.
- Instantaneous dipole induces a dipole on a neighbouring molecule.
- Induced dipole induces further dipoles on neighbouring molecules, which then attract one another.
How does increasing electrons affect London forces?
- Larger instantaneous and induced dipoles. Greater induced dipole-dipole interactions the stronger the attractive forces between molecules.
What are permanent dipole-dipole interactions?
- Act between permanent dipoles in different polar molecules. (permanent dipoles - small differences in charge between atoms bonded)
What is hydrogen bonding?
- Permanent dipole-dipole interaction found between lone pair on a oxygen, nitrogen or fluorine atom and a hydrogen atom in different molecules.
Why is ice less dense than water?
Hydrogen bonds in ice hold water molecules apart in open lattice structure.
hydrogen bonds are relatively long, so molecules on average will be further part in ice.
Why does water have relatively high melting + boiling point?
- Hydrogen bonds are extra forces, in addition to London forces.
Hydrogen bonds are strongest type of intermolecular bond. Lots of energy required to break bonds - and so higher temperature needed.
Why might a compound not have an overall dipole, even if it has polar bonds?
Can be symmterical/ dipoles cancel
When does hydrogen bonding strength increase?
Greater differences in electronegativity
