Chapter 6 - molecule shapes and intermolecular forces

Question 1

why do molecules form these shapes?

Answer 1

“molecules take the shape that minimises the repulsion between electron pairs within the molecule”

Question 2

electron pair repulsion theory and lone pairs

Answer 2

electron pairs repel and get as far away from each other as they can, lone pairs repel more

Question 3

how lone pairs change the angle

Answer 3

each lone pair reduces the angle by roughly 2.5 degrees

Question 4

drawing molecules (see poster for more details)

Answer 4

---- = going away
\_\_\_ = on the same plane as page
wedge = coming out from page

Question 5

what is the pauling scale

Answer 5

a scale of electronegativity; the values increase as you go across (right) and up the table

Question 6

define electronegativity

Answer 6

“electronegativity is the ability of an atom in a covalent bond to attract the bonded pair of electrons towards it”

Question 7

why does electronegativity increase at top and right of table

Answer 7

at top - smaller atoms so the electrons are closer to the nucleus giving a greater attraction

at right- more protons, nuclear charge increases and the atomic radius decreases giving a greater attraction

Question 8

what gives a bond polarity

Answer 8

bond polarity- when two atoms have significantly different electronegativities one of the atoms pulls the electrons towards it more making it slightly negative and the other atom slightly positive

pure covalent bonds- when two atoms haver very similar electronegativities

Question 9

what makes a molecule polar

Answer 9

polar molecules form when there is a net polarity on the molecule due to polar bonds- this can only occur in asymmetrical molecules - this gives permanent dipoles

Question 10

what is a polar solvent and how can they affect solubility

Answer 10

a polar solvent is a solvent made with polar molecules

the opposite dipoles of a polar solvent attract charged ions such as the slightly negative o in water attracts Na+ etc.

this can help break down ionic lattices

Question 11

what are london forces

Answer 11

these occur in all molecules between induced dipoles, they are a type of intermolecular force

Question 12

how do instantaneous dipoles form

Answer 12

• as electrons move at any instant there is an induced dipole in a molecule from an uneven distribution of electrons.
• these are constantly changing.
• these instantaneous dipoles induce dipoles on other molecules,
• thus you have opposite charges and an attraction

Question 13

things which affect london forces

Answer 13

more electrons/bigger molecules

Question 14

permanent dipole dipole interactions

Answer 14

permanent dipoles attract oppositely charged dipoles on other molecules to form an intermolecular attraction stronger than london forces but weaker than hydrogen bonding

Question 15

what is hydrogen bonding

Answer 15

a special case of dipole-dipole attraction where molecules must contain- a hydrogen atom attached to a very electronegative atom e.g. O,N,F

these are the strongest type of intermolecular force

Question 16

anomalous properties of water

Answer 16

relatively high mpt/bpt. - strong hydrogen bonds require a large amount of energy to overcome
ice is less dense than water- strong hydrogen bonds hold the water molecules in an open lattice with larger gaps reducing the density
high surface tension and viscosity - strong hydrogen bonds