Chapter 6

Question 1

What is electron pair repulsion theory ?

Answer 1

Pairs of electrons around a central atom repel each other, so they move as far apart as possible to minimise this repulsion, therefore molecules containing 3 or more atoms have shapes.

Question 2

What does the shape of a molecule depend on ?

Answer 2

Number of electron pairs in the outer shell
Number of these electrons that are bonded and lone pairs

Question 3

What is the shape and bond angle in a shape with 2 bonded pairs and 0 lone pairs ?

Answer 3

Linear, 180 degrees

Question 4

What is the shape and bond angle in a shape with 3 bonded pairs and 0 lone pairs ?

Answer 4

Trigonal planar, 120 degrees

Question 5

What is the shape and bond angle in a shape with 4 bonded pairs and 0 lone pairs ?

Answer 5

Tetrahedral, 109.5 degrees

Question 6

What is the shape and bond angle in a shape with 5 bonded pairs and 0 lone pairs ?

Answer 6

Trigonal pyramidal, 120 and 90 degrees

Question 7

What is the shape and bond angle in a shape with 6 bonded pairs and 0 lone pairs ?

Answer 7

Octahedral, 90 degrees

Question 8

What is the shape and bond angle in a shape with 3 bonded pairs and 1 lone pair ?

Answer 8

Pyramidal, 107 degrees

Question 9

What is the shape and bond angle in a shape with 2 bonded pairs and 2 lone pairs ?

Answer 9

Non linear, 104.5 degrees

Question 10

By how many degrees does each pair reduce the bond angle ?

Answer 10

2.5 degrees

Question 11

What ion is an exception to the “ one single, two double bonds “ rule ?

Answer 11

The NO3- ion is an exception. Because it has 3 regions of electron density around the central atom. Forms trigonal planar shapes, with bond angles of 120 degrees. Contains dative covalent bond.

Question 12

Define electronegativity

Answer 12

the ability of an atom to attract a pair of bonding electrons in a covalent bond to itself.

Question 13

What is the Pauling Scale ?

Answer 13

Used to compare the electronegativity of the atoms of different elements.

Question 14

Why type of elements have high electronegativities ?

Answer 14

Reactive non metals with small atoms eg fluorine

Question 15

Why type of elements have low electronegativities ?

Answer 15

Reactive metals with large atoms eg lithium

Question 16

In what direction does the electronegativity increase on the periodic table ?

Answer 16

Left to Right
Down to Up

Question 17

How to differentiate between ionic and covalent bonding using electronegativity numbers ?

Answer 17

If the electronegativity difference between the 2 bonded elements is more than 1.8, the electrons will be unevenly shared so the bond can be considered ionic. One atom loses an electron, other gains it.

Question 18

When is a molecule polar ?

Answer 18

If one end of the molecule is more positive than the other end of the molecule, then the molecule as a whole is polar. ( needs separation of charge )

Question 19

What does the overall polarity of a molecule depend on ?

Answer 19

There has to be polar bonds
Depends on shape of molecule
Has to be a separation of charge so dipoles do not cancel out. ( so the centre of positive charge does not coincide with the centre of negative charge )

Question 20

What is a permanent dipole ?

Answer 20

When molecules have polar bonds and an overall dipole.

Question 21

Describe permanent dipole - induced dipole interactions

Answer 21

When a molecule with a permanent dipole is close to other non polar molecules it causes the non polar molecule to become slightly polar, leading to attraction.

Question 22

Describe permanent dipole - permanent dipole interactions

Answer 22

Some molecules with polar bonds have permanent dipoles - forces of attraction between those dipoles and those of neighbouring molecules.

Question 23

Describe induced dipole - dipole interactions

Answer 23

Electrons in atom / molecule are constantly moving
Temporary uneven distribution of electrons occurs for an instant
Created dipole which induces a dipole in its neighbours
Leads to an attraction, each dipole can induce many others
Few moments later, electrons move on and dipoles are gone

Question 24

Describe how London forces arise

Answer 24

Electron fluctuations in one molecule result in a temporary dipole. The temporary dipole causes an (opposite) induced dipole in a neighbouring molecule and then the opposite dipoles attract.

Question 25

Define Hydrogen Bonding

Answer 25

Intermolecular bonding between molecules containing O, N or F and the H atom of –NH, –OH or -HF

Question 26

Name the 4 different intermolecular bonds in order of strength : from weakest to strongest

Answer 26

London forces ( induced dipole - dipole ) Permanent dipole - dipole Hydrogen bonding forces Covalent bonds

Question 27

What are the 2 main factors of deciding the strength of London forces ?

Answer 27

Number of electrons Shape of molecule

Question 28

3 anomalous properties of water due to hydrogen bonding

Answer 28

That water is a liquid at room temperature That water is less dense as a solid than a liquid (takes up more space) That water can dissolve substances which can accept/make hydrogen bonds Water has a high surface tension

Question 29

Explain the density of ice compared with water

Answer 29

Water ( liquid ) denser than ice ( solid ). In ice, hydrogen bonds hold the water molecules apart in an open lattice structure. When the ice melts, the rigid hydrogen bonds break and the water molecules come closer together, making it more dense than ice.

Question 30

Explain water's relatively high melting and boiling points

Answer 30

Due to the strong intermolecular forces of hydrogen bonding between the molecules in both ice (solid H2O) and water (liquid H2O) A lot of energy is therefore required to separate the water molecules and melt or boil them

Question 31

Explain the solid structures of simple molecular lattices

Answer 31

The atoms within a molecule are held together by strong covalent bonds but individual molecules are held together by weak intermolecular forces of attraction. Compounds with simple molecular structure have low melting and boiling points.

Question 32

Explain the effect of structure and bonding on the physical properties of covalent compounds with simple molecular lattice structures

Answer 32

Simple covalent compounds have low melting and boiling points because the intermolecular forces between the molecules are weak. This means that little energy is required to break the lattice. Simple covalent compounds do not conduct electricity in the solid or liquid state because they do not have charged particles or free electrons. However, some simple covalent compounds can conduct electricity in solution. Most covalent compounds are insoluble in water, unless they are polar and can form hydrogen bonds