Chapter 6 Shapes of molecules and intermolecular forces

Question 1

Electron-pair-repulsion theory

Answer 1

All electrons have negative charges which means they will all repel from each other. This means that they will repel each other so that they arranged as far apart from each other as possible.The arrangement of electron pairs minimises repulsion and thus hold the bonded atoms in a definite shape. Different number of electron pairs result in different shapes.

Question 2

Representing molecules in three dimension

Answer 2

a solid line represents a bond in the lane of the paper.
a solid wedge comes out of the plane of the paper.
a dotted wedge goes into the plane of the paper.

Question 3

Bonded pair and lone pair repulsions

Answer 3

A lone pair of electron i slightly close to the central atom and occupies more space than a bonded pair. This results in alone pair repelling more strongly that a bonding pair.

Question 4

Molecular shapes from four electron pairs

Answer 4

The four electron pairs around the central atom repel one another as far apart as possible into a tetrahedral arrangement.
Lone pairs repel more strongly than bonded pairs.
Therefore, lone pairs repel bonded pairs slightly close together, decreasing the bond angle - the angle between the bonded pairs of electrons.
The bond angle is reduced by about 2.5 for each lone pair.

Question 5

Molecular shapes from multiple bonds

Answer 5

In molecules containing multiple bonds, each multiple bond is treated as a bonding region.

Question 6

Molecular shapes from other electron pairs:
three, six,

Answer 6

Three bonded pairs of electrons around the atom will mean that the electron repulsion will give a trigonal planar shape with equal bond angles of 120 degrees.
Six electron pairs will give an electron repulsion which will create an octahedral shape with equal bond angles of 90 degrees.

Question 7

Shapes of ions - with methane and ammonium example

Answer 7

We can use electron-pair repulsion theory to explain and predict shapes of ions
The ammonium ion:
has the same number of bonded pairs of electrons around the central atom as a methane molecule.
Has the same tetrahedral shape and bond angles (109.5) as the methane molecule.

Question 8

Electronegativity

Answer 8

the attraction of a bonded atom for he pair of electrons in a covalent bond
only occurs when the bonded atoms are different because:
the nuclear charges are different
the atoms may be different sizes
shared pair may be closer to one nucleus than the other.

Question 9

How is electronegativity measured?

Answer 9

The pauling electronegativity values.
electronegativity increases up and across the periodic table.
Non-metals N,O,F,Cl are most electronegative
Group 1 metals - Li,Na,k, are least electronegative

Question 10

Is the molecule ionic or covalent

Answer 10

If the electronegativity difference is 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 considered ionic rather than covalent.

Question 11

Bond Polarity - Non-polar bonds

Answer 11

In a non-polar bond, the bonded electron pair is shared equally between the bonded atoms. A bond will be nonpolar when the bonded atoma are the same or the bonded atoms have the same or similar electronegativity. Thes bonds are pure covalent bonds.

Question 12

Bond polarity - polar bonds

Answer 12

In polar bonds, the electrons pair is shared unequally between the bonded atoms. A bond will be polar when the bonded atoms are different and have different electronegativity values.

Question 13

Polar solvents and solubility
NaCl example

Answer 13

e.g. Sodium chloride + water -> Na+ + Cl-
Water molecules attract Na+ ions and Cl- ions
the ionic lattice breaks down as it dissolves.
the Na+ ions attract to the oxygen and cl attract to the hydrogen.

Question 14

Intermolecular forces

Answer 14

are weak interactions between diploes of diferent molecules. The fall into three categories.
induced dipole - dipole (London forces)
permanent dipole -dipole
hydrogen bonding

Question 15

Rank intermolecular forces in level of strength

Answer 15

Hydrogen bonds > permanent dipole -dipole > London forces

Question 16

Induced dipole - dipole interactions ( London Force)

Answer 16

London forces are between all moecules, whether polar or non-polar.
Movement of electrons produces a changing dipole in a molecule. an instantaneous dipole will exist. This will then induce a dipole on a neighbouring molecule. The induced dipole further dipoles on neighbouring molecules which then attract one another.
They are temporary attractions

Question 17

The strenght of London forces

Answer 17

The more electrons in each molecule:
the large the instataneous and induced dipoles
the greater the london force interactions
the stronger the attractive forces between molecules

Question 18

What happened to van der Waals forces

Answer 18

Covers both so is ambiguous

Question 19

Permanent dipole - dipole interactions

Answer 19

Only between polar molecules.
Opposite polar end attract with other molecules.
Stronger than london forces.\

Question 20

Simple molecular substance

Answer 20

is made up of simple molecules
in the solid state simple molecules forma regular strucutre called a simple molecular lattice.
molecules held in place by week intermolecular forces.
bonded through strong covalent bonds.

Question 21

Properties of simple molecular substance

Answer 21

Low melting and boiling point
Solubility of non-polar simple molecular substances:
soluble in non-polar solvents,
insoluble in polar solvents
Solubility of polar simple molecular substance:
Hard to predict depends on strenght of the intermolecular forces.
Doesn’t conduct

Question 22

Hydrogen bonds

Answer 22

Found between permanent dipole - dipole interactions found between molecules containing:
an electronegative atom with a lone pair of electrons, e.g. O, N,fl
a hydrogen atom attached to an electronegative atom,

Acts between a lone pair in an electronegative atom and a hydrogen atom in another molecule.

shown by a dashed line

Question 23

Anomalous properties of water

Answer 23

The solid ice is less dense than the liquid water because they hydrogen bonds hold water molecules apart in an open lattice structure but in ice they are further apart.
Relatively high melting point and boiling point - each water molecule has two hydrogen bonds aswell which are strong.