6- Shapes of molecules and intermolecular forces.

Question 1

Explain the molecular geometry of CH4.

Answer 1

• Four bonded pairs of electrons surrounding central C atom.

- Tetrehedral shape, bond angle 109.5 degrees.

Question 2

What is the relative repulsion between loan pairs of electrons and bonded pairs?

Answer 2

• Loan pairs of electrons are slightly closer to the central atom and occupy more space than bonded pairs.
• So lone pairs have a greater repulsion.

Question 3

What effect does the difference in repulsion between loan pairs and bonded pairs of electrons have on the bond angle.

Answer 3

• Lone pairs repel more than bonded pairs, so they push bonded pairs of electrons slightly closer together.
• The bond angle is reduced by about 2.5 degrees with every lone pair.

Question 4

Explain the molecular geometry of NH3

Answer 4

• Three bonded pairs, 1 lone pair.

- Trigonal Pyramidal shape, bond angle 107 degrees.

Question 5

Explain the molecular geometry of H2O

Answer 5

• Two bonded pairs, two lone pairs.

- Non linear shape, bond angle 104.5 degrees.

Question 6

Explain molecular geometry of CO2

Answer 6

• Each multiple bond is treated as a single bonded region.

- Two bonding regions, no lone pairs on central atom. Bond angle 180 degrees. Linear.

Question 7

Explain molecular geometry of BF3

Answer 7

• Three bonding pairs, no lone pairs.

- Trigonal Planar shape. Bond agnle 120 degrees.

Question 8

Explain molecular geometry of SF6

Answer 8

• Eight bonding pairs, no lone pairs.

- Octahedral shape, bond angle 90 degrees.

Question 9

How would you predict the molecular shape of an ion?

Answer 9

• Find the number of regions of electron desnisty around the central atom.
• Work out which regions are bonded and which are lone pairs of electrons.

Question 10

Definition of electronegativity.

Answer 10

The ability of an atom to attract electron density towards itself in a covelant bond.

Question 11

What effects an atoms electronegativity?

Answer 11

-Nuclear charge.
-Atomic radius.
-Shell sheilding.
Shell sheilding and atomic radius change the effective nuclear charge.

Question 12

Where are the more electronegative elements on the periodic table?

Answer 12

The electronegative atoms are on the top right corner of the periodic table, fluorine has the highest electronegativity of 4.0 on paulings electronegativity scale. Nobel gases are not included as they tend to not form compounds.

Question 13

Explain why the difference in electronegativity between two atoms must be greater than 1.8 to be ionic.

Answer 13

A large electronegativty means one of the atoms will have a much greater attraction to the bonding electrons. This atom gains control of the electron and so the compound is ionic.

Question 14

What is a non polar bond?

Answer 14

In a non polar bond the bonding electron is shared equally between both atoms as the difference in electronegativity is very low. eg CH

Question 15

What is a pure covelant bond?

Answer 15

The difference in electronegativity is 0 so the two bonding elements are the same. The bonding electron is shared equally.

Question 16

What is a polar bond?

Answer 16

When the difference in electronegativity is 0-1.8. The electrons are shared unevenly creating a polar covelant bond.

Question 17

What is a dipole?

Answer 17

When a molucule has a difference in electronegativity creating a polar covelant bond. One atom will have a partially different charge to the other atom. (represented by teh delta sign). This separation in charge is called a dipole.

Question 18

What is a permenant dipole?

Answer 18

The charges on each side of the molecule’s polar covelant bond do not change.

Question 19

Explain if H2O is a polar molecule or not.

Answer 19

As H2O is a bent molecule, one side of the molecule (oxygen) has a more negativly charge than the other side (two hydrogens). This is because the dipoles act in different directions but do not exactly oppose each other.

Question 20

Explain if CO2 is a polar molecule or not.

Answer 20

CO2 is linear, this means the two delta negative charges on the carbon atoms act in opposite directions and cancel each other out. Overr all dipole is 0.

Question 21

What makes ionic compounds soluable in a polar solvent? (water)

Answer 21

Polar solvents dissolve polar molecules. The delta negative oxygen molecules attract positive ions and delta positive hydrogen attracts the negative ion. The solid lattice breaks down and each ion is surrounded by delta positive/ negative atoms from the water molecule.

Question 22

Definition of intermolecular force.

Answer 22

Weak interactions between dipoles of different molecules.

Intermolecular forces determine boiling/ melting point. Covelant bonds determine identity and reactions of a molecule.

Question 23

What are the three catagories of intermolecular forces?

Answer 23

Induce dipoles (london forces)
Permenant dipole
Hydrogen bond.

Question 24

Induced dipole definition.

Answer 24

London forces act between all molecules. Movement of electrons causes a changing dipole in a molecule. At any instant, an instantanious dipole could exist. An instantaineous dipole induces a dipole on a neighbouring molecule. The induce dipole induces further dipoles on neighbouring molecules.

Question 25

What effects the strength of an induced dipole?

Answer 25

The larger amount of electrons means a larger amount of instantaneous and induced dipoles which are also stronger.

Question 26

Why do diatomic molecules tend to have a low boiling point?

Answer 26

As both atoms are the same, the differencein electronegativity is 0. This means no dipoles can be formed. The only intermolecular force formed are london forces.

Question 27

What is a simple molecular substance?

Answer 27

It is made up of simple molecules. (small units). The molecules are held together by weak intermolecular forces but the atoms in the molecule are held together with strong covelant bond.

Question 28

Is a non polar simple molecular substance soluble?

Answer 28

Non polar substances tend to be soluble in non polar solvents (hexane). This is because intermolecular forces are able to form between the solvent and the molecules and break the intermolecular forces between the molecules.

Question 29

When is a non polar simple molecular substance not soluble?

Answer 29

It cannot be dissolved in a polar solvent. The intermolecular forces within a polar solvent are too strong for the simple molecule to break so the solvent molecules only bond between each other.

Question 30

What is a hydrogen bond?

Answer 30

A hydrogen bond is a type of permenant dipole found between a delta positive hydrogen atom and an electronegative atom in a molecule with a lone pair of electrons.

Question 31

What properties of water are due to hydrogen bonding?

Answer 31

Ice is less dense than water. (Hydrogen bonds hold water molecules apart in open lattice structure.) This allows ice to float and let fish survive (insulation). -Water also has a high melting/ boiling point due to the strong hydrogen bonds. Water lets life thrive. Hydrogen bonding holds dna together.