Chapter 6 Shapes Of Molecules And Intermolecular Forces

Question 1

What is electron pair repulsion theory?

Answer 1

Electrons have negative charges so repel eachother, this is used to create a model to accurately predict the shapes of molecules
Electron pairs surrounding a central atom determine the shape
The electron pairs repel eachother so they are arranged as far apart as possible
The arrangement minimises repulsion forming a definite shape

Question 2

How can 3Dimensions be represented in a diagram?

Answer 2

Solid line = a bond in the plane of the paper
Solid wedge = comes out of the plane of paper
Dotted wedge = goes into the plane of the paper

Question 3

How do Lone pairs and bonded pairs influence the shape?

Answer 3

A lone pair of electrons sit slightly closer to the central atom occupying more space than a bonded pair. This means they repel more strongly. This decreases the bond angle of the molecule by roughly 2.5 degrees per lone pair

Question 4

Explain linear bonding shapes

Answer 4

There are 2 bonding pairs around the central atom creating a bond angle of 180 and a straight line shape
E.g. CO2

Question 5

Explain a trigonal planar shape

Answer 5

There are 3 bonding pairs around the central atom creating a bind angle of 120 and a triangular shape
E.g. BF3

Question 6

Explain a tetrahedral shape

Answer 6

There are 4 bonding pairs around the central atoms resulting in a bond angle of 109.5 and a tetrahedral shape (like diamond)
E.g. CH4

Question 7

Explain an octahedral shape

Answer 7

There are 6 bonding pairs around the central atom resulting in a bond angle of 90 and an octahedral shape
E.g. SF6

Question 8

Explain a non linear shape

Answer 8

for water: There are 2 bonding pairs and 2 lone pair around the central atom resulting in an angle of 104.5 (109.5 -2(2.5)) and a bent line shape
E.g. H2O

Question 9

Explain a pyramidal shape

Answer 9

For ammonia: there are 3 bonding pairs and 1 lone pair around the central atom resulting in a bond angle of 107 (109.5 -2.5) and a pyramid shape
E.g. NH3

Question 10

State the shapes, type and number of electron pairs and bond angle of common ions

Answer 10

Ammonium= tetrahedral, 109.5, 4 bonding pairs
Carbonate and Nitrate= Trigonal planar, 120, 4 bonding pairs (in three regions if electron density as one is a double bond which changes nothing)
Sulphate= tetrahedral, 109.5, 6 bonding pairs ( in 4 regions of electron density as there are 2 double bonds)

Question 11

What is electronegativity?

Answer 11

In a covalent bond the nuclei of the bonded atoms attract the shared pair of electrons. Some elements are more electronegative, this is measured on the Pauling electronegativity values scale. More electronegativity means the electrons sit closer to that element within the covalent bond creating dipoles

Question 12

What are non polar bonds?

Answer 12

Electrons are shared equally because the bonding atoms have a similar or the same electronegativity e.g. Hydrogen-Hydrogen bonds

Question 13

What are polar bonds?

Answer 13

The electron pair is shared unequally between the two molecules because one of the bonding atoms is more electronegative than the other. This means that the more electronegative atom has more time with the electrons close t out creating a slightly (delta) negative charge and the other atom which has less electron time has a delta positive charge. These are permanent dipoles and create polar bonds and molecules

Question 14

What are polar molecules?

Answer 14

E,g. HCl is a simple example because it only has two atoms however large molecules may have more polar bonds which cancel the polar effect or enhance the polar effect on the molecule

Question 15

Why is H2O polar but CO2 is not

Answer 15

H2O has a non linear shape making the area near the O more negative creating a polar molecule whereas CO2 is a linear molecule to the polar bonds cancel eachother

Question 16

What are polar solvents and why are they useful?

Answer 16

Polar solvents, like H2O, are solvents made of polar molecules, this means that typically insoluble materials, like ionic compounds, are more likely to dissolve
E.g. NaCl in H2O is soluble because water molecules attract Na+ and Cl-, the ionic lattice breaks down as it dissolves and in the resulting solution water molecules surround the Na+ and Cl- ions

Question 17

What are the 3 types of intermolecular force in order of increasing strength?

Answer 17

Weakest: Induced dipole-dipole forces (London forces)
Middle: Permanent dipole-dipole forces
Strongest: Hydrogen bonding
(A single covalent bond is stronger than all of these)

Question 18

What are IM forces mostly responsible for?

Answer 18

Physical properties like bp and mp

Question 19

How does an induced dipole-dipole force occur?

Answer 19

Movement of electrons produces a changing dipole in a non polar molecule
At an instant, an instantaneous dipole will exist, but it’s position is constantly shifting
The instantaneous dipole induces a dipole in a neighbouring molecule
The induced dipole induces further dipoles on neighbouring molecules which then attract one another weakly

Question 20

What influences the strength of a London force?

Answer 20

Electrons.
If there are more electrons:
Larger instantaneous dipole,
Greater induced dipole-dipole interaction,
Stronger attractive force between molecules

Question 21

What is a permanent dipole-dipole force?

Answer 21

Two molecules with permanent dipoles (caused by differing electronegativities) interact
They line up so that the delta- of one molecule and delta+ of another are attracting eachother creating a stronger intermolecular force called a permanent dipole to dipole force which requires more energy to break than the London force

Question 22

What is a Simple molecular substance and what are it’s properties?

Answer 22

Made of simple molecules that have a definite molecular formula like H2O
They are held together by weak IM forces but the atoms within each molecule are held together by strong covalent bonds.
This means that they have low melting and boiling points

Question 23

What is the solubility of a non-polar simple molecular substance in non-polar and polar solvents? (Generally)

Answer 23

Non polar molecules tend to dissolve in non polar solvents because IM forces form between the solute and solvent which weakens the IM forces of the simple molecular lattice breaking down the compound.

Non polar molecules tend to be insoluble in polar solvents because there is little interaction between the solvent and the molecules of the lattice besucase the IM forces of the solvent are too stronge for the non polar substance to break

Question 24

What is the solubility of a polar simple molecular substance in non-polar and polar solvents? (Generally)

Answer 24

Polar substances may dissolve in polar solvents as the polar solute and solvent molecules can attract eachother and is similar to dissolving an ionic compound. The solubility depends on the strength of the dipole and can be hard to predict

Question 25

Can simple molecular structures conduct electricity?

Answer 25

No because there are no mobile charged particles meaning there is nothing to complete an electrical circuit

Question 26

What is a hydrogen bond?

Answer 26

It’s a special type of permanent dipole-dipole interaction containing:
An electronegative atom with a lone pair of electrons (e.g. O,N,F)
A hydrogen atom attached to an electronegative atom (e.g. OH, NH, FH)
The hydrogen bond acts between a lone pair form an electronegative atom in one molecule and a hydrogen atom in another molecule and is drawn with a dotted line

Question 27

What three anomalous properties of water are there?

Answer 27

The solid (ice) is less dense than the liquid (water)
Water has a relatively high mp and be
High surface tension and viscosity

Question 28

Why is ice less dense than water?

Answer 28

Hydrogen bonds hold water molecules apart in an open lattice structure
The water molecules in ice are further apart than in water
Solid ice is less dense than liquid water so floats as the holes in the open lattice structure decrease the density of water on freezing
When ice melts the lattice collapses and the molecules move closer increasing the density

Question 29

Why does water have a relatively high mp/bp?

Answer 29

Hydrogen bonds are extra forces over and above the London forces
Significantly More energy is needed to break the hydrogen bonds in water, so water has a much melting and boiling point than would be expected from just London forces
When the ice lattice breaks, the rig in arrangement of hydrogen bonds in ice is broken
When water boils the hydrogen bonds break completely