AQA AS-Level Chemistry Topic 1.3 Bonding (PT.2)

What does VSEPR stand for?

valence shell electron pair repulsion theory

What does VSEPR stand for?

-Electron pairs repel each other

-They will adopt positions where they are separated by the largest possible angle

-Lone pairs repel more strongly than bonded pairs so the repulsion between lone pair-lone pair is larger than lone pair-bonded pair which is also larger than bonded pair-bonded pair

What are the steps in predicting the shape of a molecule?

1) Draw a dot-and-cross diagram

2) Count the number of electron pairs around the central atom
-Decide the orbital arrangement that minimizes repulsion

3) Count the number of bonded pairs and lone pairs of electrons
-Decide the shape of the molecule based on where the atoms are found

Explain why graphite is used as a lubricant

Graphite has strong layers but weak London forces between the layers which need little energy to break. The layers can slide over each other easily.

What is the shape of the molecule,, number of bonded pairs and lone pairs and bond angle for… 2 ELECTRON PAIRS around the central atoms

Shape of molecule: Linear

Number of bonding pairs and lone pairs: 2bp

Bond angle: 180 degrees

What is the shape of the molecule,, number of bonded pairs and lone pairs and bond angle for… 4 ELECTRON PAIRS around the central atoms

Shape of molecule: Tetrahedral

Number of BP/LP: 4bp

Bond angle: 109.5 degrees

In which type of structure would you find intermolecular forces?

Simple covalent lattice

What is the shape of the molecule,, number of bonded pairs and lone pairs and bond angle for… 4 ELECTRON PAIRS BUT 3 bonding pairs and 1 lone pair around the central atoms

Shape of molecule: Pyramidal

Number of BP/LP: 3bp, 1 lp

Bond angel: 107 degrees

What is the shape of the molecule, number of bonded pairs and lone pairs and bond angle for… 4 ELECTRON PAIRS BUT 2 bonding pairs and 2 lone pairs around the central atoms

Shape of molecule: V shaped or non-linear

Bonding pairs/Lone pairs: 2 bp, 2lp

Bonding angle: 104.5 degrees

What is the shape of the molecule,, number of bonded pairs and lone pairs and bond angle for… 6 ELECTRON PAIRS around the central atoms

Shape of molecule: Octahedral

BP/LP: 6 bp

Bond angle: 90 degrees

Draw a dot and cross diagram of gallium chloride, assuming it to be covalent and state the number of electron pairs, shape, and bonding angle

3 (bonding) pairs

Trigonal planar

120 degrees

Draw a dot and cross diagram for Germaniumm GeCl4, and state the number of electron pairs, shape of molecule and bonding angle

-Four bonded pairs

Tetrahedral

109.5 degrees

What is the shape of the molecule,, number of bonded pairs and lone pairs and bond angle for… 3 ELECTRON PAIRS around the central atoms

Shape of molecule: Trigonal planar

Number of bonding pairs/lone pairs: 3 bp

Bond angle: 120 degrees

Draw a dot and cross diagram for PH3 and state the number of electron pairs, shape of molecule and bonding angle

-3 bonded pairs and one lone pair

Pyramidal

107 degrees bond angle

What are intermolecular forces.

Forces between molecules

In which type of structure would you find intermolecular forces?

Simple covalent lattice

What are the types of intermolecular forces that exist between all molecules

Van der Walls forces

Which can be divided in to two sub-categories, and hydrogen bonds

There are two types of Van der Waals forces that can exist within a molecule:
Induced dipole - dipole interactions
Permanent dipole - dipole interactions

What are Induced dipole - dipole forces also known as

London forces

What are London forces?

London forces are the only attractive intermolecular forces that act between non-polar molecules

How do London forces arise?

Electrons in molecules are constantly moving.
At any instant the distribution may not be symmetrical. This results in a instantaneous temporary dipole.

This dipole induces dipoles in neighboring molecules and leads to an attraction between neighboring charges in the dipoles.

These attractions between molecules are known as London forces, and are a type of Van der Waals forces.

What affects the strength of London forces

-The more electrons there are in the molecule, the stronger the fluctuations in the electron cloud, and the greater the instantaneous dipole - induced dipole forces.

-For molecules with the same number of electrons, the greater the contact area between the molecules, the stronger the induced dipoles that develop.

-Unbranched molecules have stronger London forces than branched molecules

The more electron a molecule has, the greater the __ formed

And the stronger the _ forces that arise

therefore the more energy is required to break them and the higher the / of the substance
Dipole, London, MP/BP.

Fill in the blank

How does branching affect the strength of London forces

-The more branching a molecule has, the lesser the contact area between molecules.

-The smaller the contact area between molecules and the weaker the London forces that arise

-So less energy is required to break the London forces and the lower the MP/BP of the substance

In which state will chlorine exist at 25 degrees C

Gas

In which state will bromine exist at 25 degrees C

Liquid

In which state will Iodine exist at 25 degrees C

Solid

How does the boiling point change down the halogens

Boiling point increases down the group - as the number of electrons in each molecule increases the London forces between molecules become stronger and more energy is needed to break them.

What is the trend in melting and boiling points of Period 3 elements

They increase from right to left

This is because the number of electrons per molecule increases, the London forces between molecules get stronger so more energy is needed to separate the molecules.

What is a key definition for permanent dipole-dipole forces

Electronegativity

Define the term electronegativity

The ability of an atom to attract the bonding electrons in a covalent bond

How is electronegativity measured?

The Pauling Scale is used to compare the electronegativity of atoms of different elements

Elements get more electronegative the closer you get to the top right of the periodic table

1) Find out if the bond is polar by finding the difference in electronegativity values between the 2 atoms in the bond. If the difference is….

• equal to 0 the bond is non-polar covalent
  -the only exception is C - H bond which is considered non-polar

-Less than 0 and more than 1.8 the bond is polar covalent (it has a permanent dipole_=)

-Less than 1.8 the bond is ionic

What is the relationship between the size of the Pauling value and the degree of electronegativity

the greater the value, the greater the electronegativity

What is a permanent dipole?

A small charge difference that does not change across a bond, with partial charges on the bonded atoms (the result of the bonded atoms having different electronegativities

labeling Polar bonds

δ- = the more electronegative atom (the one with the higher Pauling value)

What is a Polar covalent bond?

A bond with a permanent dipole, having positive and negative partial charges on the bonded atoms

What is a Polar molecule?

A molecule with an overall dipole, having taken in to account any dipoles across bonds and the shape of the molecule

What is a common example of a polar molecule

Water

What are the charges like on TRIGONAL PLANAR molecules

Dipoles cancel each other, therefore NOT POLAR

What are the charges like on PYRAMIDAL molecules?

δ+ is concentrated on the bottom, therefore POLAR

What are the charges like on TETRAHEDRAL molecules

Cancel so non polar

What are the charges on LINEAR molecules?

Dipoles cancel each other, therefore they are NON POLAR

What is a hydrogen bond?

A hydrogen bond is a strong dipole-dipole attraction between an electron Deficient hydrogen atom of -NH, OH or HF on one molecule and a lone pair of electrons on a highly electronegative atom (N, O or F) on a different molecule

How Melting and Boiling Points relate to Intermolecular forces

-The stronger the IMFs present the more energy is required to break them, therefore the higher the melting/boiling point of the substance

How do Hydrogen bonds arise?
e.g. water

The O-H bond is polar because of the large difference in electronegativity between hydrogen and oxygen.

If another molecule of water (B) were attracted to the existing molecule (A), the δ+ end of molecule B will be attracted to the δ- end of molecule A.

One of the lone pairs of electrons from molecule A can form a partial dative covalent bond with the δ+ H on molecule B.

This very strong dipole-dipole interaction is the H bond.

what is the trend in melting and boiling points of the hydrides, H2S, H2Se and H2Te, as the number of electrons in the molecule increases?

They increase because the strength of the intermolecular forces increases

What type of intermolecular force would you expect to find in all simple molecular substances?

London forces

Explain why the increase in strength of this type of intermolecular force is consistent with expectations with the increase in the number of electrons in the molecules.

More electrons in the molecule means more chance of larger instantaneous dipoles/induced dipoles and therefore stronger attractions between them

What is responsible for the higher than expected melting and boiling points of water for a molecule of its size

Hydrogen bonding

What is the density of ice compared with water

Ice floats on water because Ice is less dense than water.

This means for a given volume, there are fewer molecules of H2O in ice than in water.

The molecules of H2O in solid ice are less closely packed than in liquid water.

When water freezes, the molecules are pushed further apart in the structure of ice. This means there are fewer molecules per unit volume and therefore a lower density.

What is responsible for the open structure of ice which is why ice is less dense than water

Hydrogen bonding

What is the bond Angle in a water molecule?

104.5

How do hydrogen bonds relate to biological systems

Hydrogen bonds are responsible for the secondary structure of proteins, such as the formation of a-helices, B-sheets

Butane has a higher boiling temperature than 2-methylpropane. This is because butane has

A larger surface area of contact

How many hydrogen bonds happen between Adenine and Thymine bases

2 H bonds

The melting and boiling temperatures of a substance depends on the of the forces between the particles. The __ the forces, the _ energy is required to separate the particles, so the the melting and boiling points.

In a metallic substance, the forces broken are _ between the lattice of _ and the "sea" of electrons. These attractions are strong, so a lot of energy is required to overcome them.

In an ionic compound, the forces broken are _ attractions between the ions of opposite charge. These attractions _, so a lot of energy is required to overcome them.

In a covalent substance with giant structure, the forces broken are covalent bonds between the bonding atoms. These forces are __, so a lot of energy is required to overcome them.
In a covalent substance with simple structure, the forces broken are intermolecular forces between the molecules. These forces are weak, so not much energy is required to overcome them.
Fill the blanks

Strength, stronger, more, higher, electrostatic attractions, positive ions, delocalised, electrostatic, strong, strong

How many hydrogen bonds happen between Guanine and Cytosine

Solubility of Ionic compounds in water

Ionic compounds mostly dissolve well in water.

The separated ions form strong attractions to the polar water molecules.

Positive ions are attracted to the δ- O of the water molecules and negative ions are attracted to the δ+H.

Solubility of Ionic compounds in non-polar solvents

Ionic compounds are insoluble in non-polar solvents, such as hexane, because they cannot form interactions favorable enough to break up the ionic lattice.

Solubility of Simple covalent compounds in non-polar solvents

Simple covalent substances are often soluble in non-polar solvents because Van der Waals' forces form between the solvent and the solute. These forces weaken the lattice structure, allowing it to break up.

Solubility of simple covalent substances in water

Simple covalent substances do not dissolve in water unless they are polar.

However δ+ ends of polar molecules can attract the δ- O of the water molecules and the δ- ends attract the δ+ H, so polar molecules are water-soluble.

Solubility of Giant covalent substances in polar and non-polar substances

Giant Covalent substances are insoluble in both polar and non-polar solvents because the covalent bonds holding the atoms together in the lattice are too strong to be broken by the solvent.

Solubility of metals

Metals are insoluble in both polar and non-polar solvents, though some metals react with water to form a soluble (ionic) product.

For any substance to conduct electricity, there must be….

Mobile charge carriers

Electrical conductivity of metals

The delocalised electrons in the electron "sea" are free to move, so metals can conduct electricity in both in the solid and liquid states.

Electrical conductivity of Ionic solids

In an ionic solid, the Ions are held in fixed positions in the lattice and cannot move; so ionic solids are non-conductors,

However when melted or dissolved in water, the Ions can move and carry charge, so ionic substances in the liquid or aqueous state can conduct electricity.

Electrical conductivity of Covalent substances (simple or giant)

Covalent substances do not conduct electricity in any state because there are no mobile electrons. Graphite is an exception because it contains Delocalised Electrons between its layer