7 – Shapes of Molecules & Intermolecular Forces Leaving Certificate Chemistry

Using the Valence Shell Electron Pair Repulsion Theory 2007 Q5 (b) (6) 2005 Q4 (b) (6) 2004 Q4 (b) (6) 2003 Q5 (b) (12) The shape of a molecule depends on the number of pairs of electrons in the outer shell of the central atom

Using the VSEPR Theory What is the shape of a BeCl 2 (beryllium chloride) molecule? Be Cl 2 bonding pairs No Lone Pairs Beryllium chloride (BeH 2 ) is a linear molecule 2 Bonding pairs

Beryllium Chloride Bond Angle = 180 o Negatively charged electrons repel each other and arrange themselves as far apart as possible

Using the VSEPR Theory What is the shape of a BCl 3 (boron trichloride) molecule? B Cl 3 bonding pairs No Lone Pairs Boron trichloride (BCl 3 ) is a triangular planar molecule Cl

Boron trichloride Bond Angle = 120 o Negatively charged electrons repel each other and arrange themselves as far apart as possible

Using the VSEPR Theory What is the shape of a CH 4 (methane) molecule? C H H H H 4 bonding pairs No Lone Pairs Methane (CH 4 ) is a tetrahedral molecule

Methane Bond Angle = o Negatively charged electrons repel each other and arrange themselves as far apart as possible

Using the VSEPR Theory What is the shape of a NH 3 (ammonia) molecule? N H H H 3 bonding pairs 1 Lone Pair Ammonia (NH 3 ) is a pyramidal molecule Lone Pair of electrons

Ammonia (NH 3 ) Bond Angle = 107 o The presence of the lone pair of electrons distorts the tetrahedral shape to become a pyramidal shape

Using the VSEPR Theory What is the shape of a water molecule? 2 bonding pairs 2 Lone Pairs Water (H 2 O) is a V-Shaped Molecule O HH

Water (H 2 O) Bond Angle = o The presence of the 2 lone pair of electrons distorts the tetrahedral shape to become a v-shaped molecule

Learn the following table off by heart Bond Pairs Lone Pairs Molecule ShapeBond Angle 20Linear180 o 30 Triangular Planar 120 o 40Tetrahedral109.5 o 31Pyramidal107 o 22V-Shaped104.5 o 13Linear180 o

Today’s objectives To learn about the difference between a polar bond and a polar molecule To predict whether a molecule will be polar

Symmetry & Bond Polarity Some molecules may have polar bonds but they are not polar molecules....This is due to symmetry

OCO ∂ ― ∂ 2+ Since the centre of the partial negative charges coincides with that of the partial positive charges, none of these molecules are polar B Cl ∂ 3+ ∂ ― CCl ∂ 4+ ∂ ―

Since the centre of the partial negative charges does not coincide with that of the partial positive charges, none of these molecules are polar despite the presence of polar bonds

Learning objectives Difference between intermolecular and intramolecular forces 3 types of intermolecular forces and relative strengths Relationship between intermolecular forces and melting/ boiling points

Intramolecular Bonding Water O H H Intramolecular bonding is the term used to describe the bonding between atoms in a molecule

Intermolecular Forces Intermolecular forces are attractions between different molecules 2008 Q5 (a) (5)

Intermolecular Forces Van Der Waals Forces Dipole-Dipole Interaction Hydrogen Bonding

Van Der Waals Forces Let’s look at neon atoms. There are 10 electrons in every neon atom. Ne This temporary force is called a Van Der Waals Force

Van Der Waals Forces

Intermolecular Forces Van Der Waals Forces They result from the ebb and flow of electrons within these molecules, which causes temporary polarity in a molecule These weak forces are the only main forces between like non-polar molecules like N 2, O 2, H 2, I 2, CH 4. The bigger the molecule the more electrons so the greater the strength of the VDW forces.

Intermolecular Forces Van Der Waals Forces – used by Gecko’s?

A dipole-dipole interaction exists between polar molecules. They are permanent and much stronger than Van Der Waals forces. Dipole-Dipole Forces H Cl δ+ δ- HCl δ+ δ- H Cl δ+ δ- Dipole-Dipole Forces Created

Dipole-Dipole Forces

When molecules that have a hydrogen atom bonded to nitrogen, oxygen or fluorine atom come together a very strong dipole-dipole interaction is setup due to the high electronegativity difference between the atoms. Hydrogen Bonding H O δ+ δ 2- HO δ+δ 2- H O δ+ δ 2- Hydrogen Bonding Created H δ+ H H

Hydrogen bonding forces are permanent. They are and much stronger than normal Dipole- Dipole Forces Hydrogen Bonding H O δ+ δ 2- HO δ+δ 2- H O δ+ δ 2- Hydrogen Bonding Created H δ+ H H

Learning check… What is the difference between a intermolecular force and an intramolecular force? Name three different types of intermolecular force Which is the strongest? Which is temporary?

Particle arrangements: solids This animation shows a 2-D view of the motion of the atoms in a 3-D solid.

Particle arrangements: liquids This animation shows a 2-D view of the motion of the atoms in a liquid. There is no order.

Particle arrangements: gases

Hydrogen Bonding Hydrogen bonding in water high boiling point Strong forces attract molecules together and a lot of energy is needed to separate the particles to turn it to a gas

Hydrogen Bonding Kevlar is a type of fibre that is extremely strong and flexible. It is strong because long chains of molecules line up parallel to each other and are held together strongly by hydrogen bonding

Changes of state

Check your learning.. What is the relationship between boiling point and intermolecular bonding?

Boiling Points & Forces If a substance has a very high boiling point then the forces between molecules of that substance are very strong – ionic, dipole- dipole forces or hydrogen bonding. If a substance has a very low boiling point then the forces between molecules of that substance are very weak – usually Van Der Waals forces

Why does oxygen (O 2 ) have a much higher boiling point than hydrogen (H 2 )? Both are non-polar Therefore both molecules only experience weak Van Der Waals forces. But the Van Der Waals forces are stronger in oxygen because it is a molecule with more electrons (16 versus 2). More Electrons  Greater VDW Forces  Higher Boiling Point for oxygen Q4 (c) (6)

Why does methanal have a much higher boiling point than ethene? Ethene is non-polar  Weak Van Der Waals forces present Methanal is polar  Stronger dipole-dipole forces present Therefore methanal has a much higher boiling point then ethene. δ+δ-

Why does water have a much higher boiling point than hydrogen sulfide (H 2 S)? Contains very strong hydrogen bonding intermolecular forces Contains very weak Dipole-Dipole intermolecular forces 2007 Q5 (c) (12)