CfE Higher Chemistry Unit 1: Chemical Changes and Structure

Intermolecular forces

05/09/2017

Learning Outcomes :

Van der Waal’s Forces and London Dispersion Forces

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•I can explain the difference between intermolecular and intramolecular forces.•I can explain how London dispersion forces arise.•I can state where London dispersion forces arise.•I can relate the strength of London dispersion forces to the number of electrons within the atom or molecule.•I can relate London dispersion forces to other types of bonds in terms of strength.

Lesson Starter: Bonding

a) Magnesiumb) Calcium chloridec) Iron oxided) Propanee) Copper

f) Buteneg) Iodineh) Goldi) Neonj) Lithium bromide

What type of bonding is present in the following:

Intermolecular Bonding• Intermolecular bonding is bonding between molecules• Intermolecular bonding is weaker than intramolecular

bonding.• Melting point and boiling points depend on the strength

of the interaction between molecules.

Glow Science: Intermolecular Forces

Intermolecular bond

Molecules

1. London Dispersion Forces• Very weak forces of attraction exist between all

atoms and molecules e.g. when a noble gas is cooled sufficiently the atoms move together and form a solid.i.e. there must be forces holding the atoms together.

HeHe

HeHe

• Solid helium melts at -273oC (0K) so the forces of attraction must be very weak.

• Pure covalent molecules have Van der Waals forces between them- m.pts and b.pts are low.

London Dispersion forces increase with increasing atomic mass.

How do Van der Waals Forces Arise?The hydrogen atomAt any instant one area of the atom is positive and one is negative i.e. an electric dipole exists.

The electric dipole is temporary.

• These dipoles in the atoms lead to instantaneous forces of attraction between atoms and molecules caused by the movement of electronsà These are weak Van der Waals forces.

• i.e. the melting and boiling points of substances held together by Van der Waals forces are low because little energy is required to overcome the interaction.

Van der Waals Properties

• Consider the halogens F2à GasCl2à GasBr2à LiquidI2à Solid

Down a group à size of molecules increase (there are more electrons / energy levels)à so larger Van der Waals àMelting and boiling points increase.

• The attraction increases as the molecules become closer.

Success Criteria:

Next Lesson:

üI can explain the difference between intermolecular and intramolecular forces.üI can explain how London dispersion forces arise.üI can state where London dispersion forces arise.üI can relate the strength of London dispersion forces to the number of electrons within the atom or molecule.üI can relate London dispersion forces to other types of bonds in terms of strength.

Permanent Dipole-Permanent Dipole Interactions

VdW and LDF Forces05/09/2017

Learning Outcomes :

Permanent Dipole-Permanent Dipole Interactions

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•I can explain how permanent dipole-permanent dipole interactions arise.•I can use spatial arrangement of polar covalent bonds to predict whether a molecule is polar or non-polar.•I can relate permanent dipole-permanent dipole interactions to other types of bonds in terms of strength.

Lesson Starter: Polarity

3. Some covalent compounds are made up of molecules that contain polar bonds but the molecules are overall non-polar. Which of the following covalent compounds is made up of non-polar molecules?

A) Ammonia

B) Water

C) Carbon tetrachloride

D) Hydrogen fluoride

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2. Atoms of nitrogen and element X form a bond in which the electrons are shared equally. Element X could be:

A) Carbon B) Oxygen

C) Chlorine D) Phosphorus.

Polar molecules:

• Non-polar molecule• Symmetrical molecule

• Polar molecule• Non-symmetrical molecule

A molecule is described as polar if it has a permanent dipole.

In CCl4 although there is a difference in electronegativityin the polar bonds, the charge is distributed around the central carbon atom with the positive and negative charges balancing out. The molecule has no overall dipole.

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Propanoneformula mass 58

boiling point 56°C

Butaneformula mass 58boiling point 0°C

POLAR NON-POLAR

The stronger intermolecular forces are responsible for the relatively high boiling point of propanone compared with butane.

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Propanoneformula mass 58

boiling point 56°C

Butaneformula mass 58boiling point 0°C

POLAR NON-POLAR

The intermolecular forces between polar molecules containing polar bonds are stronger than LDF

05/09/2017

Propanoneformula mass 58

boiling point 56°C

Butaneformula mass 58boiling point 0°C

POLAR NON-POLAR

NOTE: We can use the boiling points of propanone and butane to compare the strengths of the intermolecular forces because these substances have the same relative molecular mass

2. Permanent Dipole-Permanent Dipole Interactions

• Occurs between polar molecules.• Polarity increases the strength of intermolecular bonding.• Polar molecules have higher melting and boiling points.• Stronger than Van der Waals.• Attraction between a permanent dipole in one molecule and a

permanent dipole in another molecule.

δ- δ-δ+ δ+

Permanent Dipole-Permanent Dipole intermolecular bond-holds molecules together (weak)

Intramolecular covalent bonds-holds atoms together (strong)

e.g.

Diatomic polar molecule

Notes:Permanent dipole-permanent dipole interactions act in addition to London Dispersion Force electrostatic attractions between polar molecules.

PDPD forces are stronger than LDF attractions for molecules of equivalent size.

Success Criteria:

Next Lesson:

üI can explain how permanent dipole-permanent dipole interactions arise.üI can use spatial arrangement of polar covalent bonds to predict whether a molecule is polar or non-polar.üI can relate permanent dipole-permanent dipole interactions to other types of bonds in terms of strength.

Hydrogen Bonding

PDPD Interactions 05/09/2017

Learning Outcomes :

Hydrogen Bonding 05/09/2017

•I can correctly describe hydrogen bonding as an example of intermolecular or intramolecular bonding.•I can identify from the structural formula or the molecular formula compounds which have hydrogen bonds.•I can relate hydrogen bonding to other types of bonds in terms of strength.

Lesson Starter:

. Rank the following by from lowest to highest anticipated boiling point: C2H4, CH4, Ne, H3COCH3.

. Explain why ethyl alcohol (C2H5OH) has a higher boiling point (78.40 C) than methyl alcohol (CH3OH; 64.70 C).

Lesson Starter:. Hydrogen cyanide, HCN, is highly toxic. Information about hydrogen cyanide is given in the table.Although hydrogen cyanide has a similar molecular mass to nitrogen, it has a much higher boiling point. This is due to the permanent dipole–permanent dipole attractions in liquid hydrogen cyanide..What is meant by permanent dipole–permanent dipole attractions?. Explain how they arise in liquid hydrogen cyanide.

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3. Hydrogen Bonding

• Strongest intermolecular bond.

• Occur due to high difference in electronegativity values of N-H, O-H andF-H bonds.

Hydrogen bonds between water molecules

δ-N-Hδ+ δ-O-Hδ+ δ+H-Fδ-

Eg. alcohols, water

Eg. ammonia, amines

Substances with hydrogen bonds between the molecules have higher than expected boiling points.

e.g. Hydrogen bonds between a hydrogen atom and a strongly electronegative atom.

e.g. methanol

Hydrogen bond

Covalent bond

Dimers and Trimers in Hydrogen Fluoride

• In hydrogen fluoride the bond is so polar that molecules of H-F move about in groups of two or three molecules bonded together.

• Dimer: two molecules linked together

Hydrogen bonding

• Trimer: three molecules linked together

Hydrogen bonding

• Dimers can also occur in alkanoic acids.

Comparing the Strength of Intermolecular Forces

Molecule Formula Mass and Boiling Point

Structure Polarity Intermolecular Forces Present

Butane

Propanone

Propan-1-ol

58 amu-1oC

58 amu56oC

60 amu97oC

Only LDF

LDF and PDPD

LDF and Hydrogen bonding

Non-polar

Polar

Polar

INCREASING STRENGTH à

1. Van der Waals (weakest)2. Permanent Dipole- Permanent Dipole 3. Hydrogen (strongest)

Behaviour in electrical fields

New Higher Chemistry E Allan J Harris

Video clip

Expt 1.11

Bonding SummaryIntramolcular

(Within molecules)Intermolecular

(Between molecules)

1Pure Covalent (e.g. H2, Cl) Same Electronegativity. Covalent Molecules.

Van der Waals (very weak)

2Polar Covalent (e.g. H – Cl) Different electronegativities.Covalent Molecules.

Permanent dipole- permanent dipole

3Ionic (between atoms of metals and non-metals not between atoms of covalent molecules)

Hydrogen bonding (weak) e.g O – H Water

N – H AmmoniaF – H Hydrogen fluoride

4 Metallic

5 Covalent Networks

Lesson Starter: Past Paper 2010

Learning Outcomes :

Relating Properties to Intermolecular Forces

05/09/2017

•I can explain the relationship between the type and strength of intermolecular bonds and melting point, boiling point and viscosity.•I can predict the strength of intermolecular forces by considering the polarity of molecules and the number of electrons.•I can explain patterns in melting and boiling points in terms of the strength of intermolecular forces.•I can explain why the boiling points of ammonia, water and hydrogen fluoride are higher than expected given the number of electrons present in the molecules.•I can explain how hydrogen bonding affects boiling points, melting points, viscosity and solubility (miscibility).•I can explain why ice is less dense than water.

Lesson Starter: Past Paper Questions 20031. Which of the following elements exist as discrete molecules?

A BoronB Carbon (diamond)C SiliconD Sulphur

2. In which of the following liquids does hydrogen bonding occur?A Ethanoic acidB Ethyl ethanoateC HexaneD Hex-1-ene

Properties of Polar Molecules1. Melting and Boiling pts:• Higher melting and boiling pts than non-polar molecules.• Due to the strong intermolecular forces (permanent dipole-

permanent dipole and hydrogen bonds).

2. Attraction to a charged rod

• Polar molecules are attracted to a charged rod whilst non-polar liquids are not.

3. Viscosity• Viscosity is the thickness of a liquid.• The more hydrogen bonds in a molecule the more the

molecules will hold together and the more viscous the liquid will be.

• Consider the following molecules:

• So more O–H bonds per molecule, the more hydrogen bonds the molecule can make and, therefore, so the more viscousthe molecule is.

4. Solubility• Polar solvents dissolve other polar substances (miscible) e.g.

ethanol and water.• The polar functional groups are able to hydrogen bond with

water molecules• Non-polar solvents dissolve other non-polar substances as

there are only Van der Waals forces between the molecules e.g. hexane and iodine.

• Polar solvents will not dissolve non-polar substances (immiscible) e.g. water (polar) and hexane (non-polar).

• Ionic compounds will dissolve in polar solvents but will not dissolve in non-polar solvents.

Like dissolves like!

Properties of WaterThe unusual properties of water are caused by hydrogen bonding.1. Anomalous high boiling pt for such a small molecule.

Boiling pt. of Group 6 Hydrides

H2O H2S H2Se H2Te

Boi

ling

poin

t (o C

)

2. Water is a solvent for ionic lattices• The slightly negative ends of the

polar water molecules are attracted to the positive ions in the crystal lattice while the slightly positive ends of the water molecule are attracted to the negative ions.

• The formation of electrostatic attractions between the ions and polar water molecules results in a release in energy.

• This energy overcomes the electrostatic attractions between oppositely charged ions in the lattice.

3. Lower density of solid ice.Water expands when it freezes. The Hydrogen bonds cause the molecules to adopt an open hexagonal shape. This means the ice is less dense than water and floats.

Learning Outcomes :

Predicting solubility from solute and solvent polarities

05/09/2017

•I can state the type of solvent which tends to dissolve ionic compounds and polar molecular compounds•I can state the type of solvent which tends to dissolve non-polar molecular compounds.•I can select groups within molecules which imply hydrogen bonding.•I can identify molecules with a permanent dipole by considering the spatial arrangement of polar covalent bonds.

Practice Question

Identify the main type of intermolecular force in each compound:1) carbon disulphide2) ammonia3) oxygen4) CH2F2

5) C2H6

Practice QuestionWhat is the strongest intermolecular force present for each of the following compounds?

1) water

2) carbon tetrachloride

3) ammonia

4) carbon dioxide

5) Phosphorus trichloride

6) nitrogen

7) ethane (C2H6)

8) acetone (CH2O)

9) methanol (CH3OH)

10) borane (BH3)

Practice Question1) nitrogen

2) carbon tetrachloride

3) H2S

4) sulfur monoxide

5) N2H2

6) boron trihydride

7) CH4O

8) SiH2O

Practice Question

a) FCN

b) HCN

c) C2H6

d) CF2H2

Practice Question(2)

Past paper 2006

Practice Question(2)

Past paper 2006

Practice QuestionPast Paper 2012

Practice QuestionPast Paper 2012

Practice QuestionPast Paper 2012

Practice QuestionPast Paper 2012

Starter

1. Write ionic formula for the following compounds:

a) Sodium chlorideb) Magnesium carbonatec) Aluminium phosphated) Calcium oxidee) Aluminium carbonatef) Iron (II) nitrate