1. Intermolecular Forces, Liquids, and Solids
SCH4U 1 1 1 1

2. Why do some solids dissolve in water, but others do not?
QUESTIONS TO CONSIDER
Why do some solids dissolve in water, but others do not?
Why are some substances gases at room temperature, but others are liquid or solid?
What gives metals the ability to conduct electricity, what makes non- metals brittle?

3. Solids, Liquids & Gases, A Comparison
The fundamental difference between states of matter is the distance between particles.

4. Solids, Liquids & Gases, A Comparison
Because in the solid and liquid states particles are closer together, we refer to them as condensed phases.
© 2009, Prentice-Hall, Inc.

5. States of Matter (Review)

6. Intermolecular Forces
Are the forces holding solids and liquids together.
much weaker than chemical bonds (< 15% as strong)
Example:
energy needed to break O-H bond in water: 934 kJ
energy needed to vaporize water: 40.7 kJ

7. Intermolecular Forces
When a substance melts or boils the intermolecular forces are broken (not the covalent bonds).
When a substance condenses intermolecular forces are formed.
Intermolecular forces in ice
Intermolecular forces between water molecules

8. Importance of Intermolecular Forces
Intermolecular forces of attraction are strong enough to control physical properties such as:
Boiling & melting points
Vapor pressures
Viscosity
Surface tension
Collectively, the intermolecular forces being discussed are called van der Waal forces.
Hydrogen Bonds
Dipole-Dipole
London Dispersion Forces
Increasing Strength

9. 1. Dipole-Dipole Exist in Polar molecules only.
The positive end of one is attracted to the negative end of the other and vice-versa.
These forces are only important when the molecules are close to each other (like in solid & liquid states).

10. Intermolecular Forces – Dipole-Dipole
In molecules of equal mass & size, the strength of the IMF increase with increasing polarity.

11. 2. Hydrogen Bonding
Special case of dipole-dipole forces when H is bonded to N, O, or F.
By experiments: boiling points of compounds with H-F, H-O, and H-N bonds are abnormally high.
Hydrogen bonds are abnormally strong but still weaker than chemical bonds (5-25 kJ/mol versus kJ/mol)

12. Hydrogen Bonds in Water

13. Hydrogen Bonds H2O 100oC H2S -60oC H2Se -40oC H2Te 0oC
Consider the boiling point of substances that contain hydrogen bonded to group VIA elements:
Substance
Boiling point
H2O
100oC
H2S
-60oC
H2Se
-40oC
H2Te
0oC

14. Hydrogen Bonds
Hydrogen bonding is present in water, but absent in H2S, H2Se, and H2Te.
This is because the electronegativity difference between H and S, Se and Te is not great enough.
Only molecules containing N-H, O-H, and F-H bonds can form hydrogen bonds

15. Methanol vs. Fluoromethane
Consider fluoromethane (CH3F)and methanol (CH3OH)
Both have identical numbers of electrons (16 e- ) and similar dipole moments
However, CH3F boils at -78oC, and CH3OH boils at 65oC
What accounts for this difference?

16. Hydrogen Bonding Hydrogen bonds are responsible for: Protein Structure
Protein folding is a consequence of H-bonding.
DNA Transport of Genetic Information

17. Hexagonal crystal lattice of H2O

18. Properties of Water Water is the solvent of Life!
Solute – substance dissolved in a solvent to form a solution
Solvent – fluid that dissolves solutes
Example: Ice Tea – water is the solvent and tea and sugar the solutes

19. Cohesion, Adhesion and Surface Tension
Properties of Water
Cohesion, Adhesion and Surface Tension
cohesion = water attracted to other water
molecules because of polar properties
adhesion = water attracted to other materials
surface tension = water is pulled together creating
the smallest surface area possible

20. Properties of Water Capillary Action
Because water has both adhesive and cohesive properties, capillary action is present.
Capillary Action = water’s adhesive property is the cause of capillary action. Water is attracted to some other material and then through cohesion, other water molecules move too as a result of the original adhesion.
Ex: Think water in a straw
Ex: Water moves through trees this way

21. Properties of Water High Heat Capacity
In order to raise the temperature of water, the average molecular speed has to increase.
It takes much more energy to raise the temperature of water compared to other solvents because hydrogen bonds hold the water molecules together!
Water has a high heat capacity.
“The specific heat is the amount of heat per unit mass required to raise the temperature by one degree Celsius.”

22. Properties of Water Density
Water is less dense as a solid! This is because the hydrogen bonds are stable in ice – each molecule of water is bound to four of its neighbors.
Solid – water molecules are bonded together – space between fixed
Liquid – water molecules are constantly bonding and rebonding – space is always changing

23. 2. Ion- Dipole Forces
Forces that occur between solutions in which ionic compounds are dissolved in polar solvents.

24. 3. London Dispersion Forces
Weakest of all intermolecular forces.
Only IMF between noble gases and nonpolar molecules.
The nucleus of one molecule (or atom) attracts the electrons of the adjacent molecule (or atom).
For an instant, the electron clouds become distorted.
In that instant a dipole is formed (called an instantaneous dipole).
Relatively weak and short-lived

25. Formation of a dipole moment+ - + -
Neon atom without a dipole moment
Neon atom with a dipole moment

26. Creating a dispersion force

27. London Dispersion Forces
One instantaneous dipole can induce another instantaneous dipole in an adjacent molecule (or atom).

28. London Dispersion
Strength of dispersion forces are directly related to:
Size, larger molecules have greater LDF
Surface area available for contact
spherical molecules have lower than between oblong molecules.

29. Comparing Intermolecular Forces

30. Intermolecular Forces – How to rank IMF
Dispersion forces exist between ALL molecules.
When comparing relative strengths of intermolecular attractions, remember:
When molecules have similar molecular weights and shapes, dispersion forces are approximately equal. The relative strength of IMF will be measured by strengths of dipole-dipole interactions (which one is more polar? ).
When molecules vary drastically in molecular weights, dispersion forces are the decisive factor in determining which substance has stronger IMF (which one is larger?).

31. Example Question
Arrange the following substance in order of increasing magnitude of the London forces: SiCl4, CCl4, GeCl4.

32. Example Question
Arrange the following substance in order of increasing magnitude of the London forces: SiCl4, CCl4, GeCl4.
Answer –
London forces increase with increasing number of electrons:
CCl4 (74e-), SiCl4(82e-), GeCl4(100e-).

33. Example Question
What kinds of intermolecular forces are expected in the following substances?
Methane
Trichloromethane (chloroform)
butanol

34. Answers
Methane is nonpolar. Therefore only London (dispersion) forces are present.
Chloroform is an unsymmetrical molecule with polar bonds. We would expect dipole-dipole forces and London forces
Butanol has a hydrogen atom attached to an oxygen atom. Therefore, you expect hydrogen bonding. You would also expect dipole-dipole and London forces