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

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?

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

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.

States of Matter (Review)

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

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

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

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).

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

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 200-1100 kJ/mol)

Hydrogen Bonds in Water

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

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

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?

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

Hexagonal crystal lattice of H2O

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

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

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

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.”

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

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

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

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

Creating a dispersion force

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

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.

Comparing Intermolecular Forces

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?).

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

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-).

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

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