IM Forces

Forces Between Particles in Solids and Liquids

• Ionic compounds– Attractive forces between oppositely charged

ions hold ionic compounds together.– Ionic bonds are the strongest interparticle

force.– Smaller the ion and the larger the charge on

the ion the stronger the attractive forces among the ions

Ionic Bonding

Forces Between Particles in Solids and Liquids

• Forces between molecular compounds– Intermolecular (IM) forces between molecules

attract molecules to each other in the liquid and solid state.

• IM forces are very weak as compared to ionic or covalent bonds

States of Matter

IM Forces

Three types of IM Forces1. Dipole-dipole force

2. Hydrogen “bonding”

3. London dispersion forces

Interparticle Forces and Physical Properties

• The stronger the attractive forces between particles in a liquid or solid, the– Higher the:

• Melting point• Boiling point• Surface tension• Viscosity

– Lower the:• Vapor pressure

IM Forces

• Dipole-dipole forces– Attractive forces between oppositely charged

dipoles.– Dipole-dipole forces are found between polar

compounds.• The more polar the compound the stronger the

dipole-dipole force.

IM Forces

• Hydrogen “bonds”– Attractive force between a + H bonded to an

O, N, or F and a - O, N, or F generally on another molecule.

• Really a relatively strong dipole-dipole force

– Hydrogen bonding is the strongest of the IM forces.

– H bonding is very important in water and in many biological molecules.

• Hydrogen “bond” is a weak attractive force between a + hydrogen and a -O, N, or F in a second polar bond

London Dispersion Forces

• London Dispersion force– Very weak and short-lasting attractive forces

between temporary dipoles• See figure 10.5

– Weakest of the IM forces

London Dispersion Forces

• London Dispersion forces– Found between all molecules in liquid/solid

state.• Of greatest significance in nonpolar molecules as

it’s the only IM force between nonpolar molecules

– The larger the molecule the stronger the dipersion forces.

Dispersion ForcesOccur between every compound and arise from the net attractive forcesamount molecules which is produced from induced charge imbalances

The magnitude of the Dispersion Forces is dependent upon how easily itis to distort the electron cloud. The larger the molecule the greater it’s Dispersion Forces are.

Dispersion Forces and Molecular Shape

• Elongated ['i:lɔŋɡeitid, i'lɔŋ-] molecules have higher dispersion forces分散力 than compact [kəm'pækt, 'kɔmpækt]简洁molecules

• Ringed structures have higher dispersion forces than straight chain molecules.– Consider:

• Hexane• Cyclohexane• 2,2 – dimethyl butane

Interparticle Forces

• Weakest to Strongest: Intermolecular forces – all relatively weak

London dispersion forces

Dipole-dipole force

Hydrogen Bonding

Ionic bond - BY FAR THE Strongest: - not an IM Force

Properties of Liquids

• Freezing and boiling point

• Surface tension

• Capillary action

• Viscosity [vi'skɔsəti]粘性， [Which are directly related to the strength

of the IM forces present between molecules?

Change of State

• Normal Freezing/Melting point – temperature at which the liquid and solid state

co-exist at 1 atm pressure

• Normal boiling point – temperature at which the liquid and gaseous

state co-exist at 1 atm pressure

• Predict the relative BP of:– Methane, acetone, methanol, ethanol, NaCl

Surface Tension

• Surface tension– Resistance of a liquid to increase its surface

area– Measure of the energy needed to break the

IM forces at the surface

Capillary Action

• Capillary action– Spontaneous rising of a liquid in a narrow

tube

• Related terms:– Cohesive forces – attractive forces among like

molecules– Adhesive forces – attractive forces among

dislike molecules

See Figure 10.7, page 444

Concave meniscus Convex meniscusAdhesion > Cohesion Cohesion > adhesion

Viscosity

• Viscosity – resistance of a liquid to flow– Highly viscous liquids are thick (syrupy)– Consider relative viscosity of:

• Propanol

• Glycerol

Graphite

• Layers of ringed carbon structures– Each C is bonded to 3 other C– Each C is sp2 hybridized

Diamond

• A diamond is a gigantic molecule, each C atom is bonded to 4 other C atoms

• Each C is sp3 hybridized

A phase diagram summarizes the conditions at which a substance exists as a solid, liquid, or gas.

Phase Diagram of Water

11.9

11.9

CH 11: Properties of Solutions

1. Describing Solutions – concentration units

2. Energetics of solution formation

3. Colligative Properties of solutions• BP elevation• FP depression• Osmotic pressure• Vapor Pressure

Terms

• Solution – homogeneous mixture

• Solvent – generally the larger component of the solution– Determines the physical state of the solution

• Solute – generally the smaller component of the solution– Solute is dispersed in the solvent

Solution Composition

• Concentrated solution – relatively large amount of solute

• Dilute solution – relatively small amount of solute

Solution Composition

• Unsaturated solution –solution with less than the maximum amount of solute that will normally dissolve at a given temperature

• Saturated solution - solution with maximum amount of solute that will normally dissolve at a given temperature

Solution Composition

• Super-saturated solution - solution with more than the maximum amount of solute that will normally dissolve at a given temperature

Concentration Units

Molarity (M) = moles solute/Liters solution

Molality (m) = moles solute/kg solvent

Mass % = Mass solute/mass solution x100%

Mole fraction () = moles A/total moles

Practice!

• Start by writing definitions for the concentration unitsM =

m =

Mass % =

Mole fraction =

Starting with Molarity

Solution: – 3.75 M H2SO4 solution with a density of 1.23

g/mL

Calculate: – Mass % – Molality

– mole fraction of H2SO4

Starting with Masses

Solution: – A solution is made by combining 66.0 grams of

acetone (C3H6 O) with 146.0 grams of water.

– Solution has a density of 0.926 g/mL

Calculate: – Molarity – need volume of solution

– Mass %

– Molality

– Mole fraction of acetone

Starting with Mass %

Solution: – 35.4 % H3PO4

– Density of 1.20 g/mL

Calculate: – Molarity – Molality

– Mole fraction of H3PO4

Starting with Molality

Solution: – 2.50 m HCl solution– Density of 1.15 g/mL

Calculate: – Molarity – need _______ – Mass %– Mole fraction of HCl

Solution Formation

Formation of a solution involves 3 steps1. Separate the solute particles

• expand the solute

2. Separate the solvent particles• Expand the solvent

3. Form the solution– Solute and solvent interact

Solution Formation

• Each step of solution formation involves energy and has a H.

H1 = energy needed to separate the solute

H2 = energy needed to separate the solvent

H3 = energy released when solution forms

Solution Formation

HsolutionH1 + H2 + H3

Solutions form when the Hsolution is a small value – see page 492

Factors Impacting Solubility

• Structure – like dissolves like– #38 on page 520

Factors Impacting Solubility

• Pressure– Pressure has little impact on the solubility of

liquids and solids– Pressure has a significant impact on the

solubility of gases in a liquid• The higher the pressure of gaseous solute above a

liquid the higher the concentration of the gas in the solution

Henry’s Law

• Henry’s Law: C = kP

C = Concentration of dissolved gas

k = solution specific constant

P = partial P of the solute gas above

the solution• No calculations required.

Page 494

Temperature and Solubility

• Temperature has variable effects on the amount of solid that will dissolve in an aqueous solution!– See figure 11.6 page 496

• Solutes do dissolve more rapidly at higher temperatures

Temperature and Solubility

• The solubility of a gas in water decreases as temperature increases.– See figure 11.7 on page 496

– Thermal pollution – read the story on page 497 when you get a chance

Vapor Pressure of Solutions

• See Raoult’s Law on page 498

• Psolution = solventsolvent P0 solventsolvent

Colligative Properties

• Colligative properties– properties of a solution that depend upon the

amount of dissolved solute, not the identity of the solute.

• Freezing point depression• Boiling point elevation• Osmotic Pressure

• Note: I will be weaving section 11.7 and the van’t Hoff factor (i) into my consideration of these properties and not consider it separately.

Colligative Properties

FP = Kf m i

BP = Kb m i

See page 505 for needed constants

1. Calculating the bp or fp of a solution

2. Calculating the molar mass of a solute from fp or bp data

Osmotic Pressure

• Osmotic Pressure () is often used to determine the molar mass of large biological molecules– See figure 11.17 on page 508

= MRTi