Ch 10Particle Forces

States of Matter

Solid- Particles moving about a fixed point

Liquid-Particles moving about a moving point

Gas-Particles filling the volume of the container with complete random motions.

Particle Forces Affect• Solubility

• Vapor Pressures

• Freezing Points

• Boiling Points

Particle Forces• Intramolecular forces (Relative strength = 100)

Ionic bonding Covalent bonding

• Interparticle forces Ion-dipole forces Dipole-dipole (Polar molecules)

(relative Strength = 1) London Forces (Dispersion forces)( Nonpolar molecules)

(relative strength = 1) Hydrogen Bonding (Relative strength = 10)

Ion-Ion Interactions• Coulomb’s law states that the energy (E) of

the interaction between two ions is directly proportional to the product of the charges of the two ions (Q1 and Q2) and inversely proportional to the distance (d) between them.

E (Q1Q2)

d

Predicting Forces of Attraction• Coulombs Law indicates the increases in the

charges of ions will cause an increase in the force of attraction between a cation and an anion.

• Increases in the distance between ions will decrease the force of attraction between them.

Size of Ions

Interactions Involving Polar Molecules

• An ion-dipole interaction occurs between an ion and the partial charge of a molecule with a permanent dipole.

• The cluster of water molecules that surround an ion in aqueous medium is a sphere of hydration.

Illustrates of Ion-Dipole Interaction

The Solution Process

Bond Breaking Processes• Break solute particle forces (expanding

the solute), endothermic• Break solvent particle forces (expanding

the solvent), endothermic

The Solution ProcessAttractive Forces• Energy released when solute solvent are

attracted, exothermic• Energy is released due to new attractions

Ion dipole if the solute is ionic and the solvent polar.

London-Dipole for nonpolar solute and polar solvent

Dipole-dipole for polar solute and polar solvent

The Solution ProcessTheromodynamics

• Enthalpy • Entropy ΔS (Perfect crystal, assumed to be

zero)• Gibbs free energy ΔG

• ΔG = ΔH - T ΔS• ΔG < 0, spontaneous change• ΔG = 0, equilibrium• ΔG > 0, nonspontaneous

The Solution ProcessOil dissolving in water• London forces holding the oil molecules

together are large do to the large surface area of the oil

• The hydrogen bonds holding water molecules together are large

• The forces of attraction of between nonpolar oil and polar water are weak at best

• Thus the overall process is highly endothermic and not allowed thermo chemically

The Solution ProcessOil dissolving in water• Entropy should be greater than zero

• Free energy should be greater than zero, since the process is highly endothermic

• Thus the overall process is nonspontaneous

The Solution ProcessSodium chloride dissolving in water

• Large amount of energy is required to break the ionic lattice of the sodium chloride (expand solute)

• Large amount of energy is required to separate the water molecules to expand the solvent breaking hydrogen bonds

• Formation of the ion dipole forces releases a large amount of energy, strong forces (why?)

• The sum of the enthalpies is about +6 kJ (slightly endothermic), which is easily overcome by the entropy of the solution formation.

Water as a Solvent• Water most important solvent, important

to understand its solvent properties

• Most of the unusual solvent properties of water stem from it hydrogen bonding nature

• Consider the following ∆S of solution

KCl →75j/K-mole

LiF→-36j/K-mole

CaS→-138 j/K-mole

Water as a Solvent• We would expect ∆S>0 for all solutions,

right?

• But two are negative, why?

• Obviously, something must be happening for the increased order.

• Ion-dipole forces are ordering the water molecules around the ions, thus causing more order in water i.e. less positions for water than in the pure liquid state

Water as a Solvent• Smaller ions, have stronger ion dipole forces,

thus pulling water closer, therefore less positions

• Also, ions with a charge greater than one will attract to water stronger than a one plus charge, thus more order due to less space between particles

Dipole-Dipole Interactions• Dipole-dipole interactions are

attractive forces between polar molecules.

• An example is the interaction between water molecules.

• The hydrogen bond is a special class of dipole-dipole interactions due to its strength.

Dipole-Dipole ForcesDipole-dipole (Polar molecules)

Alignment of polar molecules to two electrodes

charged + and δ–Forces compared to ionic/covalent are about 1 in strength

compared to a scale of 100, thus 1%

H Cl H Cl H Clδ–δ–δ– δ+δ+ δ+

Slide 21 of 35

Dipole Dipole Interactions

Hydrogen Bonding• Hydrogen bonding a stronger intermolecular

force involving hydrogen and usually N, O, F, and sometimes Cl

–Stronger that dipole-dipole, about 10 out of 100, or 10

–Hydrogen needs to be directly bonded to the heteroatom

–Since hydrogen is small it can get close to the heteroatom

–Also, the second factor is the great polarity of the bond.

Slide 23

Hydrogen Bonding in HF(g)

Slide 24

Hydrogen Bonding in Water

around a molecule in the solid in the liquid

Boiling Points of Binary Hydrides

Interacting Nonpolar Molecules• Dispersion forces (London forces) are

intermolecular forces caused by the presence of temporary dipoles in molecules.

• A instantaneous dipole (or induced dipole) is a separation of charge produced in an atom or molecule by a momentary uneven distribution of electrons.

Illustrations

Strength of Dispersion Forces• The strength of dispersion forces depends

on the polarizability of the atoms or molecules involved.

• Poarizability is a term that describes the relative ease with which an electron cloud is distorted by an external charge.

• Larger atoms or molecules are generally more polarizable than small atoms or molecules.

London Forces (Dispersion)

• Induced dipoles (Instantaneous )• Strength is surface area dependent• More significant in larger molecules• All molecules show dispersion forces• Larger molecules are more polarizable

Slide 30

Instantaneous and Induced Dipoles

The Effect of Shape on Forces

Practice Rank the following compound in order of increasing

boiling point. CH3OH, CH3CH2CH2CH3, and CH3CH2OCH3

Practice

Rank the following compound in order of increasing boiling point. CH3OH, CH3CH2CH2CH3, and CH3CH2OCH3

CH3OH

CH3CH2CH2CH3

CH3CH2OCH3

MM32.0

58.0

60.0

IM ForcesLondon and H-bonding

London, only

London and Dipole-dipole

Practice Rank the following compound in order of increasing

boiling point. CH3OH, CH3CH2CH2CH3, and CH3CH2OCH3

CH3OH

CH3CH2CH2CH3

CH3CH2OCH3

MM32.0

58.0

58.0

IM ForcesLondon and H-bonding

London, only

London and Dipole-dipole

The order is:

CH3CH2CH2CH3 < CH3CH2OCH3< CH3OH

Polarity and Solubility• If two or more liquids are miscible, they form

a homogeneous solution when mixed in any proportion.

• Ionic materials are more soluble in polar solvents then in nonpolar solvents.

• Nonpolar materials are soluble in nonpolar solvents.

• Like dissolves like

Polarity and Solubility

• If two or more liquids are miscible, they form a homogeneous solution when mixed in any proportion.

• Ionic materials are more soluble in polar solvents then in nonpolar solvents.

• Nonpolar materials are soluble in nonpolar solvents.

Polarity and Solubility

How does polarity effect solubility?

The thermodynamic argument, is that the lower the potential energy, the more stable the system. If subtracting the potential energy of the solute from the potential energy of the original solute and solvent is negative (exothermic) then solution is thermodynamically favored.

Polarity and Solubility

How does polarity effect solubility?

Non polar solute and solvent: The forces holding these particles together are London Dispersion forces, the weakest of all of the inter-particle forces. The strength of these forces are relative to the surface area if solute and solvent are of similar size, then about the same amount of energy is required to separate solute and solvent particles from each other. And about the same amount of energy is released when solute and solvent are attracted to each other forming a solution. Thus we predict non polar solutes and solvents should dissolve

Polarity and SolubilityHow does polarity effect solubility?

Non polar solute and polar solvent: Considering solutes and solvents of similar surface area it should be noted that more energy is required to separate the polar solvent molecules from each other, since dipole-dipole interactions are stronger. The only interaction between a nonpolar solute and polar solvent would be London Dispersion forces, so the energy released is much less than required for separating the solvent and solute. Subtracting the potential energy of the products from reactants would give a positive (endothermic) result and the solution would be less stable than the dissolution.

Practice Rank the following compound in order

of increasing boiling point. CH3OH, CH3CH2CH2CH3, and CH3CH2OCH3

Solubility of Gases in Water• Henry’s Law states that the solubility of a

sparingly soluble chemically unreactive gas in a liquid is proportional to the partial pressure of the gas.

• Cgas = kHPgas where C is the concentration of the gas, kH is Henry’s Law constant for the gas.

Henry’s Law Constants

Henry’s Law Constants

Gas kH[mol/(L•atm)] kH[mol/(kg•mmHg)]

He 3.5 x 10-4 5.1 x 10-7

O2 1.3 x 10-3 1.9 x 10-6

N2 6.7 x 10-4 9.7 x 10-7

CO2 3.5 x 10-2 5.1 x 10-5

Terms• A hydrophobic (“water-fearing) interaction

repels water and diminishes water solubility.

• A hydrophilic (“water-loving”) interaction attracts water and promotes water solubility.

Phase Diagram of WaterNote: Negative

slope

Phase Diagram of Water

Phase Diagram CO2Note:

Posititve Slope

Phase Diagram CO2

Special Properties of WaterHydrogen bonding found in water gives it special properties listed below:

• Surface tension

• Capillary Rise

• Viscosity

Surface Tension

Surface Tension

Surface Tension units J/m2

Surface TensionAdhesive forces stronger in red aqueous solution

Cohesive forces stronger in mercury liquid

Capillary Rise

Viscosity

Cohesive and Adhesive Forces Produce a Meniscus

Terms• Capillary action is the rise of a liquid up

a narrow tube as a result of adhesive forces between the liquid and the tube and cohesive forces within the liquid.

• Viscosity is a measure of the resistance to flow of a fluid.

ChemTour: Lattice Energy

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Students learn to apply Coulomb’s law to calculate the exact lattice energies of ionic solids. Includes Practice Exercises.

ChemTour: Intermolecular Forces

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This ChemTour explores the different types of intermolecular forces and explains how these affect the boiling point, melting point, solubility, and miscibility of a substance. Includes Practice Exercises.

ChemTour: Henry’s Law

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Students learn to apply Henry’s law and calculate the concentration of a gas in solution under varying conditions of temperature and pressure. Includes interactive practice exercises.

ChemTour: Molecular Motion

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Students use an interactive graph to explore the relationship between kinetic energy and temperature. Includes Practice Exercises.

ChemTour: Raoult’s Law

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Students explore the connection between the vapor pressure of a solution and its concentration as a gas above the solution. Includes Practice Exercises.

ChemTour: Phase Diagrams

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Students use an interactive phase diagram and animated heating curve to explore how changes in temperature and pressure affect the physical state of a substance.

ChemTour: Capillary Action

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In this ChemTour, students learn that certain liquids will be drawn up a surface if the adhesive forces between the liquid on the surface of the tube exceed the cohesive forces between the liquid molecules.

ChemTour: Boiling and Freezing Points

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Students learn about colligative properties by exploring the relationship between solute concentration and the temperature at which a solution will undergo phase changes. Interactive exercises invite students to practice calculating the boiling and freezing points of different solutions.

ChemTour: Osmotic Pressure

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Students discover how a solute can build up pressure behind a semipermeable membrane. This tutorial also discusses the osmotic pressure equation and the van’t Hoff factor.

 

Solubility of CH4, CH2Cl2, and CCl4

Which of the following three compounds is most soluble in water?

A) CH4(g) B) CH2Cl2(λ) C) CCl4(λ)

Solubility of CH4, CH2Cl2, and CCl4

Consider the following arguments for each answer and vote again:

A. A gas is inherently easier to dissolve in a liquid than is another liquid, since its density is much lower.

B. The polar molecule CH2Cl2 can form stabilizing dipole-dipole interactions with the water molecules, corresponding to a decrease in ΔH°soln.

C. The nonpolar molecule CCl4 has the largest molecular mass, and so is most likely to partially disperse into the water, corresponding to an increase in ΔS°soln.

The End