Introduction to

Chemical Principles

Chapter 6:States of Matter

Property ↓ State

Volume Shape Compress-ibility Thermal Expansion

Solid Definite Definite Very small Very small(.01%/ °C)

Liquid Definite Indefinite Small (but > solid) Small(0.10%/ °C)

Gas Indefinite Indefinite Large Moderate(about .30%/ °C)

First lets compare the three states of matter

The three states can be changed.

Change of State – is a process in which a substance is transformed from one physical state to another physical state. This is a physical change.

Now look at the terms involved in Changes of State. You may have known many of the terms.

These three processes are Endothermic: heat/energy is absorbed from the surrounding environment into the system.

SolidLiquid

Gas

Melting

EvaporationSu

blimati

on

Changes of Statethe following processes are Exothermic: heat/energy is released from the system into the surrounding environment.

SolidLiquid

Gas

Freezing

CondensationDeposition

Boiling and Boiling Points

Boiling occurs when a liquid is turning to a gas rapidly

The normal boiling point of a substance is determined by its molecular mass and its intermolecular forces.

Intermolecular Forces in Liquids

Intermolecular forces are attractions between molecules.

Intermolecular forces play a role in determining the physical properties of a substance – melting point, boiling point, shape.

5 Types of Intermolecular Forces

1. Dipole-Dipole Interactions: attractions between polar molecules.

Polar molecules are electrically uneven. This creates a dipole: a positive end and a negative end within a molecule.

The molecules tend to orient themselves such that the opposite poles are next to one another.

2. London forces: weak temporary dipole-dipole interactions that occur because of momentary uneven electron distributions .

These are the most common intermolecular forces.

These are the only attractive forces in nonpolar molecules

London Forces / Induced Dipole

Electrons move quickly around nuclei.

At some point, most of the electrons will be on one side of the molecule, resulting in temporary polarity.

The temporary polarity will induce a similar temporary polarity in a nearby molecule.

Although London forces are fleeting interactions, they do explain the extreme increase in elemental boiling points as you move down the periodic table.

3. Ion-Dipole InteractionsAn ion-dipole is between an ion (formula unit) and a polar molecule.

Think about salt (NaCl) in water -

Na+ ion

Cl- ion

H2O molecules

Ion-dipole interactions: an attraction based on a charges/partial charges.

Such attractions facilitate ionic compounds dissolving in water.

4. Ion-Ion Interactions

• Ion-Ion interactions are attractions between oppositely charged ions in liquid state (high temperature) ionic compounds.

5. Hydrogen bonding: an especially strong form of dipole-dipole interaction.For a hydrogen bond to occur, you must have hydrogen covalently bonded to F, O, or N. The H in this covalent bond can then bond with another F, O or N to form a hydrogen bond.

Water is the most common compound that has hydrogen bonds.

Why H won’t form H bond with Cl, which is electronegatively equivalent to N?

Would Hydrogen Bonding exist for these molecules?

Boiling point comparison:

1. Ionic compounds have higher boiling point than molecular (covalent) compound

2. Next, among molecular compounds, if H-bond exists in one compound, that one has higher boiling point

3. Thirdly, if two molecular compounds have similar molecular weight but no H-bonds, then the polar (bent, trigonal pyramidal rather than linear, trigonal planar, or tetrahedron if peripheral atoms are symmetrically identical; any shape if peripheral atoms are not symmetrically identical) one has higher boiling point.

4. For two molecular compounds that have no H-bonds, the one that has significant higher molecular weight would have higher boiling point.

Boiling Points of Nonpolar vs. Polar Substances

Nonpolar Substance Polar Substance

FormulaMolecular

WeightBoiling Point

°C K FormulaMolecular

WeightBoiling Point

°C K

N2 28 -196 77 CO 28 -192 81

SiH4 32 -112 151 PH3 34 -85 188

GeH4 77 -90 183 AsH3 78 -55 218

Br2 160 59 332 ICl 162 97 370

1. Hope that you still remember how to determine nonpolar or polar substances. Please indicate which group is nonpolar?

2. Between these two groups, when molecular weight is close, which one has higher boiling point?

3. What trend do you observe within each group? (hint: consider molecular weight)

Problems:How would you expect a boiling point of NO, to compare to the boiling point N2 and O2?

If molecular weight were the only factor the Bp for NO should be halfway between N2 (-196°C)and O2 (-183°C). However NO is a polar molecule and its Bp is higher than predicted (-153°C)

Definitions of evaporation, melting, freezing, boiling point, and sublimation

The normal boiling point of a substance is determined by its _____________ and its_________. Considering these two factors, predict the order of increasing boiling (or melting) points for the following substances: CH4, CO2, KCl, NH3. [Hint: 1) ionic compounds have higher boiling points than molecular compounds; 2) H-bond molecular compounds have second highest boiling points; 3) polar molecular compounds next. If both are polar, then higher molecular weight leads to higher boiling point.

Definition of intermolecular force, London forces, dipole-dipole interactions, hydrogen bond

Boiling points depend on what factors? (two)List the type of molecules (compound) in which each intermolecular force exists?In which of the following liquids would dipole-dipole interactions be the predominant intermolecular force?A) O2 B) HCl

The predominant intermolecular force in CH3COOH (or CH3CH2OH, etc) is ________ (how to draw the structures of these molecules)

Which of the following can form H bond?A) HFB) CH3NH2

Which of the substances shown in the pure liquid state, would hydrogen bonding occur?

Predict which member of the pair would be expected to have higher boiling point, and indicate why: HF and HBr (both are polar), CO and N2 (CO is polar and N2 is nonpolar)