Welcome to Chem 2010 General chemistry (I) For third Level Chemistry, Biology and Physics Departments Chapter 8 1.

Welcome to Chem 2010

General chemistry (I)

For third Level

Chemistry, Biology and Physics Departments

Chapter 8 1

Chapter 8: STATES OF MATTER AND ENTERMOLECULARE FORCES

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Matter is anything that has mass and occupies space.Mass is a measure of the quantity of matter in a sample of any

material.

Matter

STATES OF MATTER

solid liquid gas

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In the solid state, substances are

1- rigid and have definite shapes.

2- Volumes of solids do not vary much with changes in temperature (Expansion on Heating slight)

3- Volumes of solids do not vary much with changes in pressure (slight compressability).

4- Intermolecular spaces are smoll

5- Attractive force between atoms is very big.

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6- Attractive force between atoms is middle.

5- Intermolecular spaces are middle.

4- Liquids are very hard to compress. (slight compressability)

3- Expansion on Heating slight

2- A liquid flows and assumes the shape of its container up to the volume of the liquid.

1- the individual particles are confined to a given volume.

In the liquid state,

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In the gas state,

1- Gases are much less dense than liquids and solids.

2- They occupy all parts of any vessel in which they are confined.

3- Gases are capable of infinite expansion

4- Gases are compressed easily.

5- Intermolecular spaces are big

6- Attractive force between atoms is very small

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LIQUIDSOLID GAS

Tightly packed, in a regular

patternVibrate, but do not move from place to place

Close together with no regular arrangement.Vibrate, move

about, and slide past each other

Well separated with no regular arrangement.Vibrate and

move freely at high speeds

States of Matter

7Lecture7: state of matter &

intermolecular forces

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PHASE CHANGES

Sublimation is the conversion of a solid directly to a gas without passing through the liquid state;

the reverse of that process is called deposition. The changes shown in blue are endothermic

(absorb heat); those shown in red are exothermic (release heat).

relatively fixed in position, but those in the liquid and gas can

flow around each other.

Physical changes that occur among the three states of matter.

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INTERMOLECULAR FORCES

• Why is water usually aliquid and not a gas?• Why does liquid water boilat such a high temperature for such a small molecule?• Why does ice float on water?• Why do snowflakes have 6sides?• Why is I2 a solid whereasCl2 is a gas?• Why are NaCl crystals little?

WHY?

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The answers have to do with…

INTERMOLECULAR FORCES

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Intermolecular ForcesIntermolecular Forces

INTRAmolecularforces —the forces holding atoms together

to form molecules.

INTERmolecular forcesForces between molecules,between ions, or betweenmolecules and ions.

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Intermolecular ForcesIntramolecular forces determine such molecular properties as molecular geometries and dipole moments

Intermolecular

forces determine the macroscopic physical properties of liquids and solids

Intermolecular ForcesIntermolecular Forces

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1- Ion–Ion Interactions

INTERMOLECULAR ATTRACTIONS AND PHASE CHANGESINTERMOLECULAR ATTRACTIONS AND PHASE CHANGES

2- Dipole– Dipole Interactions

3- Hydrogen Bonding

4- Dispersion Forces

Intermolecular ForcesIntermolecular Forces

Types of intermolecular forces( physical bond)

(between 2 non-polar molecules)

(between 2 “special” polar molecules)

(between 2 polar molecules)

(between ions in an ionic compound)

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Intermolecular ForcesIntermolecular Forces1- Ion-Ion interaction1- Ion-Ion interaction

The attractive force between ions in an ionic compound.

Ion – Ion interaction exist in ionic compounds such as NaCl, CaBr2, and K2SO4

Ion-Ion Forces for comparison of magnitude

Na+—Cl- in salt These are the strongest forces. Lead to solids with high meltingtemperatures. NaCl, mp = 800 C0 MgO, mp = 2800 Co

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2-Dipole-Dipole Forces

Such forces bind molecules having permanent dipoles to one another.

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2- Dipole-Dipole Forces2- Dipole-Dipole Forces

Attractive forces between polar covalent molecules

Intermolecular ForcesIntermolecular Forces

The positive end of the polar compound is attracted to the negative end of a nearby molecule.

dipole–dipole interactions become less important as temperature increases. 19

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Intermolecular ForcesIntermolecular Forces

The hydrogen bond is a special case of dipole-dipole interaction. occurs between hydrogen atom in a polar N-H, O-H, or F-H bond and an electronegative O, N, or F atom.

3- Hydrogen bonding3- Hydrogen bonding

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Intermolecular ForcesIntermolecular Forces

4- Dispersion (London) Forces4- Dispersion (London) Forces

Non-polar molecules ( as hydrocarbons) do not have dipoles like polar molecules. How, then, can non-polar compounds form solids or liquids?

1- Because electrons are moving around the nucleus , sometimes, the charge around the nucleus is not symmetrical

and a temporary dipole occur.

2- The temporary dipole forms in one atom or molecule, induce a dipole in the neighboring atoms.

Intermolecular ForcesIntermolecular Forces

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Intermolecular ForcesIntermolecular Forces

The larger the molecule, the greater it’s Dispersion Forces are.

Dispersion (London) forces are present between all types of molecules in condensed phases

Because the boiling point of alkanes increase with the length of the carbon chain. i.e. Long-chain alkanes have larger dispersion forces than short-chains.

Decane C10H22 has greater boiling point than C5H12 Why?

Intermolecular ForcesIntermolecular Forces

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Intermolecular ForcesIntermolecular Forces

Dispersion forces usually increase with molar mass.

To detect the type of intermolecular forces in a compound , It is important to be able to tell whether a substance is ionic, nonpolar covalent, or polar covalent.

Compound in Question

Ionic or Molecular?

ionic

• has Ion –ion interactions

Molecular

Polar or NonPolar?

Polar Non-Polar

•Has Dipole-dipole force •Has dispersion forces

Can it hydrogen bond ?

hydrogen bonding will be formed if compound has H and O, F, or N26

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Intermolecular ForcesIntermolecular Forces

920 kJ is required to break the covalent bonds between H and O in one mole of water.

Intermolecular forces ( physical bonds) are weaker than

intramolecular forces( chemical bonds), Example

Whiles only 40.7 kJ is required to convert one mole of liquid water into steam at 100°C.

SO

O

What type(s) of intermolecular forces exist between each of the following molecules?

HBr1- HBr is a polar molecule: dipole-dipole forces. 2- There are also dispersion forces between HBr molecules.

CH4

CH4 is nonpolar: only dispersion forces.

SO21- SO2 is a polar molecule: dipole-dipole forces. 2- There are also dispersion forces between SO2 molecules.

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Properties of the liquid state

We will briefly discuss the evaporation process and vapor pressure

Capillary actionSurface tensionviscosity

Evaporation Vapor pressure

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VaporizationVaporizationVaporization ( evaporation), is the process by which molecules on the surface of a liquid break away and go into the gas phase.

To break away, the molecules must have enough kinetic energy to overcome the intermolecular forces.

The rate of vaporization increases as temperature increases.

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“Amount of heat needed to change 1 gram of liquid to gas at its boiling point.”

Boiling (Condensing) Point of Water = 100°CHeat of Vaporization (water) = 2260 J/g

Heat of vaporizationHeat of vaporization

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•Dispersion forces generally increase with increasing molecular size, so substances composed of larger molecules have lower vapor pressures.

The relation between the intermolecular forces

and vapor pressure of a liquid

The relation between the intermolecular forces

and vapor pressure of a liquid

•The very strong hydrogen bonding in water is the reason for its unusually low vapor pressure .

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1- Detect the intermolecular forces that are present in each of the following compounds.

(a) CH3OH (methyl alcohol)

(b) NH3

( c ) HCl

(d) C6H14 (hexane)

(e) H2O

ExerciseExercise

(g ) NaCl

(h) BrF

(f ) SO2

A, b, e Hydrogen bonding + dispersion force

d Dispersion force

g Ion –ion interaction

c, f, h Dipole- dipole interaction + dispersion force

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Explain why?

(a) CH3OH boils at 65°C while CH4 boils at -161°C .

(b) Boiling point of C5H12 is greater than that of C2H6

( c ) at definite temperature, vapor pressure of C5H12 is lower that vapor pressure C2H6

ExerciseExercise