LỰC LIÊN kết PHÂN tử, TRẠNG THÁI NGƯNG kết của vật CHẤT (hóa đại CƯƠNG)

Intermolecular Forces, Liquids, and Solids Lực liên kết liên phân tử Trạng thái ngưng kết của vật chất... Liquids and Solids: A Molecular ComparisonTính chất vật lý của các chất được hiể

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Intermolecular Forces,

Liquids, and Solids

Lực liên kết liên phân tử Trạng thái ngưng kết của vật chất

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Liquids and Solids: A Molecular Comparison

Tính chất vật lý của các chất được hiểu theo khái niệm của thuyết động học phân tử ( kinetic molecular theory ) như sau:

• Chất khí dễ bị nén, có hình dạng và thể tích của vật chứa

:

• Các phân tử chất khí cách xa nhau, không tương tác với nhau

• Chất lỏng hầu như không bị nén, có hình dạng (nhưng

không có thể tích) của vật chứa :

• Các phân tử chất lỏng được giữ gần nhau hơn các phân tử chất khí,

nhưng không chắc đến mức chúng không thể trượt qua nhau (slide past each other)

• Chất rắn không bị nén, có hình dạng và thể tích xác định

• Các phân tử chất rắn được giữ gần nhau Các phân tử chất rắn được

chắc đến mức chúng không thể trượt qua nhau được

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A Molecular Comparison of Liquids and Solids

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A Molecular Comparison of Liquids and Solids

• Converting a gas into a liquid or solid requires the molecules to get closer to each other/ Để biến đổi một chất khí thành lỏng hay rắn:

– cool or compress/ làm lạnh hay nén

• Converting a solid into a liquid or gas requires the molecules to move further apart:/Để biến đổi một chất rắn thành lỏng hay

khí:

– heat or reduce pressure/ gia nhiệt hay giảm áp suất

• The forces holding solids and liquids together are called

intermolecular forces ( lực giữ các chất lỏng và rắn lai với nhau

được gọi là lực liên phân tử).

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Intermolecular Forces

• The covalent bond holding a molecule together is an

intramolecular forces (lục nội phân tử)

• The attraction between molecules is an intermolecular force (lực liên phân tử)

• Intermolecular forces are much weaker than

intramolecular forces (e.g 16 kJ/mol vs 431 kJ/mol for HCl).

• When a substance melts (nóng chảy) or boils (sôi) the

intermolecular forces are broken (not the covalent

bonds).

• When a substance condenses (ngưng tụ)

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Ion-Dipole Forces ( l ực ion-lưỡng cực)

• Interaction between an ion (e.g Na+) and a dipole (e.g water).

• Strongest of all intermolecular forces:

– Since Q1 is a full charge and Q2 is a partial charge, F is

comparatively large.

• F increases as Q increases and as d decreases:

– the larger the charge and smaller the ion, the larger the

F =

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Dipole-Dipole Forces (l ực lưỡng cực-lưỡng cực)

• Interaction between a dipole (e.g water) and a

dipole (e.g water)

• Dipole-dipole forces exist between neutral polar molecules (giữa các phân tử phân cực trunghoà).

• Polar molecules need to be close together.

• Weaker than ion-dipole forces :

2 2

1 Q

Q k

F =

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• If two molecules have about the

same mass and size, then dipole-dipole forces increase with increasing polarity.

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London Dispersion Forces

• Weakest of all intermolecular forces.

• It is possible for two adjacent (kề nhau) neutral

molecules to affect each other.

• 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-lưỡng cực tạm thời) .

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• One instantaneous dipole can induce (tác động)

another instantaneous dipole in an adjacent

molecule (or atom).

• The forces between instantaneous dipoles are

called London dispersion forces.

• Polarizability is the ease with which an electron

cloud can be deformed.

• The larger the molecule (the greater the number

of electrons) the more polarizable

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• London dispersion forces increase as molecular weight increases.

• London dispersion forces exist between all

molecules.

• London dispersion forces depend on the shape

of the molecule.

• The greater the surface area available for

contact, the greater the dispersion forces.

• London dispersion forces between spherical

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• Special case of dipole-dipole forces

• By experiments: boiling points of compounds

with H-F, H-O, and H-N bonds are abnormally high.

• Intermolecular forces are abnormally strong.

• H-bonding requires H bonded to an

electronegative element (most important for

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Hydrogen Bonding

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Hydrogen bonds are responsible for:

– Ice Floating

• Solids are usually more closely packed than liquids;

• therefore, solids are more dense than liquids.

• Ice is ordered with an open structure to optimize H-bonding.

• Therefore, ice is less dense than water.

• In water the H-O bond length is 1.0 Å.

• The O…H hydrogen bond length is 1.8 Å.

• Ice has waters arranged in an open, regular hexagon.

• Each δ+ H points towards a lone pair on O.

• Ice floats, so it forms an insulating layer on top of lakes, rivers,

etc Therefore, aquatic life can survive in winter.

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• Hydrogen bonds are responsible for:

– Protein Structure

• Protein folding is a consequence of H-bonding.

• DNA Transport of Genetic Information

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Comparing Intermolecular Forces

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Some Properties of Liquids

Viscosity (độ nhớt)

• Viscosity is the resistance of a liquid to flow.

• A liquid flows by sliding molecules over each

other.

• The stronger the intermolecular forces, the

higher the viscosity.

Surface Tension (sức căng bề mặt)

• Bulk molecules (those in the liquid) are equally

attracted to their neighbors.

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Surface Tension

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Surface Tension

• Surface molecules are only attracted inwards towards the bulk molecules.

– Therefore, surface molecules are packed more

closely than bulk molecules.

• Surface tension is the amount of energy

required to increase the surface area of a liquid.

• Cohesive forces (lực cố kết) bind molecules to each other.

• Adhesive forces (lực kết dính ) bind molecules to

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Surface Tension

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Surface Tension

• Meniscus is the shape of the liquid surface

– If adhesive forces are greater than cohesive forces, the liquid surface is attracted to its container more than the bulk molecules Therefore, the meniscus is U-shaped (e.g water in glass).

– If cohesive forces are greater than adhesive forces, the

meniscus is curved downwards

• Capillary Action: When a narrow glass tube is

placed in water, the meniscus pulls the water up the tube.

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Phase Changes

• Surface molecules are only attracted inwards

towards the bulk molecules.

• Sublimation: solid → gas.

• Vaporization: liquid → gas.

• Melting or fusion: solid → liquid.

• Deposition: gas → solid.

• Condensation: gas → liquid.

• Freezing: liquid → solid.

Energy Changes Accompanying Phase Changes

• Energy change of the system for the above

processes are:

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Phase Changes

Energy Changes Accompanying Phase

Changes

– Sublimation: ∆Hsub > 0 (endothermic)

– Vaporization: ∆Hvap > 0 (endothermic).

– Melting or Fusion: ∆Hfus > 0 (endothermic).

– Deposition: ∆Hdep < 0 (exothermic)

– Condensation: ∆Hcon < 0 (exothermic).

– Freezing: ∆Hfre < 0 (exothermic).

• Generally heat of fusion (enthalpy of fusion) is

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Phase Changes

• All phase changes are possible under the right

conditions (e.g water sublimes when snow

disappears without forming puddles).

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Phase Changes

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Phase Changes

Heating Curves / Cooling curves

• Plot of temperature change versus heat added is

a heating curve.

• During a phase change, adding heat causes no

temperature change.

– These points are used to calculate ∆Hfus and ∆Hvap

• Supercooling : When a liquid is cooled below its melting point and it still remains a liquid.

• Achieved by keeping the temperature low and

increasing kinetic energy to break

intermolecular forces.

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Phase Changes

Heating Curves

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Phase Changes

Critical Temperature and Pressure

• Gases liquefied by increasing pressure at some

temperature.

temperature for liquefaction of a gas using pressure.

for liquefaction.

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Vapor Pressure

Explaining Vapor Pressure on the Molecular

Level

• Some of the molecules on the surface of a liquid have

enough energy to escape the attraction of the bulk

liquid.

• These molecules move into the gas phase.

• As the number of molecules in the gas phase increases,

some of the gas phase molecules strike the surface and return to the liquid.

• After some time the pressure of the gas will be

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Vapor Pressure

Explaining Vapor Pressure on the Molecular Level

• Dynamic Equilibrium: the point

when as many molecules escape the surface as strike the surface.

• Vapor pressure is the pressure

exerted when the liquid and vapor are in dynamic

equilibrium.

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Vapor Pressure

Volatility, Vapor Pressure, and Temperature

• If equilibrium is never established then the liquid

evaporates.

• Volatile substances evaporate rapidly.

• The higher the temperature, the higher the average

kinetic energy, the faster the liquid evaporates.

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Vapor Pressure

Volatility, Vapor Pressure, and Temperature

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Vapor Pressure

Vapor Pressure and Boiling Point

• Liquids boil when the external pressure equals the

reducing the cooking time required.

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Phase Diagrams

• Phase diagram: plot of pressure vs Temperature

summarizing all equilibria between phases.

• Given a temperature and pressure, phase diagrams

tell us which phase will exist.

• Features of a phase diagram:

– Triple point: temperature and pressure at which all three

phases are in equilibrium

– Vapor-pressure curve: generally as pressure increases,

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Phase Diagrams

• Any temperature and pressure combination not on a

curve represents a single phase.

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– Triple point occurs at 0.0098°C and 4.58 mmHg.

– Normal melting (freezing) point is 0°C.

– Normal boiling point is 100°C.

– Critical point is 374°C and 218 atm.

• Carbon Dioxide:

– Triple point occurs at -56.4°C and 5.11 atm.

– Normal sublimation point is -78.5°C (At 1 atm CO 2 sublimes

it does not melt.)

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Phase Diagrams

The Phase Diagrams of H2O and CO2

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Structures of Solids

Unit Cells

• Crystalline solid: well-ordered, definite arrangements

of molecules, atoms or ions

• Crystals have an ordered, repeated structure.

• The smallest repeating unit in a crystal is a unit cell.

• Unit cell is the smallest unit with all the symmetry of

the entire crystal.

• Three-dimensional stacking of unit cells is the crystal

lattice.

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Unit Cells

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Unit Cells

• Three common types of unit cell.

– Primitive cubic, atoms at the corners of a simple cube,

• each atom shared by 8 unit cells;

– Body-centered cubic (bcc), atoms at the corners of a cube plus one in the center of the body of the cube,

• corner atoms shared by 8 unit cells, center atom completely enclosed in one unit cell;

– Face-centered cubic (fcc), atoms at the corners of a cube plus one atom in the center of each face of the cube,

• corner atoms shared by 8 unit cells, face atoms shared by 2 unit

cells.

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Unit Cells

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Crystal Structure of Sodium Chloride

• Face-centered cubic lattice.

• Two equivalent ways of defining unit cell:

– Cl - (larger) ions at the corners of the cell, or

– Na + (smaller) ions at the corners of the cell.

• The cation to anion ratio in a unit cell is the same for

the crystal In NaCl each unit cell contains same number of Na+ and Cl- ions.

• Note the unit cell for CaCl2 needs twice as many Cl

-ions as Ca2+ ions.

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Crystal Structure of Sodium Chloride

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Close Packing of Spheres

• Solids have maximum intermolecular forces.

• Molecules can be modeled by spheres.

• Atoms and ions are spheres.

• Molecular crystals are formed by close packing of the

molecules.

• We rationalize maximum intermolecular force in a

crystal by the close packing of spheres.

• When spheres are packed as closely as possible, there

are small spaces between adjacent spheres.

• The spaces are called interstitial holes.

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• A crystal is built up by placing close packed layers of

spheres on top of each other.

• There is only one place for the second layer of spheres.

• There are two choices for the third layer of spheres:

– Third layer eclipses the first (ABAB arrangement) This is

called hexagonal close packing (hcp);

– Third layer is in a different position relative to the first

(ABCABC arrangement) This is called cubic close packing (ccp).

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• Each sphere is surrounded by 12 other spheres (6 in

one plane, 3 above and 3 below).

• Coordination number: the number of spheres directly

surrounding a central sphere.

• Hexagonal and cubic close packing are different from

the cubic unit cells.

• If unequally sized spheres are used, the smaller

spheres are placed in the interstitial holes.

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X-Ray Diffraction

• When waves are passed through a narrow slit

they are diffracted.

• When waves are passed through a diffraction

grating (many narrow slits in parallel) they

interact to form a diffraction pattern (areas of

light and dark bands).

• Efficient diffraction occurs when the wavelength

of light is close to the size of the slits.

• The spacing between layers in a crystal is 2 - 20

Å, which is the wavelength range for X-rays.

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• X-ray diffraction (X-ray crystallography):

– X-rays are passed through the crystal and are

detected on a photographic plate.

– The photographic plate has one bright spot at the

center (incident beam) as well as a diffraction pattern.

– Each close packing arrangement produces a different

diffraction pattern.

– Knowing the diffraction pattern, we can calculate the

positions of the atoms required to produce that

pattern.

– We calculate the crystal structure based on a

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Bonding in Solids

– Molecular (formed from molecules) - usually soft

with low melting points and poor conductivity.

with very high melting points and poor conductivity.

points and poor conductivity.

high melting points, good conductivity, malleable

and ductile.

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• Room temperature gases and liquids usually

form molecular solids at low temperature.

• Efficient packing of molecules is important

(since they are not regular spheres).

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Covalent Network Solids

• Intermolecular forces: dipole-dipole, London dispersion and H-bonds.

• Atoms held together in large networks.

• Examples: diamond, graphite, quartz (SiO2),

silicon carbide (SiC), and boron nitride (BN).

• In diamond:

– each C atom has a coordination number of 4;

– each C atom is tetrahedral;

– there is a three-dimensional array of atoms.

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Covalent Network Solids

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