Liquids and Solids: AP Chemistry Unit 10

Liquids and Solids: AP Chemistry Unit 10

Sections

· Intermolecular Forces

· Liquid state

· Solid Structures

· Metal Structures

· Carbon and Silicon Networks

· Molecular Solids

· Ionic Solids

· Vapor pressure and State Change

· Phase Diagrams

States of Matter

When considering the three states of matter, properties of gases are strikingly different than solids and liquids. Liquids and solids share many similar characteristics

· compressibility

· density

· intermolecular forces

H2O(s) ® H2O(l) ΔH°fus = 6.02 kj/mol

H2O(l) ® H2O(g) ΔH°vap = 40.7 kj/mol

Water densities:

25°C and 1atm .99707g/cm3

25°C and 1065 atm 1.046g/cm3

400°C and 1atm 3.26x10-4 g/cm3

400°C and 242 atm .157g/cm3

Intermolecular Forces 10.1

· Electrons shared within the molecule are called intramolecular bonding.

· In the condensed states of matter the attraction between molecules are called intermolecular forces.

It is important to realize that when a molecule changes state, the molecule stays intact. The changes in state are due to the change in forces surrounding the molecule not from changes within the molecule.

· 40.7kj needed to vaporize water

· 934kj to break the O-H bond

Dipole–Dipole Forces

Dipole-dipole forces occur when polar molecule (molecules with dipole moments) electrostatically attract each other by lining up the positive and negative ends of the dipoles.

· Dipole-dipole forces….

· In a condensed state, …

Dipole-Dipole Forces continued…

Some dipole-dipole forces are unusually strong. These usually form between H and another very electronegative atom.

o These strong attractions have a strong impact on melting points and boiling points.

Boiling Points of Covalent Hydrides

Hydrogen bonds

Hydrogen bonds are the strongest in the smallest and lightest of the covalent molecules. This is primarily due to two factors:

Hydrogen bonds continued….

Hydrogen Bonds and Organics

Methanol (CH3OH) and ethanol (CH3CH2OH) have much higher boiling points than would be expected from their molar masses because of the O-H bonds that produce hydrogen bonding.

London Dispersion Forces

Even without dipoles, molecules exert forces on each other.

Usually it is assumed that electron dispersion is uniform throughout the molecule, but this is not always the case.

· Since the movements of the electrons around the nucleus are somewhat random, a momentary nonsymmetrical electron distribution can develop that creates a temporary dipolar arrangement of charge.

· This phenomenon leads to an inter-atomic attraction that is relatively weak and short-lived, but can be significant in larger atoms at lower temperatures.

London Dispersion Forces continued…

Polarizability is the ease at which an electron cloud can be distorted into a temporary dipole.

· This also applies to molecules like H2, CH4, CCl4 and CO2; smaller molecules, but nonpolar.

The Liquid State 10.2

Liquid Characteristics

· lack of rigidity

· low compressibility

· high density

Rounded Droplets

· Occur due to the intermolecular forces of the liquid. The liquid molecules are subject to attraction from the side and from below, so liquid tends to form a shape with the minimum surface area – sphere.

Capillary Action

Capillary action is the spontaneous rising of a liquid in a narrow tube. This action is due to two forces

· cohesive forces-

· adhesive forces –

Adhesive forces

Adhesive forces happen when bonds within the container have polar bonds

· For example: glass has O atoms that carry a partial negative charge that attracts the partial positive charge of the hydrogen in water. This balance between the strong cohesive forces and the strong adhesive forces produce a meniscus.

· A nonpolar

Meniscus: Water vs. Mercury

Viscosity

Viscosity is a fluids resistance to flow.

· Example: glycerol is highly viscous because of its ability to create hydrogen bonds.

Viscosity continued…

· Example: Gasoline has carbon chains from 3-8C long and is nonviscous. Grease is 20-25C long and is very viscous.

Introduction to Structures and Types of Solids 10.3

Types of Solids

· Crystalline solids

· Amorphous solids

Crystalline Solids

Crystalline solids have a regular arrangement of components at a microscopic level and produce beautiful, characteristic shapes of crystals:

The positions of components are usually represented by a lattice.

Amorphous Solids

Amorphous solids have considerable disorder in their structures.

· Example: Common glass looks like a solution frozen in place. It has a rigid shape but a great deal of disorder within its structure.

X-ray Analysis of Solids

The structures of crystalline solids are commonly determine by X-ray diffraction.

· This type of diffraction occurs when beams of light are scattered as they go through spaces between substances. Light scatters when the size of the spaces are similar to the wavelength of light.

· A single wavelength is directed at the crystal and a diffraction pattern is obtained. The diffraction pattern is a series of light and dark areas on a photographic plate from constructive and destructive interference from waves of light.

· A diffractometer is a computer-controlled instrument used for carrying out the X-ray analysis of crystals

o It rotates the crystal with respect to the X-ray beam and collects the data produced by the scattering. The techniques have been refined to the point that very complex structures can be determined, such as large biological enzymes.

The Bragg equation combines trigonometry and physics to determine the atomic spaces between crystals:

· nλ = 2d sin θ

· d is the distance between atoms and θ is the angle of incidence and reflection of the light. n is an integer, most commonly 1. (n is usually given)

X-ray Analysis of Solids continued….

Example Problem: X-rays of wavelength 1.54 Â were used to analyze an aluminum crystal. A reflection was produced at θ = 19.3°. Assuming n=1, calculate the distance d between the planes of atoms producing this reflection

Types of Solids

· Ionic solids

· Molecular solids

· Atomic solids

Atomic Solids

Atomic solids are broken down into subgroups depending on the bond that exists in the solid:

· Metallic solids

· Network solids

Atomic Solids continued….

· Group 8A solids

Classification of Solids

Structure and Bonding in Metals 10.4

Metal Characteristics

Most of the properties that we see in metals is due to the nondirectional covalent bonding found in metal crystals.

· High thermal conductivity

· Electrical conductivity

· Malleability

· Ductility

Metallic Crystals

Metallic crystals can be pictured as containing spherical atoms packed together that can be bonded to each other equally in all directions.

Closest Packing

· The spheres pack in layers. Each sphere is surrounded by six others. These layers do not lie directly over those in the first layer, instead they fill the indentations of the layer below. The third layer is in the same position as the first. This is called aba arrangement.

· The abc arrangement has a face-centered cubic unit cell and the resulting structure is called the cubic closest packed (ccp) structure. This has a repeating vertical placement every fourth layer.

Closest Packing: Hexagonal

Closest Packing: Cubic

Knowing the net number of atoms in a particular unit cell is important for many applications involving solids.

Example: A face centered cube (unit cell) is defined by the centers of the spheres on the cube’s corners. Therefore 8 cubes share a given corner sphere, so 1/8 of this sphere lies inside the unit cell. (8 corners x 1/8 sphere = 1sphere). The sphere at the center of each face is shared by two cubes. (6 faces x ½ sphere = 3 spheres). The total number of spheres for a face centered cube is 4.

Face – Centered Cubic Unit Cell

Cubic Substances

Metals that form cubic closest packed solids are:

Hexagonal Substances

Metals that form hexagonal closest packed solids are:

Other Metal Solids

· Some metals, including many alkali metals, have structures that are characterized by a body-centered cubic (bcc) unit cell. In this structure, each sphere has 8 neighbors.

Example Problem: Silver crystallizes in a cubic closest packed structure. The radius of a silver atom is 144pm. Calculate the density of solid silver?

Bonding Models for Metals

In order to determine bonding for metals, one must account for the typical properties: durable, high melting point, malleable, ductile, and efficient in uniform conduction of heat and electricity in all directions.

Electron Sea Model

Metal cations ‘swim’ in a sea of valence electrons that are mobile and shared.

Band Model (MO Model)

In this model, the electrons are assumed to travel around the metal crystal in molecular orbitals formed from the valence atomic orbitals of the metal atoms.

· When metals atoms interact, the large number of resulting molecular orbitals become more closely spaced and finally form a virtual continuum of levels, called bands.

Band Model continued…

Metal Alloys

An alloy is best defined as a substance that contains a mixture of elements and has metallic properties. There are two types of alloys:

· Substitutional alloy–

· Interstitial alloy –

Substitutional Alloy

Example: brass: 1/3 of copper metal atoms are replaced by zinc atoms

· Sterling silver-

· Pewter-

· Plumbers solder –

Interstitial Alloy

Example: Steel contains carbon atoms in the holes of an iron crystal. The presence of the interstitial atoms changes the properties of the host metal. Iron is relatively soft, ductile and malleable, but when carbon (which forms directional bonds), is introduced into the crystal, it makes the iron bonds stronger and less ductile.

The amount of carbon directly affects the properties of steel:

· Mild steels-

· Medium steels-

· High-carbon steel –

Mixed Alloys

Some steels contain elements in addition to iron and carbon. These are called alloy steels and are viewed as being mixed interstitial and substitutional alloys.

· Bicycle frames are usually constructed from a wide variety of alloy steels.

Carbon and Silicon Network Atomic Solids 10.5

Network Solids

Many atomic solids contain strong directional covalent bonds to form a solid that might be viewed as a “giant molecule.” These materials are typically brittle and do not efficiently conduct heat and electricity. Two examples of these network solids are carbon and silicon.

Carbon

Two most common forms of carbon are diamond and graphite. They are typical network solids.

· Diamond

· Graphite

Diamond

· Each carbon is surrounded by a tetrahedral arrangement of other carbon atoms to form a large molecule. Diamond is an insulator not a conductor. Each carbon is sp3 hybridized with localized bonding and therefore does not conduct.

· The application of 150,000 atm at 2800°C can break graphite bonds and rearrangement into a diamond structure.

Graphite

· The structure of graphite is based on layers of carbon atoms arranged in fused 6 C rings. The unhybridized p orbitals allow for delocalized electrons and therefore conductivity.

Carbon: Graphite layers

Silicon

Silicon is an important constituent of the compounds that make up the earth’s crust. Silicon is to geology what carbon is to biology and is fundamental to most rocks, sands and soils found in the earth’s crust.

· Carbon compounds typically have long strings of C-C bonds

Silica

· The fundamental silicon-oxygen compound is silica, which has the empirical formula SiO2. The structure that is formed is based on a network of SiO4 tetrahedra with shared oxygen atoms rather than smaller SiO2 units.

Silica continued…

When silica is heated above its melting point (1600°c) and cooled rapidly, an amorphous solid called glass results. Glass has a lot of disorder as opposed to the crystalline nature of quartz. Glass, also homogeneous, more closely resembles a very viscous solution than it does a crystalline solid.

Glass

The properties of glass can vary greatly depending on the additives.

· Common glass results when…

· B2O3 produces borosilicate glass which…

· K2O produces ….

Silicates

Compounds closely related to silica and found in most rocks, soils and clays are the silicates. Like silica, the silicates are based on interconnected SiO4 tetrahedra, but instead of a O/Si ratio of 2:1, the ratio is typically higher. This higher ratio tends to make silicon-oxygen anions.

Ceramics are typically made from clays (which contain silicates) and hardened by firing at high temperatures. They tend to be strong, brittle and heat and chemical resistant.

Clays

Clay comes from the weathering of feldspar, an Aluminosilicate (Na2O/K2OŸAl2O3Ÿ6SiO2). This weathering produces kaolinite, that consists of tiny thin platelets of Al2Si2O5(OH)4. When dry these platelets cling together and lock into place; when wet they can slide over one another. During firing, these platelets bind and form a glass.

Ceramics

Ceramics constitute one of the most important classes of ‘high-tech” materials. Their stability at high temperatures and resistance to corrosion, make them an obvious choice for constructing jet and car engines.

Ceramics continued…

Organoceramics are taking form by the addition of organic polymers to ceramics. This reduces some of the brittle nature of ceramics and allows them to be used for things such as flexible superconducting wire, microelectronic devices, prosthetic devices and artificial bones.

Semiconductors

Elemental silicon has the same structure as diamond. The structure is different in that the energy gap between filled and empty MO’s is not as large and electrons can delocalize and make silicon a semi-conductor. At higher temperatures, more electrons get excited in the conduction bands and the conductivity of silicon increases.

N-type Semiconductor

P-type Semiconductor

· When small fraction of silicon atoms are replaced by boron atoms (one less valence electron), an electron ‘vacancy’ is made. As electrons move, the fill the ‘hole’ and make a new one. This movement of electrons can therefore carry a current. This type of conductor (less electrons) is called a p-type semiconductor.