# Chaper 12 notes.docx - Chapter 12 Notes Video 1 Phase...

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Chapter 12 Notes Video 1 Phase diagrams They describe the different states and state changes occurring at various temps and pressure conditions Regions represent states Lines represent state changes The liquid/gas line is the vapor pressure curve On the line, both phases exist simultaneously (or in equilibrium with each other) Points The critical point is the farthest point on the vapor pressure curve The triple point is the temp/pressure condition in which all three states exist simultaneously For most substances, the freezing point increases as pressure increases Diamonds are NOT forever They are thermodynamically unstable at STP. They will spontaneously change into graphite at room temp and 1 ATM. The rate just happens to be very, very, very slooowwww!! Solids Crystalline Ionic crystals(salt) Covalent(diamond) Liquid crystals Regular covalent(graphite) Amorphous (without form: soot, polymers) Crystalline solids and their structure Crystalline solids are orderly geometric structures whose lattice points are occupied by atoms, ions, or molecules Understanding of solid structure is often done using the technique of x-ray diffraction crystallography DNA Rosalind Franklin Photographed molecules using x-rays (1953). This is what she observed These patterns indicated DNA was like a coil. The DNA molecule had a constant diameter of 2 nm. It did not get wider or narrower in some parts of the molecule Video 2 Diffraction from a crystal When x-rays strike parallel planes of atoms or molecules in a crystal, constructive interference occurs if the difference in path length between means reflected from adjacent planes is an integral number of wavelengths Constructive interference “adds” to the amplitude of the electromagnetic wave whereas destructive interference “subtracts” from the wave’s amplitude Bragg’s law allows scientists to determine the distance between layers of atoms or molecules in a crystal lattice n(upside down y) =2d sin θ
Crystal lattice When allowed to cool slowly, the particles in a liquid will arrange themselves to give the maximum attractive forces They minimize the energy The result will generally be a crystalline solid The arrangement of the particles in a crystalline solid is called the crystal lattice The smallest unit that slows the pattern of arrangement for all the particles is called the unit cell Unit cells They are three-dimensional, usually containing two or three layers of particles Repeated over and over to give the macroscopic crystal structure of the solid Starting anywhere within the crystal results in the same unit cell