Chem 105a Week 8 Notes
Chem 105a Week 8 Notes CHEM 105A
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This 7 page Class Notes was uploaded by Emma Morrissey on Friday October 14, 2016. The Class Notes belongs to CHEM 105A at University of Southern California taught by Thomas Michael Bertolini in Fall 2016. Since its upload, it has received 12 views. For similar materials see General Chemistry in Chemistry at University of Southern California.
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Date Created: 10/14/16
10 October 2016 Chapter 6 continued ● Calculating change in enthalpy in reaction ○ There are three quantitative relationships between a chemical equation and ΔH ■ If a chemical equation is multiplied by some number, ΔH is also multiplied by the same number ■ If a chemical equation is reversed, then ΔH changes sign ● Remember that negative enthalpy means that the function is exothermic ○ Hess’s Law ■ If a chemical reaction can be expressed as the sum of a series of steps, then ΔH for the overall equation is the sum of the ΔH s for each step ○ Since enthalpy is a state function, ΔH is dependent only on initial and final states ● Standard States ○ A set of conditions to measure state properties ○ Don’t confuse STP with standard state. SS doesn’t include a specific temperature ○ Standard states include ■ Gases (1 atm) ■ Solutions (1 M) ■ Liquids, solids (Pure, 1 atm) ○ Recall the state of each element at 25 C (room temperature) ● Standard Enthalpy of Formation (can use fractional reaction coefficients) ○ ΔH f The ΔH when 1 mol of a compound forms from its elements in their standard states ○ For a pure element in its standard state, the enthalpy of formation is zero ○ The reaction coefficient is for the product must be 1 ● Standard Enthalpy of Reaction ○ The enthalpy change for a given reaction is calculated by subtracting the enthalpies of formation of the reactants from the enthalpies of formation of the products ○ ΔH ° = Σn ΔH (products) −Σn ΔH (reactants) rxn p f r f ○ For these problems on exams, data for the enthalpy of formation will be given. Consult the appendix of the textbook for practice problems. ● Keep the ΔHs straight ○ ΔH - enthalpy or heat of reaction. Units in kJ. Calculated by rxn Hess’ Law or measured by calorimetry ° ○ ΔH -fstandard enthalpy of formation. 1 mol of a compound forms from its elements in their standard states; elements may have fractional coefficients. Units are kJ/mol. Elements in standard state have ΔH = 0 ° f ○ ΔH °rxn standard enthalpy of reaction. Hypothetical decomposition of reactants into their elements and reconstitution of elements into products. Units are kJ. ° ° ° ΔH rxn= Σn pH (pfoducts) −Σn ΔH (reacfants) Chapter 7 The Quantum Mechanical Model ● Two branches of mechanical physics ○ Classical mechanics- laws describing the motion of macroscopic objects (planets have o rbits) ○ Quantum mechanics- principles of electrical and magnetic properties at the atomic and subatomic level (electrons have orbitals) ● Electromagnetic radiation ○ EMR- a form of energy transmission ○ Wave- a disturbance that transmits energy through space or matter ○ All electromagnetic radiation travels in waves. They have three basic characteristics ■ Wavelength (λ)- the distance between the crests or troughs of a wave. Measured in meters ■ Frequency (nu, ν )- the number of waves per second 1 passing a certain point. Measured in Hertz, or , or s s ■ Speed (c) - all electromagnetic radiation travels at the speed of light (c= 2.9979 E 8 m/s) ○ λ*ν = c ■ As wavelength increases, frequency decreases and vice versa ● The Electromagnetic Spectrum ○ Electromagnetic radiation is classified based on wavelength −12 ○ Smallest wavelength: Gamma rays (10 m) ○ Longest wavelength: Radio waves ● Infrared (IR) ○ If you don’t have a fever, you emit IR radiation at wavelengths of about 12 nanometers ● Interference ○ Waves interact with each other via interference ○ Constructive interference- two waves add to make a larger wave (with a larger amplitude) ○ ○ Destructive interference- two waves cancel each other out (because they are out of phase) ○ ● Diffraction ○ waves (not particles) bend around an opening in a barrier ○ Diffraction through two slits gives an interference pattern of the diffracted waves ● Photons ○ Quanta (discrete amounts) of energy ○ Electromagnetic radiation is photons. A specific type of EMR has photons with the same energy ○ Atoms can absorb and emit photons ● The photoelectric effect ○ When exposed to certain light, metals eject electrons from their surface ● The photoelectric effect ○ The PE requires a minimum or threshold frequency of EMR (not intensity) to occur ○ Metals emit electrons when exposed to EMR at or above the threshold frequency ● Einstein and the PE effect ○ Einstein proposed electromagnetic radiation (EMR) is a stream of photons ○ Energy of a photon is proportional to its frequency ○ Intense EMR can dislodge more electrons, it has more photons than dim EMR ● PE in Summary ● v>>v0 v>v0 v<v0 Dim EMR Fewer Fewer None (fewer photons) electrons, electrons, lower higher velocity velocity Bright EMR More electrons, More electrons, None (more photons) higher velocity lower velocity ● ● Quantum mechanics ○ Energy is quantized; it can only be released or absorbed in specific amounts called quanta (hv) ○ This equation solves energy per particle ○ Ephoton hv =hλ -34 2 ○ Planck’s constant h= 6.62607004 × 10 m kg / s ● Photon energy ○ The equation accounts for the energy of a photon itself or an electron transition ○ A photon is a quantity of energy, so the energy is only positive ○ The energy of an electron can be positive or negative, but the equation is the same ΔE = E = hv =hc electron photon λ ● Electron transitions ○ Electrons can absorb a photon and more to a higher energy level ○ Electrons in high energy states are unstable and lose energy as photons, dropping to lower energy states ○ ΔE electronv =hλ= Enfinal ninitial ○ When an electron moves to a higher energy level, it is called absorption. The photon is consumed ○ When an electron moves to a lower energy level, a photon is emitted 14 October 2016 ● A mole of photons is called an Einstein ● Evidence for Electronic Transitions ○ Atoms- line spectra ● Atomic spectrum of Hydrogen ○ When white light is concentrated through a prism, the entire visible light spectrum is ○ When H2 is refracted through a prism, only certain pieces of the spectrum are seen ● Line spectra ○ Unique to each element ○ Line spectra are not the continuous spectrum because electron energies are quantized ○ Each line corresponds to an electronic transition ○ Four elements are named after prominent colors in their line spectra ■ Thallium is Greek for green stem (only one green line) ■ Indium has a very strong indigo line ■ Rubidium is Latin for deep red ■ Cesium is Latin for sky blue ● Flame tests ○ Elements display various colors of light upon exposure to heat ○ the color is characteristic of the element and represents one or more strong lines in its line spectra ● Fireworks ○ Large scale flame tests ○ Blue fireworks shouldn’t exist; all elements with a blue flame are toxic to the environment ■ Selenium, indium, lead, arsenic ● The luminol reaction ○ Luminol and hydrogen peroxide yields light, oxygen, and water ○ This is chemiluminescence ● Wave-Particle Duality ○ All matter exhibits both particulate and wave properties ○ Large objects generally display undetectable behavior ○ Electromagnetic ● Electrons as particles ○ Mass and charge ● Electrons as waves ○ Can be observed behaving like waves ● The de Broglie Equation ○ Einstein’s theory of relativity reveals that energy has mass ■ E = mc^2 ○ The energy of a photon is proportional to its frequency ○ De Broglie: λ = mν ○ Using the de Broglie equation, the wavelength can be calculated from velocity. Everything obeys this equation. ● The width of an atom is an Angstrom, or 10 E-10 m ● The Heisenberg Uncertainty Principle ○ An orbital is a volume of space with a high probability of finding an electron ■ How the electron moves… TBD ■ We are limited in our knowledge of both the position and momentum of a particle at any one time ■ Δx *Δν ≥ h 4Π ■ Cannot know both the location and the velocity at any one point ■ Δx= uncertainty of particle position ■ mΔν = uncertainty of particle momentum ■ ν = velocity (not frequency)
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