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Chapter 7

Chemistry Review Chapter 7.docx

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University of Toronto St. George
James Thomson

Chemistry Review CHAPTER 7 The Nature of Light - Electromagnet radiation: has energy made by electric + magnetic fields that increase and decease in intensity as they move through space - Ex) visible light, xrays, microwaves - The Wave Nature of Light Wave prop of electromagnetic radiation described by 3 variables and 1 constant -1 1) Frequency (v): the number of cycles a wave does per second (s /Hz) - shorter Y = higher freq/longer Y = higher freq 2) Wavelength (m): the distance between any point on a wave and the corresponding point on the next crest/trough) - AKA, the distance the wave travels during one cycle 3) Speed(m/s): the distance it moves per under time 8 - In a vacuum, ER moves at the speed of light: 3.00 x 10 m/s - C = v x Y , since product of v and Y are constant, relationship = reciprocal - v Y and v Y 4) Amplitude: height of the crest, related to intensity of radiation (brightness of light) - Light of a certain color has a specific frequency (dimmer at low amp, brighter at high amp) Electromagnetic Spectrum - Visible light represents a small region - All waves on the spectrum travel at the same speed thru vacuum BUT differ and frequency AND SO wavelength - Continuum of radiant energy - (L to R): Radio freq – Microwave – infrared – visible – UV – Xray – Gamma Ray - We see diff Y as visible colors, from red (Y=750nm) to violet (Y=400nm) - Monochromatic: light of single Y - Polychromatic: light of may wavelengths (white light) - Short Y, Higher Frequencies from L to R *Diagram in notes of Wavelength Colors* Distinction Between Energy and Matter - Matter = chunks that you can + weigh + change the quantity of matter piece Light = radiant energy by piece + moves in specific paths - Energy = “massless” and its quantity can change continuously + light moves in waves 1) REFRACTION + DISPERSION: light of a given Y travels at different speeds thru various media (ex: water, air, vacuum) - Y’s speed change as light passes from one medium to another - Refraction!!!? Speed of a light wave passing between media changes immediately, which bends its path - Strikes at an angle other than 90 - Angle depends on 2 media + light’s Y - Particles of matter don’t do this (ex: pebble) - Dispersion !!!? White light sep into its component colors when it passes thru a prism because each incoming wave is refracted at a slightly diff angle - Rainbow/diamond = light going thru near surface of drops dispersed + reflected out far surface - Red light bent least (higher)… Violet light bent most (lower) 2) DIFFRACTION + INTERFERENCE: when wave strikes the edge of an object, it bends around it - Particle’s don’t, some go thru opening and move @ straight path sometimes bumping into eachother - When waves of light pass thru 2 adjacent slits, emerging circular waves interact thru interference!!! - Crests of waves coincide = in phase = constructive interference: amps add to form brighter region - Crests coincide with troups = out of phase = deconstructive: amps cancel to form dark region - Result = diffraction pattern!!! (figure 7.5) Particle Nature of Light - ???? = blackbody radiation. Photoelectric effect, atomic spectra 1) BLACKBODY RADIATION + QUANTUM THEORY OF EN - Blackbody: idealized object that absorbs all radiation incident on it. Like a hollow cube with a small hole in one wall Light given off - OBSERVED: When a solid object is heated to about 1000K, it begins to show while object visible light heated - 1500K light is brighter + orange….2000K > light is brighter + whiter - Changes in intensity + Y of emitted light as an object is heated are characteristic of blackbody rad - EXPLAIN? quantum theory! Planck “hot, glowing object could emit/absorb only certain quantities of energy: E=nhv (p.266) - According to quantum theory, atom has only vertain quantities of en (e=nhv) and can change its energy only by ab/emitting a photon whose energy = change in atom’s energy - SO, atom’s en = QUANTIZED: occurs in fixed quantities instead of being continuous - Change in atom’s en when atom absorbs/emits “packets” - Packets = definite amounts of energy, called a quantum (fixed quant equal to hv) Atom changes its energy state by emitting/absorbing one (+) quanta + energy of emitted/absorbed raduatuib = diff in atom’s energy states: E atom = E emitted/ab radiation = nhv - smallest change = when atom in given energy state changes to an adjacent state… n = 1 E = hv 2) PHOTOELECTRIC EFFECT + PHOTON THEORY OF LIGHT Wave model can’t explain flow of current when light strikes metal - OBSERVED: Photoelectric effect!!!! = when monochromatic light of sufficient frequency shines on a metal plate, a current flows - CONFUSING: THRESHOLD FREQUENCY + ABSENSE OF A TIME LAG Presence of Threshold freq: current can only flow if light shining on metal has a min freq - diff metals have diff min freq - WAVE THEORY, energy of light assc with amp (intensity) not freq (color). SO, theory says e will break free when it absorbed enough en from light of any color Absence of a time lag: current flows the moment light of min freq shines on metal, LIGHT INTENSITY nnnnnnnnnnnnnnnnnDOESN’T MATTAHHH!!! - WAVE THEORY, with dim light there would be a time lag before current flows cause e’s would have to absorb enough en to break free - EXPLAIN? Photon theory! Einstein “ light itself is particulate, quantized into tiny “bundles” of energy CALLED PHOTOS - Atom’s change energy when they absorb/emit 1 photon (“1 particle of light”) whose energy is related to its frequency not amplitude: E photon = hv = E atom HOW PHOTON THEORY EXPLAINS PHOTO ELECTRIC EFFECT: Q1) WHY IS THERE A THRESHOLD FREQUENCY? Beam of light has many photos - Intensity, aka brightness, of light related to # of photos, NOT energy of each - SO, a photon of a certain minimum energy must be absorbed to free an electron from the surface - Since energy depends on frequency (hv), there is a threshold freq Q2) WHY IS THERE NO TIME LAG? An e breaks free when it absorbs a photon of enough energy… can’t save up energy! - Current is weak in dim light cause fewer photos of enough energy can free fewer e’s per unit time - SOME current flows as soon as light of sufficient energy (freq) strikes the metal ATOMIC SPECTRA: LIGHT EMITTED WHEN AN ELEMENTS IS VAPORIZED AND THEN EXCITED ELECTRICALLY Line Spectra + Rydberg Equation - Line spectrum made when light made from electrically excited gas atoms pass through a slit and refracted by a prism - NOT CONTINUOUS SPECTRUM - Line spectrum: series of find lines at specific frequencies separated by black spaces… looks depend on element making it - GO TO LEC NOTES: LOOK AT FREQUENCIES! Rydberg Eqwashunnn: relationship that predicted the position + Y of any line in a given series: (p.268) Problems with Rutherfod’s Nuclear Model: subat. Behavior violates principles - (+) nucleus and (-) e attract each other and to stay apart, the KE of e’s motion hasta counterbalance the PE of attraction - BUUUT! (-) particles moving in curved path around (+) particle MUST emit radiation, SO it must lose energy... E’s would crash into nurcleus and kill atom - BUUUUT! Also says freq of light should change smoothly as (-) particles spirals inward = CONTINUOUS SPECTRUM…. NOT LINE SPECTRUM! Bohr Model of H-atom POSTULATES: (P.269) 1) H atom has only certain en lvls “stationary states”, associated with fixed circle orbit of e’s around nucleus 2) Atom does not radiate energy while in one of its stationary states: does not change energy while e moves in orbit (violates principles) 3) Atom changes to another stationary states AKA e MOVES TO ANOTHER ORBIT, only by absorbing/emitting a photon. Energy of photon (hv) equals the diff in energies of the 2 states: FEATURES 1) Quantum #s + e Orbit: quantum # (n) is (+), associated with the radius of an e orbit (related to e’s energy) - Lower the n value, the smaller the r of orbit, and the lower the en lvl 2) Ground State: when e in first orbit (n=1) is closest to nucleus. H atom in lowest (1 ) energy level = ground state 2 orb (n=2) 3) Excited States: when e is in any orbit farther from nucleus atom is in first4) Absorption (makes farther): H atom absorbs a photon whose en equals the diff excited state, etc. between lower + higher en lvls, e moves to outer orbit 5) Emission (makes closer): H atom in higher en lvl returns to lower en lvl by emitting a photon whose en equals the diff between the 2 levels How Model Explains Line Spectra: - When photons of specific en (so, freq) are emitted = spectral line - Emission = when e moves to orbit closer to nucleus as atom’s energy changes from higher state to lower one * An atomic spectrum is not continuous because the atom’s energy is not continuous, instead it has certain
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