Chemistry 141 Wednesday, October 25, 2017 Lecture 21 Chemistry 11 - Lecture 11 9/30/2009 Chemistry 141 Wednesday, October 25, 2017 Lecture 21 Enthalpy of Formation Waves and Light

Hess’s Law If a reaction is carried out in a series of steps, H for the overall reaction will be equal to the sum of the enthalpy changes for the individual steps.

Enthalpies of formation ΔHf = Enthalpy of formation = Enthalpy to make compound from its elements Standard state = most stable form of pure substance at 25 ˚C and 1 atm ΔHf˚ = Standard enthalpy of formation = Enthalpy to make 1 mole of compound from its elements, all at standard conditions ΔHf˚ for elements in their standard states is ZERO

ΔHf˚

Example Calculate ΔH˚rxn for the following: CaO (s) + 2 HCl (g)  CaCl2 (s) + H2O (g)

Simple model of the atom ~10-8 cm 10-13 cm Particle Mass (amu) Charge (eV) Proton 1.00728 +1 Neutron 1.00866 Electron 1/1822.9 -1 Do electrons, protons, neutrons, and atoms behave the same as larger objects (like a baseball)? (Recall results of Rutherford’s experiments) Can we apply the laws of classical physics to these small particles? Can we write equations to describe the forces on and motion of an electron?

Light and Matter

Light and matter What distinguishes matter and energy? matter – has mass, consists of particles, location in space can be specified light – is described as a wave, is mass-less and delocalized (position in space cannot be specified) How do light and matter interact on an atomic scale?

Properties of waves Animations of Waves What is a wave? “A disturbance from an equilibrium condition which travels, or propagates, from one region of space to another” -- Young, H. D., University Physics, 8th ed., Addison-Wesley, Reading, MA, 1992. Animations of Waves

Properties of waves Wavelength: distance between adjacent peaks/troughs units – length: m, cm, nm symbol – lambda: λ Frequency: number of peaks/troughs of a wave that cross a fixed point in space in one second units – cycles/second: s-1, Hertz (Hz) symbol – nu: ν (not v!) Amplitude: maximum displacement from equilibrium Speed: rate at which a peak/trough moves through a medium

Wavelength – frequency relationship

Electromagnetic waves (light) Consist of oscillating electric and magnetic fields All electromagnetic waves travel at the speed of light  = wavelength (m) n = frequency (Hz, s-1) c = speed of light = 2.99792458 ×108 m/s (in vacuum) Copyright © 2002 by Houghton Mifflin Company. All rights reserved.

The electromagnetic spectrum 400 nm 700 nm Examples of electromagnetic radiation: visible light x-rays microwave ovens radio and TV signals

Color and Light  = wavelength (m) n = Frequency (Hz, s-1) A traffic light emits light of frequency 4.28×1014 Hz. What is the wavelength of this radiation? Should you go or stop? How does the wavelength of red light compare to the wavelength of blue light? What about the frequency?  = wavelength (m) n = Frequency (Hz, s-1) c = 2.99792458 ×108 m/s (in vac.)

Blackbody radiation Classical theory cannot explain the wavelengths of light emitted from a blackbody

Quantization of energy

Photon energies  = wavelength (m) n = Frequency (Hz, s-1) c = 2.99792458 ×108 m/s (in vac.) h = 6.626×10-34 J·s Earlier, we calculated the wavelengths of red light (700 nm). What is the energy of a photon with this wavelength?