1. 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

2. 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.

3. 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

4. ΔHf˚

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

6. Simple model of the atom
~10-8 cm
10-13 cm
Particle
Mass
(amu)
Charge
(eV)
Proton +1
Neutron
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?

7. Light and Matter

8. 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?

9. 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

10. 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

11. Wavelength – frequency relationship

12. 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
= ×108 m/s (in vacuum)
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13. The electromagnetic spectrum
400 nm
700 nm
Examples of electromagnetic radiation:
visible light x-rays
microwave ovens radio and TV signals

14. 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 = ×108 m/s (in vac.)

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

16. Quantization of energy

17. Photon energies
 = wavelength (m)
n = Frequency (Hz, s-1)
c = ×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?