General Chemistry Principles, Patterns, and Applications, 2011

a. Explain why the emitted light shifts to longer wavelength at higher temperatures. (Hint:
consider the expansion of the crystal and the resulting changes in orbital interactions when
heated.)
b. Why does the relative intensity of the emitted light decrease as the temperature
increases?
2. A large fraction of electrical energy is currently lost as heat during transmission due to the electrical
resistance of transmission wires. How could superconducting technology improve the transmission of
electrical power? What are some potential drawbacks of this technology?
3. Light-emitting diodes (LEDs) are semiconductor-based devices that are used in consumer electronics products
ranging from digital clocks to fiber-optic telephone transmission lines. The color of the emitted light is
determined in part by the band gap of the semiconductor. Electrons can be promoted to the conduction
band and return to the valence band by emitting light or by increasing the magnitude of atomic vibrations in
the crystal, which increases its temperature. If you wanted to increase the efficiency of an LED display, and
thereby the intensity of the emitted light, would you increase or decrease the operating temperature of the
LED? Explain your answer.
4. ♦ Strips of pure Au and Al are often used in close proximity to each other on circuit boards. As the boards
become warm during use, however, the metals can diffuse, forming a purple alloy known as “the purple
plague” between the strips. Because the alloy is electrically conductive, the board short-circuits. A structural
analysis of the purple alloy showed that its structure contained a face-centered cubic (fcc) lattice of atoms of
one element, with atoms of the other element occupying tetrahedral holes. What type of alloy is this? Which
element is most likely to form the fcc lattice? Which element is most likely to occupy the tetrahedral holes?
Explain your answers. What is the empirical formula of the “purple plague”?
5. ♦ Glasses are mixtures of oxides, the main component of which is silica (SiO 2 ). Silica is called the glass former,
while additives are referred to as glass modifiers. The crystalline lattice of the glass former breaks down
during heating, producing the random atomic arrangements typical of a liquid. Adding a modifier and cooling
the melt rapidly produces a glass. How does the three-dimensional structure of the glass differ from that of
the crystalline glass former? Would you expect the melting point of a glass to be higher or lower than that of
pure SiO 2 ? Lead glass, a particular favorite of the Romans, was formed by adding lead oxide as the modifier.
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