Back
- The semiconductor CdSe has a band gap of 1.74 eV. What wavelength of light would be emitted from an LED made from CdSe? What region of the electromagnetic spectrum does this correspond to?
Problem 77
- The first LEDs were made from GaAs, which has a band gap of 1.43 eV. What wavelength of light would be emitted from an LED made from GaAs? What region of the electromagnetic spectrum does this light correspond to: ultraviolet, visible, or infrared?
Problem 78
- GaAs and GaP make solid solutions that have the same crystal structure as the parent materials, with As and P randomly distributed throughout the crystal. GaPxAs1 - x exists for any value of x. If we assume that the band gap varies linearly with composition between x = 0 and x = 1, estimate the band gap for GaP0.5As0.5. (GaAs and GaP band gaps are 1.43 eV and 2.26 eV, respectively.) What wavelength of light does this correspond to?
Problem 79
Problem 81
(a) What is a monomer? (b) Which of these molecules can be used as a monomer: ethanol, ethene (also called ethylene), methane?
- The molecular formula of n-decane is CH3(CH2)8CH3. Decane is not considered a polymer, whereas polyethylene is. What is the distinction?
Problem 82
- State whether each of these numbers is a reasonable value for a polymer’s molecular weight: 100 amu, 10,000 amu, 100,000 amu, 1,000,000 amu.
Problem 83
- Indicate whether the following statement is true or false: For an addition polymerization, there are no by-products of the reaction (assuming 100% yield).
Problem 84
- An ester is a compound formed by a condensation reaction between a carboxylic acid and an alcohol that eliminates a water molecule. Read the discussion of esters in Section 24.4 and then give an example of a reaction forming an ester. How might this kind of reaction be extended to form a polymer (a polyester)?
Problem 85
Problem 86
Write a balanced chemical equation for the formation of a polymer via a condensation reaction from the monomers succinic acid 1HOOCCH2CH2COOH2 and ethylenediamine 1H2NCH2CH2NH22.
- An addition polymerization forms the polymer originally used as Saran™ wrap. It has the following structure −[CCl2−CH2]−n. Draw the structure of the monomer.
Problem 87
Problem 88b
Write the chemical equation that represents the formation of (b) polyacrylonitrile from acrylonitrile (polyacrylonitrile is used in home furnishings, craft yarns, clothing, and many other items).
- (a) What molecular features make a polymer flexible? (b) If you cross-link a polymer, is it more flexible or less flexible than it was before?
Problem 91
Problem 92
What molecular structural features cause high-density polyethylene to be denser than low-density polyethylene?
- If you want to make a polymer for plastic wrap, should you strive to make a polymer that has a high or low degree of crystallinity?
Problem 93
Problem 94
Indicate whether each statement is true or false: (a) Elastomers are rubbery solids. (b) Thermosets cannot be reshaped. (c) Thermoplastic polymers can be recycled.
- Explain why “bands” may not be the most accurate description of bonding in a solid when the solid has nanoscale dimensions.
Problem 95
Problem 96a
CdS has a band gap of 2.4 eV. If large crystals of CdS are illuminated with ultraviolet light, they emit light equal to the band gap energy. (a) What color is the emitted light?
Problem 96b
CdS has a band gap of 2.4 eV. If large crystals of CdS are illuminated with ultraviolet light, they emit light equal to the band gap energy. (b) Would appropriately sized CdS quantum dots be able to emit blue light?
Problem 96c
CdS has a band gap of 2.4 eV. If large crystals of CdS are illuminated with ultraviolet light, they emit light equal to the band gap energy. (c) What about red light?
- Indicate whether each statement is true or false: (a) The band gap of a semiconductor decreases as the particle size decreases in the 1–10-nm range. (b) The light that is emitted from a semiconductor, upon external stimulation, becomes longer in wavelength as the particle size of the semiconductor decreases.
Problem 97
- Indicate whether this statement is true or false: If you want a semiconductor that emits blue light, you could either use a material that has a band gap corresponding to the energy of a blue photon or you could use a material that has a smaller band gap but make an appropriately sized nanoparticle of the same material.
Problem 98
Problem 101
Which statement correctly describes a difference between graphene and graphite? (a) Graphene is a molecule but graphite is not. (b) Graphene is a single sheet of carbon atoms and graphite contains many, and larger, sheets of carbon atoms. (c) Graphene is an insulator but graphite is a metal. (d) Graphite is pure carbon but graphene is not. (e) The carbons are sp2 hybridized in graphene but sp3 hybridized in graphite.
- What evidence supports the notion that buckyballs are actual molecules and not extended materials? (a) Buckyballs are made of carbon. (b) Buckyballs have a well-defined atomic structure and molecular weight. (c) Buckyballs have a well-defined melting point. (d) Buckyballs are semiconductors. (e) More than one of the previous choices.
Problem 102
- Selected chlorides have the following melting points: NaCl (801 °C), MgCl2 (714 °C), PCl3 (-94 °C), SCl2 (-121 °C). (b) Predict which of the following compounds has a higher melting point: CaCl2 or SiCl4.
Problem 103
Problem 103a
Selected chlorides have the following melting points: NaCl (801 °C), MgCl2 (714 °C), PCl3 (-94 °C), SCl2 (-121 °C) (a) For each compound, indicate what type its solid form is (molecular, metallic, ionic, or covalent-network).
- A face-centered tetragonal lattice is not one of the 14 three-dimensional lattices. Show that a face-centered tetragonal unit cell can be redefined as a body-centered tetragonal lattice with a smaller unit cell.
Problem 104
Problem 105
Imagine the primitive cubic lattice. Now imagine pushing on top of it, straight down. Next, stretch another face by pulling it to the right. All angles remain 90°. What kind of primitive lattice have you made?
Problem 106
Pure iron crystallizes in a body-centered cubic structure, shown in the figure. but small amounts of impurities can stabilize a facecentered cubic structure. Which form of iron has a higher density?
- Introduction of carbon into a metallic lattice generally results in a harder, less ductile substance with lower electrical and thermal conductivities. Explain why this might be so.
Problem 107
- For each of the intermetallic compounds shown in Figure 12.17 determine the number of each type of atom in the unit cell. Do your answers correspond to the ratios expected from the empirical formulas: Ni3Al?
Problem 109
