Chapter 1 Basic Concepts of Chemistry

Transcription

1 Chapter 1 INSTRUCTOR S NOTES This chapter discusses a wide range of basic subjects needed for the study of chemistry. How you use the material will depend on the preparation level of your students. Essentially all of the topics in this chapter would be found in a typical high school chemistry class. However, if students took that course two or three years earlier, or if their performance at that time was inconsistent with their present goal of a professional study for science or engineering, an extensive review might be necessary. Such a review could take two or three lecture periods. If your class consists of students who have been screened by a placement exam or other process to reasonably ensure that they have good abilities in high school chemistry, you may assign most or even all of the chapter for outside reading with only a single class to review and stress key points. The chapter can stand on its own without direct instruction if your students learned the topics previously. For an intermediate approach you could assign sections for student reading then take one lecture for the rest of the chapter. A pre-quiz selected from Chapter 1 Study Questions could help establish what degree of review is needed for your students. SUGGESTED DEMONSTRATIONS 1. Density Kolb, K. E.; Kolb, D. K. Method for Separating or Identifying Plastics, Journal of Chemical Education 1991, 68, 348. Franz, D. A. Densities and Miscibilities of Liquids and Liquid Mixtures, Journal of Chemical Education 1991, 68, 594. Checkai, G.; Whitsett, J. Density Demonstration Using Diet Soft Drinks, Journal of Chemical Education 1986, 63, 515. Shakhashiri, B. Z. Density and Miscibility of Liquids, Chemical Demonstrations: A Handbook for Teachers of Chemistry; University of Wisconsin Press, 1989; Vol. 3, pp Properties of Elements Pictures of the elements and their uses can be found on the Periodic Table Videodisc and on the Periodic Table Live! CD-ROM available from JCE Software. 1

2 3. Illustration of Physical Change Liquid nitrogen is always a favorite of students. We freeze a banana, a hot dog, a flower, or similar object. 4. Illustration of Chemical Change The first lecture in the course is often begun with a bang by setting off several hydrogen-filled balloons in a darkened lecture room. The demonstration is described in Chemical Demonstrations: A Handbook for Teachers of Chemistry; University of Wisconsin Press, 1983; Vol. 1, pp The reaction is also shown on the General ChemistryNow CD-ROM, the Periodic Table Videodisc and the Periodic Table Live! CD-ROM. Other reactions could be done as well, depending on the facilities available. Possibilities include placing small pieces of potassium in water, the thermite reaction (Shakhashiri, Volume 1, page 85), or the reaction of zinc and ammonium nitrate (Shakhashiri, Volume 1, page 51). The latter reaction gives off a large amount of ZnO dust and other irritating fumes. It is not suitable for a room without good ventilation. For fun, and to give some color, as well as talking about our future study of acid-base reactions, we add aqueous NH 3 to separate flasks containing (i) very dilute acid with phenolphthalein, (ii) Al(NO 3 ) 3, and (iii) dilute CuSO 4. We often have a student contribute a penny to put into concentrated HNO 3. The reaction brings up a brief discussion of oxidation reduction processes. The NO 2 gas generated prompts a discussion of air pollution problems, as well as the fuels used in the Lunar Lander and in the Space Shuttle. (CAUTION: NO 2 is a very corrosive gas. Use only in a well ventilated room. We do the reaction by putting the penny in a few milliliters of acid in a 2-L Erlenmeyer flask that is lightly stoppered. This effectively contains the gas.) The Periodic Table Videodisc and the Periodic Table Live! CD-ROM can be used to explore reactions of the elements with air, water, acids, and bases. Examples include the reaction of potassium with water and the reaction of cobalt with HCl and HNO 3. 2

3 SOLUTIONS TO STUDY QUESTIONS 1.1 (a) C Carbon (c) Cl Chlorine (e) Mg Magnesium (b) K Potassium (d) P Phosphorus (f) Ni Nickel 1.2 (a) Mn Manganese (c) Na Sodium (e) Xe Xenon (b) Cu Copper (d) Br Bromine (f) Fe Iron 1.3 (a) barium Ba (d) lead Pb (b) titanium Ti (e) arsenic As (c) chromium Cr (f) zinc Zn 1.4 (a) silver Ag (d) tin Sn (b) aluminum Al (e) technetium Tc (c) plutonium Pu (f) krypton Kr 1.5 (a) NaCl is a compound; sodium is an element (b) Sugar is a compound; carbon is an element. (c) Gold chloride is a compound; gold is an element. 1.6 (a) Pt(NH 3 ) 2 Cl 2 is a compound; Pt is an element (b) Copper is an element; copper(ii) oxide is a compound (c) Silicon is an element; sand is a compound 1.7 (a) physical property (b) chemical property (c) chemical property (d) physical property (e) physical property (f) physical property 1.8 (a) chemical change (b) physical change (c) chemical change (d) physical change 1.9 (a) Physical properties: color (colorless), physical state (liquid) Chemical property: reactivity (burns in air) (b) Physical properties: color (shiny metal, orange), physical state (liquid) Chemical property: reactivity (aluminum reacts readily with bromine) 1.10 (a) Physical properties: color (white), physical state (solid), density (2.71 g/cm 3 ) Chemical properties: reactivity towards acid (reacts to produce gaseous carbon dioxide) (b) Physical property: color (gray zinc, purple iodine, white compound) Chemical property: reactivity (zinc and iodine react to give a white compound) 3

4 1.11 (a) Qualitative observations: blue-green color, solid physical state Quantitative observations: density of 2.65 g/cm 3, mass of 2.5 g, length of 4.6 cm (b) Mass and length are extensive properties, color, physical state, and density are intensive properties (c) 2.5 g 1 cm g = 0.94 cm Physical properties: Hydrogen and oxygen are colorless gases while iron and sodium are both solids at room temperature. Chemical properties: Hydrogen and oxygen react to form water. Sodium reacts violently with water. Iron reacts with oxygen to form rust Observations (c), (e), and (f) identify physical properties (a) Copper, Cu Carbon, C Oxygen: O (b) Oxygen is a gas, while copper, carbon, and aquamarine are solids at room temperature. Oxygen is colorless, while copper has a reddish color and carbon is gray. The gemstone is a bluish color Calcium: Ca Fluorine: F The shape of the fluorite crystals can be described as interwoven cubes. The overall shape of the crystals indicates that the ions in the solid matrix arrange themselves with alternating calcium and fluoride ions to produce the crystal appearance The non-uniform appearance of the mixture indicates that samples taken from different regions of the mixture would be different a characteristic of a heterogeneous mixture. Recalling that iron is attracted to a magnetic field while sand is generally not attracted in this way suggests that passing a magnet through the mixture would separate the sand and iron The large colorless block of salt represents the macroscopic view. The spheres represent the microscopic or particulate view. If one can imagine producing multiple copies of the particulate view, the macroscopic view will result Liquids: mercury and water Solid: copper Of the substances shown, mercury is most dense and water is least dense The plastic (with a much lower density than CCl 4 ) will float. The aluminum (which is more dense than CCl 4 ) will sink Melting point. Sugar melts around ºC while salt melts at 800 ºC The two liquids will be layered, with hexane floating on top of perfluorohexane. HDPE and PVC will lie at the bottom of the hexane layer (HDPE on top of the PVC). Teflon will sink to the bottom of the perfluorohexane layer Milk is mostly water. When water freezes its volume increases (its density decreases). When the milk froze, the increase in volume was so great that it pushed out of the bottle. 4

5 1.23 HDPE will float in liquids with a density higher than 0.97 g/ml, ethylene glycol, water, acetic acid, and glycerol The mass of the object is determined and then the volume is determined by submersion in a known volume of liquid. The increase in volume would be equal to the volume of the irregularly shaped object. The density could be calculated by dividing its mass by its volume (a) solid iron (b) liquid water (c) water vapor 1.26 (d) water vapor, helium (e) liquid water, solid aluminum (f) brass 1.27 The sample's density and melting point could be compared with that of silver to prove whether or not the sample is silver One could check for an odor, check the boiling or freezing point, or determine the density. If the density is approximately 1 g/cm 3 at room temperature, the liquid could be water. If it boils at about 100 ºC and freezes at about 0 ºC, that would be consistent with water. To check for the presence of salt, boil the liquid away. If a substance remains, it could be salt, but further testing would be required If too much sugar is excreted, the density of urine will increase. If too much water is excreted, the density of urine will decrease Least dense liquid: water Medium density: carbon tetrachloride Most dense liquid: mercury 5

6 1.31 (a) Solid potassium reacts with liquid water to produce gaseous hydrogen and a homogeneous mixture of potassium hydroxide in liquid water. (b) The reaction is a chemical change. (c) Potassium and water are reactants, hydrogen and potassium hydroxide are products. (d) Among the qualitative observations are (i) the reaction is violent; and (ii) heat and light (a purple flame) are produced A copper-colored metal could be copper, but it may also be an alloy of copper, for example, brass or bronze. Testing the material s density and melting temperature would be one way to find out if it is copper (a) The water could be evaporated by heating the solution, leaving the salt behind. (b) Use of a magnet would attract the iron filings away from the lead. (c) Mixing the solids with water would dissolve only the sugar. Filtration would separate the solid sulfur from the solution. Finally, the sugar could be separated from the water by boiling the solution Any balloon filled with a gas having a density less than 1.12 g/l will float in air. Helium and neon balloons will float One possible method is outlined on Screen Separate the iron from a weighed sample of cereal by passing a magnet through a mixture of cereal and water after the flakes have disintegrated. Remove the iron flakes from the magnet and weigh them to determine the mass of iron in the mass of cereal (a) P 4 and Cl 2 are reactants, PCl 3 is the product (b) P 4 molecules are a tetrahedron (a four-sided polyhedron) and Cl 2 molecules consist of two chlorine atoms. PCl 3 molecules contain a phosphorus atom bonded to three chlorine atoms in a triangular pyramid Physical change 6