The Composition of Metals and Alloys

Transcription

1 1 The Composition of Metals and Alloys Metals are shiny, malleable substances that conduct heat and electricity. They comprise the larest class of elements in the Periodic Table. All metals except mercury are solids at room temperature. Solid metals can be melted at sufficiently hih temperatures. Once melted, metals can be mixed with other metals to produce alloys, whose properties are different from those of the individual metals. Bronze (an alloy of copper and tin) and brass (copper and zinc) are two examples. Both bronze and brass are harder than pure copper and thus more useful in applications where strenth is needed. The density of any substance is the mass of the substance per unit volume (d=m/v). Generally this is reported in units of rams per milliliter (/ml) or rams per cubic centimeter (/cm 3 ). Recall that 1 cm 3 is equal to 1 ml. Volume measurements are quite easy for liquids but are a bit more complicated for solids, especially for irreularly shaped solids. If the solid has a reular shape (cube, sphere, cylinder, etc) then it is a relatively simple matter of carefully measurin the objects dimensions. The volume of an irreular object, such as a stone, coin, or metal ornament, must be determined indirectly by displacement of a volume of water correspondin to the volume of the object. After some pre-lab calculations, you will calculate the density of the a quarter usin three different methods. Then you will calculated the density of pre-1982 pennies and post-1983 pennies to see if you can determine what they are made of. Finally, you will determine the density of the bronze you made in the last class period and share your results with the class. Useful information metal density metal density Al 2.70 /ml Cu 8.92 /ml Sn 5.75 /ml A 10.5 /ml Zn 7.10 /ml Pb 11.3 /ml Fe 7.87 /ml Au 19.3 /ml Ni 8.90 /ml Pt 21.5 /ml Quarters are composed of nickel and copper, the densities of which are almost identical, which is not surprisin, since they are adjacent in the periodic table. To measure the density of a quarter you need to know its weiht and volume. The weiht can be accurately measured on an electronic balance. Measurin volume accurately is more difficult. You will measure the volume usin two different methods. First you will measure the thickness and radius of a quarter, then in a second method you will measure its volume by the amount of water a quarter displaces. Experimental Uncertainty A basic principle of measurement is that a final answer is only as precise as the least precise measurement that went into it. This can be estimated with the concept of sinificant fiures. A measurement has as many sinificant fiures as there are readable diits in the answer, not countin any precedin or trailin zeros that are used to mark the position of the decimal point. For example, if you can read an answer as 31.2, it has three sinificant fiures. The number 45 and both have two sinificant fiures. The number 3200 may have 2, 3, or 4 sinificant fiures, dependin on whether one or both of the zeroes were actually measured. If we write it as 3.2 x 10 3, none of the zeros were measured, but simply mark space until the (implied) decimal point. If we write it as 3.20 x 10 3, the number has 3 sinificant fiures.

2 2 measured value assumed uncertainty 47 ± ± ± ± ± ± 0.1 One can estimate the error in a quantity based on the assumed uncertainty. The estimated error as a percentae is the assumed uncertainty divided by the total value, and converted to a percent. For example, the % error in 46 ± 1 is (1/47) x 100 = 2.1%. Likewise, the % error in 46.7 ± 0.1 is (0.1/46.7) x 100 = 0.2%. One can also calculate the % actual error, if one knows the actual (true) value of a quantity. The accepted true value for the density of copper is 8.92 /cm 3. If you measure a value of 8.85 /cm 3 in your experiment, the actual % error is the difference divided by the true value: % actual error = (true value measured value) / (true value) = ( ) / 8.92 = = 0.78 % Time Manaement This experiment will take one full class period. Overview Do pre-lab calculations Calculate density of a quarter by measurin dimensions Calculate density of a quarter usin water displacement method Calculate density of a quarter usin your own method Calculate density of pure copper Calculate density of pre-1982 pennies Calculate density of post-1983 pennies Calculate density of pure your bronze from last experiment Graph bronze density as a function of alloy composition A. Pre-lab Assinments 1. When you made bronze how much malachite did you use? What is the weiht percentae of copper in malachite? % How many rams of copper went into your bonze mixture? How much cassiterite did you use? What is the weiht percentae of tin in cassiterite? % How many rams of tin went into your bonze mixture? What is the percentae tin and copper in your bronze alloy? % Sn % Cu

3 3 2. Electrum is an alloy of old and silver. An ancient coin made of electrum was found to have a density of 11.8 /ml. What is the percentae of old in the coin? B. Measure the density of a quarter (method 1) Materials 100 ml raduated cylinder a quarter electronic balance 1. Weih a quarter on an electronic balance. Note: Be sure to note the exact amount in your lab notebook. 2. Determine the volume of a quarter by measurin its diameter and thickness. The formula for the volume of a cylinder is πr 2 h. 3. Calculate the density. Questions: Hint convert your units so the volume is /cm 3. Show all your calculations in your lab notebook. Of all the numbers used in calculatin the density, which has the smallest number of sinificant fiures? How many sinificant fiures are in this number? Your calculated density will have the same number of sinificant fiures. Rewrite the density, if necessary to show this. 4. Estimate the error as a percentae. Be sure to record all calculations in your lab notebook. C. Measure the density of a quarter (method 2) 1. Fill a 100 ml raduated cylinder to around the 95 ml mark. Note: Be sure to note the exact amount in your lab notebook. 2. Record the water level in the cylinder. Hint read from the bottom of the meniscus. 3. Carefully drop the same quarter you used in the previous density determination into the cylinder without splashin. You may want to use tweezers. Interpolate between the raduations as best you can. Hint If water splashes out of the cylinder, start over. Watch out for air bubbles! They will throw your calculations off.

4 4 4. Re-read the water level. The difference is the volume of the quarter. 5. Use your previously measured mass to find the density. 6. Pour the water out, dry the quarter, and repeat the volume measurement twice more. 7. Calculate the density for each run, then calculate the averae of the three densities. Be sure to record all measurements in your lab notebook. Questions: Sinificant fiures are counted to the last diit that stays constant or varies only slihtly. Which diit is this? This means you should assume how many sinificant fiures in your answer? Rewrite the density, if necessary, with the proper number of sinificant fiures. 8. Estimate the % error from the sinificant fiures. Be sure to record all calculations in your lab notebook. D. Measure the density of a quarter (method 3) 1. Form a roup with two or three other students. Write down the names of your partners. Compare results. 2. As a roup decide on yet another procedure to determine the density, one that you expect to be more accurate than the method in parts B & C. Present your proposal to your instructor. After approval carry it out. Record the procedure carefully enouh that another roup of students could repeat it and check your numbers. Question Is there any reason to prefer one person s results? Explain. E. Measure the density of a copper shot Materials copper shot raduated cylinder 1. Weih a weih boat full of dry copper shot. Do not select pieces with dimples as they may lead to air bubbles. 2. Fill the raduated cylinder about half full of water. Usin the water displacement method, determine the volume of the shot. Hint Watch out for air bubbles! off. 3. Report the density and % actual error. Hint density of copper is iven in the table of densities in the introduction.

5 5 F. Measure the density of pre-1982 pennies Materials 30 pre-1982 pennies raduated cylinder 1. Weih 30 pre-1982 pennies. Hint Watch out for air bubbles! off. Tap the side of the cylinder if bubbles form. 2. Usin the water and raduated cylinder method, determine the volume of the pennies. 4. Based on the density you measured what is the major component in the alloy used to make old pennies? Hint consult the table of densities in the introduction. G. Measure the density of post-1983 pennies Materials 30 post-1983 pennies raduated cylinder 1. Weih 30 post-1983 pennies. Hint Watch out for air bubbles! off. Tap the side of the cylinder if bubbles form. 2. Usin the water displacement method, determine the volume of the pennies. 4. Based on the density you measured what is the major component in the alloy used to make new pennies? Hint consult the table of densities in the introduction. H. Measure the density of your bronze Materials your bronze lump raduated cylinder 1. Calculate the density of the bronze lump you made in the last experiment usin the water displacement method. 2. Collect the class data on the bronze alloy composition (as calculated in the pre-lab) and the density. 3. Usin Excel, or some other spread sheet, plot the density of the bronzes as a function of the % tin. Question How ood is the classes data? Adapted from Chemistry in the Ancient World by Patrick Hoard and The Chemistry of Art and Artifacts by Ruth Beeston