Introto Chem Penny Lab PDF

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TURNING A PENNY SILVER AND GOLD

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Before coming to the laboratory, read the Discussion Section below and complete the PreLab Exercise found in the Florida Online system (under “Quizzes” or “Content” tab)! OBJECTIVES: When this experiment is completed you should have: 9

Observed the behaviors of pennies and steel washers treated with zinc and heating.

9

Learned about some of the physical properties of copper and steel.

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Developed lab techniques to handle hot substances.

DISCUSSION: Physical Properties of Metals and Alloys Timberlake &Timberlake (course textbook) references: Chapter 4, Sections 1-4. For hundreds of years, the main goal of alchemists was to find a way to turn inexpensive metals like lead or copper into valuable metals such as silver and gold. In this lab, you will have the opportunity to turn an inexpensive penny into a “silver” and “gold” coin. Of course, it will only look silver or gold in appearance, since we now know that you cannot fundamentally change the chemical identity of one element into another by simply heating or applying a chemical solution! You might wonder why two elements are so different from each other and what makes them unique. The answer is in the makeup of the atoms, the smallest building blocks of matter. Every element is a pure substance containing only one kind of atom and the atoms of each element are unique from the atoms of all other elements. The key to their differences lies in the smaller subatomic particles which are found in the atoms. These are the protons, neutrons, and electrons. Protons are positively charged particles which are found only in the atom’s nucleus, the very small, dense center of an atom. Neutrons are uncharged particles which are also only found in the nucleus. Both of these particles are much heavier than the electrons, which are negatively charged particles found moving around in the very large, outer space surrounding the nucleus. The identity of an element is determined by the number of protons it contains. This number is called the atomic number and is displayed on the Periodic Table of the Elements along with the element’s chemical symbol (the one- or two-letter abbreviation for the name of an element). For example, gold has a chemical symbol of Au and an atomic number of 79. Thus, every gold atom must contain 79 protons. If we could add another proton to a gold atom, it would become an atom of mercury (chemical symbol Hg) since it would then have 80 protons. Clearly, the properties of gold and mercury are very different from one another! Indeed, each element has unique chemical and physical properties. One category of elements, the metals, shares some similar properties. In general, metals are shiny solids (at room temperature) which can be drawn into wires (ductile) and hammered into a flat sheet (malleable). They also are good conductors of heat and electricity. If two or more metals are mixed together Copyright 2013, Daytona State College. May not be reproduced or altered without permission from the authors. For information, email [email protected]. Revised: 8/21/2015

TURNING A PENNY SILVER AND GOLD

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in their molten state, the resulting homogeneous mixture is referred to as an alloy. Some common alloys are steel (mainly iron with carbon), stainless steel (iron, carbon, nickel, and chromium), bronze (copper and tin), brass (copper and zinc), and pewter (tin, lead, antimony, copper, and zinc). The property which we will be concerned with in this lab is the melting point of various metals. Shown below is a table of melting points for some common metals: Melting Points of Some Common Metals*** Metal or Alloy Temperature (oC) 232 tin (pure Sn) 420 zinc (pure Zn) 660 aluminum (pure Al) 962 silver (pure Ag) 1064 gold (pure Au) 1083 copper (pure Cu) 1453 nickel (pure Ni) ~1315 - 1510 steel (alloy of Fe, C, and sometimes Cr) 1535 iron (pure Fe) 1857 chromium (pure Cr) 3500 carbon (pure C) ***Source: www.chemicalelements.com When we look at a U.S. penny, we see its outer copper color and might assume that it is made of pure copper. However, pure copper pennies were only made from 1793 to 1837. Since then, the composition of pennies has been changed over the years for various reasons. It has been made from alloys such as bronze, brass, NS-2 (copper and nickel), and zinc-coated steel (during World War II). Currently (since 1982), pennies contain a zinc core (97.6%) with a copper plating (2.4%). Scientific Observations vs. Conclusions Often times as scientists, we conduct experiments and watch to see what happens. When we describe our results, it is important to differentiate between what we observe (our observations) and what we think our data is telling us about the experiment (our conclusions). Students often confuse these two when they are watching something familiar and already understand what is happening. For example, it you are watching a candle burning, your observations would be that the candle gets very hot, the wax melts into a liquid, light is given off by the flame, and you see smoke emitted from the candle. If you watch long enough, you would observe that the candle gets shorter. If you test to see what happens when the candle is placed inside of a sealed container, the observation would be that the flame is extinguished. Based upon your experiments, then, your conclusions might be that the burning candle consumes wax (it gets shorter) and that it requires some sort of gas. From this limited experimentation, you would not be able to conclude that the gas was oxygen, even though you have learned this before! You could propose that the gas is oxygen as a hypothesis (a tentative, unproven explanation), but you would need to conduct some additional experiment in order to prove this. Be careful that you distinguish between what you see (observations) and what you can logically conclude based ONLY on what you see (conclusions) as you think about your experiments. Copyright 2013, Daytona State College. May not be reproduced or altered without permission from the authors. For information, email [email protected]. Revised: 8/21/2015

TURNING A PENNY SILVER AND GOLD

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PROCEDURE: 1.

Obtain 2-clean 250 mL beakers and a stir rod from your tray. Mark one beaker ‘C’ and use it for the cleaning solution. Mark the other ‘D’ and use it for the distilled water rinse. Also, obtain a pair of tongs or forceps from the tray or the common equipment drawer.

2.

Pour about 15 mL of sodium chloride/vinegar cleaning solution into the beaker marked ‘C’. Fill the beaker marked ‘D’ about half full with distilled water.

3.

Clean two pennies by placing them in the sodium chloride/vinegar cleaning solution. Stir the pennies using the stir rod until they are shiny (it will take about 30-40 sec.).

4.

Remove the pennies using tongs or forceps and rinse them off thoroughly with distilled water in the beaker ‘D’. Dry the pennies completely with a paper towel. Note: Do not handle the clean pennies with your hands. The oils from your skin may interfere with the zinc-plating reaction.

5.

Obtain two washers and clean them using sodium chloride/vinegar cleaning solution and repeating the steps 3 and 4 above.

6.

Record your initial observations of the appearance of the clean pennies and washers in Table 1 of your Post-Lab Report.

7.

Place the 250 mL beaker containing the zinc chloride and granular zinc solution on a hot plate set to a medium heat setting. Carefully and gently heat the mixture until the solution is just under the boiling point. Do not boil the solution.

8.

Using tongs, carefully place the two pennies and washers in the zinc chloride/zinc solution. Using the tongs, gently turn the pennies and the washers over several times until you see a change in color on both sides of the pennies. This may take 4 – 5 minutes. Do not boil the solution.

9.

Remove the pennies and the washers using the tongs. Caution: They will be very hot. Carefully dip each of them in the beaker of distilled water to cool them off. Remove the pennies and washers from water and shine them with a paper towel.

10.

Record your observations of the appearance of the pennies and the washers after treatment with zinc chloride and granular zinc solution in Table 1 of your Post-Lab Report.

11.

Fill the beaker ‘D’ with a fresh portion of distilled water. Using tongs, carefully place a treated penny and a washer on the hot plate. To avoid burning, flip the penny and the washer every 30 seconds. Continue heating until you see a change in color of the penny.

12.

Carefully remove the penny and the washer from the hot plate using your tongs and immediately dip both in the distilled water in beaker ‘D’. Place them on a paper towel to

Copyright 2013, Daytona State College. May not be reproduced or altered without permission from the authors. For information, email [email protected]. Revised: 8/21/2015

TURNING A PENNY SILVER AND GOLD

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dry. The penny and the washer will be extremely hot and should be handled with tongs until they are cooled for several minutes. Remove the zinc chloride/zinc beaker from the hot plate and allow it to cool. 13.

Record your observations of the appearance of the penny and the washer after gentle heating in Table 1 of your Post-Lab Report.

14.

Obtain a ring stand from the side bench. Connect a Bunsen burner to the gas line and light it using a striker. You can adjust the flame height by turning the needle valve at the base of the burner. You may also need to rotate the barrel of the burner (long tube with holes at the base) to change the air intake until you get a proper blue flame (with an inner and outer blue cones). Remember to use good safety practices while working with your open flame (e.g., tie long hair back, remove stray papers from the bench, keep any bottles of acetone away from the flame, etc.).

15.

Hold the Bunsen burner at the base in a slight (60o) angle as shown in the Figure. Using your tongs, take the heated penny (from Step 12) and heat it vigorously in the hottest part of the flame. Once it is very hot, try pushing the penny against the base of your ring stand (NOT against the bench top or your plastic tray!) and see what happens to it. Record your observations in Table 1 of your Post-Lab Report. Be careful that the hot penny will take more time than you think for it to cool down, so wait to touch it (or dip it in the water again to be sure!).

16.

Try vigorously heating the washer from Step 12 in the flame and pushing it against the base of the ring stand, as in Step 15. Record your observations for the washer in Table 1.

17.

Turn off your Bunsen burner. Return the zinc chloride/zinc solution and ring stand to the side lab bench for the next class. DO NOT dispose the zinc chloride/zinc solution.

18.

Dispose of the sodium chloride/vinegar cleaning solution down the sink. Place used pennies and washers in appropriate waste beakers in hood.

19.

Wash the beakers and the stir rod and return all equipment to its proper location. Clean your work area and wash your hands before leaving the laboratory.

Copyright 2013, Daytona State College. May not be reproduced or altered without permission from the authors. For information, email [email protected]. Revised: 8/21/2015

SILVER & GOLD PENNY: POST-LABORATORY REPORT Name

Date

Lab Partner Table 1. Observations of Pennies and Steel Washers Type of Treatment

Observations for Pennies

Observations for Steel Washers

Initial Appearance After Cleaning

After Treatment with Zinc chloride/Zinc Solution

After Gentle Heating on the Hot Plate

After Vigorous Heating Using Bunsen Burner

Copyright 2013, Daytona State College. May not be reproduced or altered without permission from the authors. For information, email [email protected]. Revised: 8/21/2015

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SILVER & GOLD PENNY: POST-LABORATORY REPORT Name

Date

Lab Partner DATA ANALYSIS: 1.

Fill in the following table:

What color was the penny after you cleaned it with the sodium chloride/vinegar solution? What color was the penny after treatment with the zinc/zinc chloride solution? Was the color change CHEMICAL or PHYSICAL? Briefly describe your evidence (remember from the last lab what kinds of evidence support each type of change):

What color was the penny after you heated it gently on the hot plate? Was the color change CHEMICAL or PHYSICAL? Briefly describe your evidence (remember from the last lab what kinds of evidence support each type of change):

2.

What is an alloy?

3.

What is the alloy formed when copper and zinc are mixed?

4.

List 3 physical characteristics of the alloy you wrote in #3.

Copyright 2013, Daytona State College. May not be reproduced or altered without permission from the authors. For information, email [email protected]. Revised: 8/21/2015

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SILVER & GOLD PENNY: POST-LABORATORY REPORT Name

Date

Lab Partner 5.

Fill in the following table:

What color was the washer after you cleaned it with the sodium chloride/vinegar solution? What color was the washer after treatment with the zinc/zinc chloride solution? Was the color change CHEMICAL or PHYSICAL? Briefly describe your evidence (remember from the last lab what kinds of evidence support each type of change):

What color was the washer after you heated it gently on the hot plate? Was the color change CHEMICAL or PHYSICAL? Briefly describe your evidence (remember from the last lab what kinds of evidence support each type of change):

6.

Fill in the following table: Substance

Melting point (°C)

Zinc Copper Steel

Copyright 2013, Daytona State College. May not be reproduced or altered without permission from the authors. For information, email [email protected]. Revised: 8/21/2015

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SILVER & GOLD PENNY: POST-LABORATORY REPORT Name

Date

Lab Partner 7.

Fill in the following table:

What happened to the penny after vigorous heating? What happened to the washer after vigorous heating The flame in a Bunsen burner can reach a temperature of approximately 750 - 950 oC. Explain your observations about the penny and the washer.

8.

Using the table of elements located in the front cover of your textbook (or a similar reference), fill in the chemical symbols and atomic numbers in the table below. Name of Element

Chemical Symbol

Atomic Number

# of Protons

Lead Tin Copper Gold Silver

9.

Suppose that you wanted to convert an atom of copper into an atom of gold, and you had a nuclear reactor handy to accomplish it. What would you have to do to the number of protons in the copper atom to accomplish this task?

Copyright 2013, Daytona State College. May not be reproduced or altered without permission from the authors. For information, email [email protected]. Revised: 8/21/2015

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