What Can Destroy a Metal?

Transcription

1 Activity 4 What Can Destroy a Metal? GOALS In this activity you will: Use proper materials to light an LED and explain the procedure. Use the Metal Activity Series to determine which metal of a given pair of metals is most reactive. What Do You Think? Since you have to light an LED as the final step of your Chapter Challenge, you will have to build an electrical circuit somewhere in your apparatus. If you wanted to destroy a metal wire in a circuit by pouring a chemical on it, what kind of chemical would you use? Would all metals be destroyed equally well, or would there be some difference between metals? Record your ideas about these questions in your log. Be prepared to discuss your responses with your small group and the class. Investigate Safety goggles and a lab apron must be worn at all times in a chemistry lab. Part A: Destroy the Circuit and Light the LED 1. You will use the LED (light-emitting diode) that you need to light as part of your Chemical Dominoes apparatus in this activity. The first thing you need to figure out is how to get an LED to light. For electricity to flow there must be a complete circuit that starts with a power source and ends back at that power source. Efficient circuits use good conductors such as metals. Set up a circuit with a battery, a 100- (ohm) resistor, a 10- resistor, a piece of metal (marked with an X), and the LED as shown on the next page. 287

2 Chemical Dominoes If you have not learned how to read a schematic diagram, the same circuit is repeated with all the wires shown. (Notice that one end of the LED is longer than the other end. LEDs must be in the correct orientation to work properly because they only let electricity pass through in one direction.) piece of metal LED LED Now that you have learned how to create a complete circuit to light an LED, you need to learn how different chemicals affect materials you have used in your circuit. Your teacher will give you three dropper bottles containing mystery chemicals 1, 2, and 3. These mystery chemicals will all destroy the magnesium in the circuit. First, practice dropping some of the mystery chemical on tiny pieces of magnesium on a safe surface. a) Record all observations that will help you tell the mystery chemicals apart, both in how they look and in how they interact with the magnesium. b) Next, decide which mystery chemical does the best job of destroying the circuit. Write down what it does, and why it is the best one. 3. Dispose of the materials as directed by your teacher. Clean up your workstation. Be careful with the chemicals. Do not get them on your skin. Do not breathe fumes from the chemicals or the reaction with the aluminum. Clean up all spills immediately. Wash your hands and arms thoroughly after the activity. In this design, there are two circuits. Some of the electricity flows through the 10 resistor and other electricity flows through the LED. By removing the piece of metal, all electricity flows through the LED. This extra electricity effectively turns the LED on. If your circuit does not behave in this way, you may have hooked up the LED incorrectly. Reverse the LED and try it again. a) Record your results. 2. In your Chemical Dominoes challenge, you can break the circuit by destroying the piece of metal chemically rather than removing it by hand. To do this, you must investigate metals and their behavior. Part B: What Kinds of Chemicals Affect Metals? 1. Chemists use a list called the activity series of metals (as shown on the next page) to help them figure out which metals are more reactive with other chemicals and which ones are less reactive. Read the following statements. Wires in electrical circuits are usually made of copper. Gold, silver, and platinum are considered precious metals and are used for making jewelry. When you use an aluminum pot for cooking, it soon gets dull. As a cheaper alternative to making pennies, the U.S. government now makes pennies 288

3 Activity 4 What Can Destroy a Metal? using a zinc filling that is covered with a copper shell. If they used just zinc, the pennies wouldn t last very long. 2. On the basis of the statements in Step 1, in which direction does the activity run in the table. Which end is most active and which end is least active? a) Record your answer in your log. 3. When a metal atom reacts, it gives up one or more electrons and becomes a positively charged ion. For example, when sodium and chlorine react, an ionic compound is formed, NaCl (table salt). Metals can exist in two forms: as neutral metal atoms in a piece of shiny metal as positively charged metal ions in salts that can be dissolved in water to form a solution 4. Different metal atoms give up different numbers of electrons. a) Copy the Metal Activity Series table into your log and add another column titled, Changes when atoms give up electrons. 5. The equation that shows how a silver atom gives up electrons and becomes a silver ion is Ag Ag 1e What this means is that when a neutral atom of Ag gives up one electron, the Ag ion has a charge of 1. The information will be useful as you compare how much one chemical substance can affect another. You can reverse a chemical equation. The reversed equation shows how a positively charged metal ion gains electrons. Ag 1e Ag 6. In order for electrons to be available to be gained by an ion, another atom has to have given up the electrons. Below is an example of how electrons are transferred between silver ions and zinc atoms. Ag 1e Ag Zn Zn 2 2e a) How many electrons does each zinc atom give up? b) How many electrons does each silver ion accept? c) Can one zinc atom transfer its electrons to one silver ion? d) If one zinc atom gives up electrons, how many silver ions will have to be present to accept electrons? 7. This change is represented in the chemical equation below. Since the zinc atoms and silver ions interact with each other, the two equations that represent the interaction can be combined. The combined equation (Net Ionic Equation) is shown in the box. The combined reaction could also be written the other way around as: 2Ag Zn 2 2Ag Zn This equation represents the change that occurs when two neutral silver atoms lose one electron each to a zinc ion. Activity Series of Metals Name Symbol lithium Li potassium K calcium Ca sodium Na magnesium Mg aluminum Al zinc Zn iron Fe tin Sn lead Pb hydrogen* H copper Cu mercury Hg silver Ag platinum Pt gold Au * Hydrogen is not a metal but it behaves like a metal in some chemical reactions so it is included in the activity series. (Ag 1e Ag) 2 2Ag 2e 2Ag (Zn Zn 2 2e ) 1 Zn Zn 2 2e 2Ag Zn 2Ag Zn 2 289

4 Chemical Dominoes Be careful with the HCl. Do not get it on your skin. Do not breathe fumes from the chemicals or the reaction with the aluminum. Clean up all spills immediately. Only one of these two reactions actually occurs in nature. The activity series tells you which one will occur. a) The activity series shows that some metals (toward the top of the list) react more easily with most metal ion solutions than other metals do. Use the activity series to determine which of the two reactions actually occur in nature. Look at which metal is closer to the top of the activity series. 8. Now use aluminum and copper as an example. When Al gives up its electrons: Al Al 3 3e When Cu 2 receives the electrons: Cu 2 2e Cu The same number of electrons must be given up as are gained, so you have to use the least common multiple of 2 and 3, which is 6. If the interaction occurs as represented by the equation below, aluminum atoms would give up electrons to copper ions. (Al Al 3 3e ) 2 2Al 2Al 3 6e (Cu 2 2e Cu) 3 3Cu 2 6e 3Cu 2Al 3Cu 2 2Al 3 3Cu An equation that represents the opposite change is shown below. That equation represents copper atoms giving up electrons to aluminum ions. 2Al 3 3Cu 2Al 3Cu 2 a) As before, only one of these two reactions occurs naturally. Use the activity series to predict which one will occur. Part C: Investigating the Activity Series of Metals 1. You will receive a set of six neutral metals and solutions of salts that contain metal ions. With this set, you will be able to investigate the activity series of metals. The set should contain most or all of the chemicals listed in the table. You will also experiment with hydrogen, to compare to the metals. You will not use neutral hydrogen (which occurs as H 2 gas) in this experiment because it s too dangerous. However, you will be provided with hydrogen ions in solution, made from HCl dissolved in water. a) First, list all the possible pairs of metals to try and all possible metals to try out with hydrogen. For example, you will test aluminum metal with copper ions from CuCl 2 and you will test copper metal with aluminum ions from AlCl 3. There should be 21 possible pairs if you have all the metals on the list, plus hydrogen. b) Next, divide up all of the possible pairs among groups in your class, so that each group has about the same number of pairs to test. Record the pairs your group will use. c) Write both possible interactions for each pair that your group will test. d) Use the activity series to predict which interaction will occur for each pair of metals you will test. For each prediction, explain your reasoning. 2. Pair up with a member of another group and walk that person through what you did for one of the pairs of metals. Include an explanation 290

5 Activity 4 What Can Destroy a Metal? of how you predicted which interaction would occur naturally. 3. Return to your group, and test the interactions your group is assigned. For hydrogen, you will only be able to test the interaction in one direction. Place about 10 drops of the solution in one well in a 24-well microplate. Then, using forceps, place the piece of metal into the well. Make sure the metal piece breaks the surface of the liquid so that there is opportunity for the metal and the metal ions to interact. When observing, sometimes it is helpful to view the wells with different colors of paper underneath. It is also helpful to look at the well from below to look for changes. a) Record your observations if a reaction occurred. If nothing happened note that as well. b) Compile the entire class s results. Now compare all of the predictions based on the activity series with all of the results that you observed. Do they match? 4. Dispose of the materials as directed by your teacher. Clean up your workstation. 5. Look back at the data you recorded about the mystery chemicals when you tested them on magnesium (Part A, Step 3). Use those observations and results you just collected to figure out what mystery chemicals 1, 2, and 3 were. a) Record your findings. Safety goggles and a lab apron must be worn at all times in a chemistry lab. If you spill any chemicals on your skin, wash your skin with water. Report any spills to your teacher. Wash your hands and arms thoroughly after the activity. Neutral metals aluminum copper iron magnesium silver zinc Metal ions in solution aluminum ions from AlCl 3 crystals that break apart into ions when added to water copper (II) ions from CuCl 2 crystals in water iron (II) ions from FeCl 2 crystals in water magnesium ions from MgCl 2 crystals in water silver ions from AgNO 3 crystals that break apart into ions when added to water (AgCl doesn t break apart well enough to give enough Ag ions) zinc ions from ZnCl 2 crystals in water 291

6 Chemical Dominoes Chem Words metals: a class of materials that exhibit the properties of conductivity, malleability, reactivity, and ductility. Metallic elements lose electrons more easily than nonmetallic elements, so they form positive ions in ionic compounds. REDOX REACTIONS What is a Metal? Metals are shiny (or can be polished to shine). They also have other important properties. They conduct electricity, so they are used in electrical circuits. They conduct heat, so they are used in cookware. Since most metals can withstand high temperatures, they are used to build strong structures. They can be pounded into different shapes with a hammer, and can be made into nails, flat surfaces, or boxes. Most of the elements in the periodic table are metals. If you draw a diagonal line from boron (B, # 5) down to astatine (At, # 85), the metals are all the elements to the left of the line, including elements and at the bottom of the table. Most of these elements don t occur very commonly in nature as pure metals. The majority of metals are more reactive than hydrogen, and are most commonly found in nature in their ionic forms, as positively charged ions involved in solid crystals or dissolved in water. The Origin of the Activity Series Chemists have experimented with the activities of metals for many hundreds of years. Much of chemistry grew out of the field of alchemy, which was popular in medieval times. The goal of an alchemist was to create gold out of other metals. You now know that it is not possible to do this by chemical means, because the identity of an atom is specified by how many protons are in its nucleus (the meaning of the atomic number on the periodic table). When an atom interacts with another, only the arrangement of electrons is changed. In the process of experimenting with metals, however, alchemists discovered that some metals react more easily with most metal ion solutions than other metals do. They developed the activity series from 292

7 Activity 4 What Can Destroy a Metal? their observations. Since then, chemists have measured these more accurately. There are now ways to measure numbers and figure out where a metal (or any other chemical) belongs in the activity series. Special Reactions between Metals and Metal Ions The reactions that can occur between neutral metal atoms, such as Zn, and metal ions, such as Cu 2, are part of a special class of reactions called oxidation-reduction reactions ( redox for short). Oxidation is defined as giving up electrons, so all of the equations you listed in the activity series represented oxidations. Chem Words oxidation-reduction (redox) reaction: a chemical reaction (change) in which electrons are transferred from one substance to another. oxidation: a loss of one or more electrons. reduction: a gain of one or more electrons. Zinc metal oxidizes: Zn Zn 2 2e Reduction is receiving electrons. An oxidation equation turned backwards represents a reduction. Copper ions reduce: Cu 2 2e Cu As you found out in the activity, when an oxidation happens, a reduction also occurs. Electrons that are given up have to be received by something. So, oxidation changes and reduction changes are often called half-reactions because you need both halves for a reaction to happen. You can use half-reactions to balance redox equations by making sure the number of electrons lost is equal to the number of electrons gained. For example, when zinc metal oxidizes, copper ions reduce: Zinc metal oxidizes: Zn Zn 2 2e Copper ions reduce: Cu 2 2e Cu Since the same number of electrons are lost as are gained, the half-reactions can be added as they are, and the electrons will cancel. This gives the redox equation: Zn Cu 2 Zn 2 Cu You learned that although you can write the equation in two directions and you can test both equations, the reaction will occur naturally in only one direction. For example, for the pair of metals Zn and Cu, the two possible equations are as follows: Zn Cu 2 Zn 2 Cu or Cu Zn 2 Cu 2 Zn The activity series can be used to predict which equation represents a reaction that will occur. From the activity series you can see 293

8 Chemical Dominoes Checking Up 1. List four properties that define metals. For each property, give one way that metals are used by humans to take advantage of that property. 2. Where are most metals found and in what form? 3. How was the activity series developed? 4. a) Define oxidation. b) Define reduction. c) How are they related? 5. What is the activity series useful for predicting, in terms of redox reactions? that zinc is a more reactive metal than copper. That is, copper is less likely to react than zinc. The first reaction on the previous page is zinc metal reacting with copper ions. The second reaction is copper metal reacting with zinc ions. Since zinc is more reactive than copper, the activity series leads you to predict that the first reaction will occur and the second reaction will not. You were able to test for this during the activity. When Is a Metal Not a Metal? You may wonder why hydrogen is included in the Metal Activity Series when it is clearly not a metal. Metals, in general, are solid, shiny, ductile, malleable, and conductive of heat and electricity. Hydrogen is none of these since it is a gas. However, hydrogen takes on the positive nature of a metal in its role in strong acids, such as hydrochloric (HCl vs. NaCl), sulfuric (H 2 SO 4 vs. Na 2 SO 4 ), and nitric (HNO 3 vs. NaNO 3 ) acids. The most important reason to include hydrogen in the Metal Activity Series is because these acids are simple and convenient reagents which can quickly establish where an unknown metal stands in the series. For example, if a metal reacts with dilute HCl, then it is above hydrogen in the series. If it does not react with dilute HCl, then it lies below hydrogen in the series. What Do You Think Now? At the beginning of this activity you were asked: If you wanted to destroy a metal wire in a circuit by pouring a chemical on it, what kind of chemical would you use? Would all metals be destroyed equally well, or would there be some difference between metals? Now that you have studied circuits and the metals that can be used to make them, how would you answer these questions? 294

9 Activity 4 What Can Destroy a Metal? What does it mean? Chemistry explains a macroscopic phenomenon (what you observe) with a description of what happens at the nanoscopic level (atoms and molecules) using symbolic structures as a way to communicate. Complete the chart below in your log. How do you know? You learned in this activity that a redox reaction will only occur naturally in one direction. You then tested various combinations of metals and metal ions to compare experimental results with the activity series. How consistent were your results with the activity series? Why do you believe? MACRO NANO SYMBOLIC In this activity, you tested a variety of combinations of metals, metal ions, and acids to determine reactivity. Describe some of your observations that indicated a reaction had occurred. The activity series determines how humans can use a variety of metals. Examples given in the text include using very unreactive metals for wiring, jewelry, building structures, etc. Describe other situations where it would be necessary to use a very reactive metal to achieve a desired result. Why should you care? Explain how electrons, atoms, and ions were involved in the chemical reactions you observed in this activity. You have learned how to light an LED using a circuit. You have also learned you can make circuits out of aluminum and then destroy them. Lighting the LED is a mandatory part of your challenge. You may also want to destroy a circuit in your apparatus. From what metals other than aluminum could you make circuits? What characteristics should a metal have if it is to be used in a circuit? Do these characteristics change if you plan to destroy the metal, causing electricity to flow through a different circuit? How could you make a circuit that will suddenly begin to operate when you destroy some metal somewhere else in the circuit? Include a diagram of the circuit including the wires, LED, and battery. Show how half-reactions can be represented in equations that show the gain and loss of electrons. 295

10 Chemical Dominoes Reflecting on the Activity and the Challenge You have learned how to light an LED using a circuit. You have also learned you can make circuits out of aluminum and then destroy them. The final step of your dominoes apparatus must be the lighting of an LED. At this point, you can choose between completing a circuit or destroying an existing circuit to cause the LED to light. You might make a wire fall into place to complete a circuit that includes the LED and battery. You could also cause one path through an already operating circuit to be destroyed. If you have designed your circuit correctly, that would force the electricity to flow through the path that includes the LED, which would cause it to light. In a later activity, you will learn about how batteries are constructed and how they operate. This will give you additional options for how to light the LED. 1. Using the periodic table, identify which of the following elements are metals: Na, Cl, Ni, Co, Ar, Sn, Cd, Sr, P. 2. Some metals are more reactive than hydrogen, and others are less reactive. This is explained in greater detail in the Chem Talk section. a) Based on your experiments in this activity, which metals that your class tested are more reactive than hydrogen? b) Which metals that your class tested are less reactive than hydrogen? 3. For each pair below, indicate which metal is more reactive. a) Al or Pb b) Au or Fe c) Sn or Al 4. Choose one of the pairs in the previous question and write the balanced redox reaction that should occur naturally. You will need to research how many electrons the metal you haven t worked with gives up. Show your work. 5. Why is it incorrect to say, When aluminum metal and copper ions were mixed, the aluminum melted? What is correct to say? Explain why. 6. Many nails and other types of hardware are made out of steel, which contains mostly iron metal. The trouble is that iron rusts, especially when it is exposed to water and air. Galvanized nails contain zinc to prevent the nails from rusting. Explain how this works, using what you know from the activity series and your experiments. (Hint: What happened with your Fe/Zn pair in the lab?) 7. Given the balanced ionic equation: 2Al(s) 3Cu 2 (aq) 2Al 3 (aq) 3Cu(s) Compared to the total charge of the reactants, the total charge of the products is a) less b) greater c) the same 296

11 Activity 4 What Can Destroy a Metal? 8. Given the balanced ionic equation: Zn(s) Cu 2 (aq) Zn 2 (aq) Cu(s) Which equation represents the oxidation half-reaction? a) Zn(s) 2e Zn 2 (aq) b) Zn(s) Zn 2 (aq) 2e c) Cu 2 (aq) Cu(s) 2e d) Cu 2 (aq) 2e Cu(s) 9. When a lithium atom forms an Li ion, the lithium atom a) gains a proton b) gains an electron c) loses a proton d) loses an electron 10. Given the redox reaction: Cr 3 Al Cr Al 3 As the reaction takes place, there is a transfer of a) electrons from Al to Cr 3 b) electrons from Cr 3 to Al c) protons from Al to Cr 3 d) protons from Cr 3 to Al 11. Which metal reacts spontaneously with a solution containing zinc ions? a) magnesium b) nickel c) copper d) silver Inquiring Further Reacting metals with bases While acids can be defined as chemicals that split into H ions and an anion when dissolved in water, bases can be defined as chemicals that split into a cation and OH ions when dissolved in water. You learned in this Investigate section that acids react with some metals, including aluminum. Some metals also react with bases. Design an experiment to test which metals react with bases, and whether the activity series can also help predict this. Have your experiment approved by your teacher, and then carry it out. This will give you more options for reactions you can use in your Chemical Dominoes apparatus. Be careful with chemicals. Clean up spills and wash hands and arms well. 297