In this lesson students will perform and analyze two single displacement reactions and prepare and manipulate Test Tube Diagrams to depict the activity at the molecular level. Using manipulatives representing individual ions, atoms and molecules for the various reactants and products, they will accurately represent species in the solid, gaseous and aqueous states by correlating the Test Tube Diagram to the complete ionic equation for each reaction. They will determine the reactants and products responsible for color, as well as identify which species is oxidized and which is reduced.
This lesson will help prepare your students to meet the performance expectations in the following standards:
- HS-PS1-5: Apply scientific principles and evidence to provide an explanation about the effects of changing the temperature or concentration of the reacting particles on the rate at which a reaction occurs.
- HS-PS1-7: Use mathematical representation to support the claim that atoms, and therefore mass, are conserved during a chemical reaction.
- HS-PS2-6: Communicate scientific and technical information about why the molecular-level structure is important in the functioning of designed materials.
- Scientific and Engineering Practices:
- Using Mathematics and Computational Thinking
- Developing and Using Models
- Analyzing and Interpreting Data
By the end of this lesson, students should be able to
- Determine whether a single displacement reaction will occur.
- Accurately represent the activity of a single displacement reaction occurring at the molecular level.
- Identify the species that is oxidized and the species that is reduced.
- Demonstrate the Law of Conservation of Mass using manipulatives and relate it to a balanced chemical equation.
- Depict the progress of a reaction using postulates of the Kinetic Molecular Theory.
This lesson supports students’ understanding of
- Single Displacement Reactions
- Activity Series
- Law of Conservation of Mass
- Redox Reactions
- Net Ionic Equations
- Kinetic Molecular Theory
- Evidence of a Chemical Reaction
Teacher Preparation: 30 minutes–2 hours (dependent on need to create manipulatives)
- Engage: 5–10 minutes
- Explore: 60–100 min
- Explain: 20–30 minutes
- Elaborate: 30–60 minutes
- Evaluate: 20–30 minutes
Note: Ideally Explore through Evaluate are happening simultaneously. The writing prompt (Elaborate) may be completed after the activity.
- 10 cm piece of copper wire, sanded (wire coiled around a pencil is more interesting visually.)
- 10 mL of 0.1 M silver nitrate solution
- 1 gas collection bottle of oxygen
- 20 cm rolled piece of steel wool
- Bunsen burner
- Glass plate
- Gas collection trough
- 1 cm x 10 cm Zn strip
- 0.2 M CuSO4
- 10 cm Cu wire
- 0.2 M ZnSO4
(Explore & Explain)
Per lab group:
- 2, 5cm strips of Magnesium ribbon
- 2, 1cm x 5cm Aluminum strips/foil
- 15 mL of 1 M Nitric acid
- 30 mL of 0.2 M Copper (II) chloride solution
- Before & After Test Tube Diagram sheets (cardstock and/or laminated)
- Various colors of cardstock/paper for cut-outs:
- Silver or gray (Al & Mg)
- Brown (Cu)
- Blue (Cu2+)
- White (Al3+, H1+, Mg2+, NO31-, Cl1-, and H2)
- Student Kit Contents Procedure 1 (see teacher notes for photo example)
- 4 silver Mg atom cards, 4 white Mg2+ ion cards, 8 white H1+ ion cards, 4 white H2 molecule cards, and 8 white NO31- ion cards.
- Student Kit Contents Procedure 2 (see teacher notes for photo example)
- 4 silver Al atom cards, 4 white Al3+ ion cards, 8 brown Cu atom cards, 8 blue Cu2+ ion cards, and 16 white Cl1- ion cards.
- Students should wear proper safety gear during chemistry demonstrations. Safety goggles and lab apron are required. Gloves may also be worn.
- Procedure 1 produces hydrogen gas. If a burning splint is brought to the mouth of the test tube during the first several minutes of production, it may be possible to ignite the hydrogen gas. There should be no open flames at the benches during Procedure 1. Magnesium is the limiting reactant, approximately 0.07 g, and is capable of producing only 0.13 L of hydrogen. Twelve lab pairs will produce only 1.6 L of hydrogen gas over the five to ten minutes required for the reaction to go to completion.
- Always wear safety goggles when handling chemicals in the lab.
- When working with acids, if any solution gets on students’ skin, they should immediately alert you and thoroughly flush their skin with water.
- Procedure 2 requires a hot plate or a burner.
- Always use caution around open flames. Keep flames away from flammable substances.
- Always be aware of an open flame. Do not reach over it. Tie back hair, and secure loose clothing.
- Open flames can cause burns.
- Exercise caution when using a heat source. Hot plates should be turned off and unplugged as soon as they are no longer needed.
- When lighting a match, be cautious with the flame.
- An operational fire extinguisher should be in the classroom.
- When students complete the lab, instruct them how to clean up their materials and dispose of any chemicals.
- Students should wash their hands thoroughly before leaving the lab.
- For more chemical information refer to this link.
- For disposal information refer to this link.
- Preparation: Like any other chemistry lab, metal and solution preparation will require 15 – 30 minutes depending on whether the teacher cuts the Mg and Al into strips or has students cut them.
- The atom/molecule/ion cards for each lab pair could require a couple hours to prepare the first time if you do all the cutting yourself. Make extra sheets while initially copying on the appropriately colored cardstock or paper to replace lost cards in the student kits. Zip-Lock bags or baby food jars can be used for the lab pair sets. Numbering them may make students feel more accountable for maintaining the inventory in each student set. Keep Procedure 1 and Procedure 2 cards in separate containers.
- Each class can make sure the correct number of each card is in the kit and replace missing cards. Have the extra cards in Ziplocs at the ready for this. The kit for Procedure 2 only contains 40 cards and is still a quick count for two people, especially since there are several colors and both sides of the cards have the formula. Each kit has several extra cards for each species, just in case. This also allows the kits to be used to illustrate limiting/excess reactants during a unit on stoichiometry. Once the student atom/molecule/ion card sets are originally prepared, they can be used year after year.
- For Procedure 1 you will need one discard beaker labeled Discard Procedure 1: Mg + HNO3. This beaker should contain only Mg(NO3)2, the excess HNO3, and perhaps some Mg (unlikely).
- For Procedure 2 you will need three discard beakers labeled 1) Discard Procedure 2 Reaction: Al + CuCl2, 2) Discard Procedure 2: Reference CuCl2, and Discard Procedure 2: Reference Al.
- Engage: Although the focus here is on single displacement reactions, the thinking can and should be extended more generally to any oxidation/reduction process. Three quick demos will catch their attention.
- Demo 1: The classic copper wire in a solution of silver nitrate can be used. Write the following equation on the board with the help of the students and the Activity Series.
- Cu (s) + AgNO3 (aq) → Cu(NO3)2 (aq) + Ag (s)
- Dip the sanded Cu wire in and immediately take it out.
- The color change on the copper wire indicates just how quickly the reaction occurs.
- Replace it in the solution and pass it around the class.
- Warn students that silver nitrate solution will stain the skin.
- Place a stopper on the test tube before passing it around if you wish. Have students pass it quickly the first time around. Then pass it around again so they can inspect it more thoroughly. Students are amazed that the “fuzz, mold” growing on the clean copper wire is actually pure silver.
- Considering that the chemical equation is on the board, their inability to correlate the chemical equation (the abstract) with their observations (the concrete) is apparent. One of the goals of this activity is to address these gaps in knowledge and to correlate students’ observations of the chemical reaction with the written chemical equation.
- Demo 2: Similarly, write the equation for the reaction between Zn and CuSO4 solution on the board.
- Dip the zinc strip into the CuSO4 solution. Students will observe an immediate reaction.
- Consult the Activity Series.
- Now write the equation for the reaction between Cu and ZnSO4 on the board.
- Dip the Cu wire into the ZnSO4 solution. Students will observe no reaction.
- Consult the Activity Series.
- Demo 3:
- Using a gas collection trough, prepare a bottle of oxygen by the catalytic (MnO2, KI or yeast) decomposition of 6% hydrogen peroxide.
- Ignite a 20 cm long rolled piece of course steel wool in a burner, quickly place it into the bottle of oxygen, and observe the fireworks.
- Metals love oxygen and oxygen loves metals.
- Explore: This is where students observe, at the macroscopic level, two single displacement reactions and try to reconcile their observations with the molecular level activity responsible for the changes they witness. They will perform each reaction, write molecular and ionic equations for each reaction and prepare Test Tube Diagrams. The instructor can view the diagrams and quickly ascertain student understanding of the difference between a neutral atom and its aqueous ion, and a solid, gaseous or aqueous species, what species are responsible for the colors observed, electron activity and collision theory.
- Students can predict products when given reactants and balance chemical equations without having any real sense of the activity at the molecular level, without understanding the chemistry occurring.
- Ask students where the piece of aluminum is in the test tube, or the hydrogen or the copper ion. Ask them how they know. Ask them if they have realistically shown each of those in their Test Tube Diagrams.
- Were copper atoms or aluminum atoms created or destroyed? Were more electrons lost than gained? What do their diagrams say about these issues?
- Ask them to move a hydrogen ion around until it is in a position to accept an electron from a magnesium atom.
- Help the students focus on the main ideas – atoms have to collide in order to react; atoms, compounds and solutions are electrically neutral; atoms are conserved; mass is conserved; oxidation/reduction are components of a single process; some species is responsible for the color we see, or none of the species yield a color; and that we need to somehow protect metals from reacting when we use them to construct bridges, buildings, cars, tableware …
- Prior to this lab I have already introduced students to general reaction types including single displacement reactions. They are familiar with the Activity Series and how it is used. They have predicted whether a reaction will occur and have seen examples that did not yield a reaction, copper in zinc sulfate or my wedding ring in hydrochloric acid, for example.
- Students have also hand drawn test tube diagrams representing the species present in before and after test tubes. However, as this lab activity demonstrates, they have very limited understanding of the abstraction that is a balanced complete ionic equation. As they play with the manipulatives at each bench there are numerous “Ah-Hah!” moments followed by “That makes sense!” comments. Physical states, oxidation states, collision theory all come to light as they physically run and observe the progress of the reactions while simultaneously and accurately creating diagrams to account for the chemistry they are witnessing.
- Student lab Procedure #1: Single Displacement / Test Tube Diagram Manipulatives:
- These Test Tube Diagrams show the ions in solution well organized, nice to read but unrealistic. The Mg and H2 are realistically placed. Note Mg (s) is on the bottom and H2 (g) is up and out of the test tube. H1+ (aq) and NO31- (aq) are floating in the solution. Also note that the Mg (s) has been completely converted to Mg2+ in the After Test Tube. It was the limiting reactant. If you would like to show that HNO3 is in excess, add another molecule or two of it to both the Before and After test tubes. For this lab have students place stoichiometric amounts, as indicated in the balanced chemical equation (1 Mg & 2 HNO3; 1 Mg2+ 2 NO31- & 1 H2 ), in the Before & After test tubes.
- These Test Tube Diagrams show ions more randomly placed, more realistically. Note Mg (s) is on the bottom, H2 (g) is up and out of the test tube and Mg2+ (aq) and NO31- (aq) are floating in the solution.
- This Before Diagram has the H1+ ions striking the Mg (s) providing for the opportunity for electron exchange leading to the formation of H2 (g) and Mg2+. Students can randomly move the H1+ ions around in the beaker until they strike the Mg emphasizing that reaction occurs when these species collide. The Mg atoms (solid) cannot move about in the beaker. However, the aqueous H1+ ions are free to move throughout the solution and will eventually bump into the Mg ribbon. In practice, students should note that this occurs instantaneously and continuously. And that collisions become less likely as the concentration of the H1+ decreases.
- Student lab Procedure #2: Single Displacement / Test Tube Diagram Manipulatives:
- These Test Tube Diagrams show ions well organized, nice to read but unrealistic. For problem set, quiz and test questions, this may be a good way to have students draw them for ease of counting for both the student and the teacher. Note Al (s) & Cu (s) are on the bottom. Cu2+ (aq), Cl1- (aq) and Al3+ are floating in the solution. Also note that the Al (s) is completely gone. This time, in reality, the solid is not the limiting reactant, the metal ions in solution are limiting. CuCl2 is the limiting reactant. If you would like to show that Al is in excess, add another atom or two of it to both the Before and After test tubes. Since Al remains after reaction ceases and since the Cu2+ ions are responsible for the blue color of the solution, which has completely disappeared after 5 or 10 minutes, the Cu2+ ions (CuCl2) must be limiting.
- These Test Tube Diagrams show ions more randomly placed, more realistically. Note Al(s) & Cu (s) are on the bottom. Cu2+ (aq), Cl1- (aq) and Al3+ (aq) are floating in the solution. The teacher can evaluate this aspect of the student Test Tube Diagrams as he/she walks about the lab checking and discussing student diagrams. At the end of the class, the teacher can highlight or have students highlight all of the main points while the diagrams are still intact.
- This Before Diagram has the Cu2+ ions striking the Al0 (s) providing for the opportunity for electron exchange leading to the formation of Al3+ (aq) and Cu0 (s) Students can randomly move the Cu2+ ions around in the beaker until they strike the Al emphasizing that reaction occurs when these species collide. The Al atoms (solid) cannot move about in the beaker. However, the aqueous Cu2+ ions are free to move throughout the beaker and will eventually bump into the Al strip. In practice, students should note that this occurs instantaneously and continuously. As this reaction progresses students can witness the disappearance of the blue color as the concentration of Cu2+ decreases.
- Explain: I find it convenient to have students artificially line the ions up on the diagrams. As you walk around the room you can quickly count ions this way and then randomize the diagram yourself, or start to and have the students finish randomizing it. You can ask the students questions about the locations, changes in colors, charges, etc. of the various species. Use the student handout to guide you in asking questions of the students. If a misconception arises consistently, you can address the entire class at once and have them adjust their diagrams to make a point or to clarify something. You can emphasize the balance of charge that has to exist in both Before and After test tubes. You can use the diagrams to illustrate the conservation of atoms and of mass. All of these concepts are addressed on their lab reports. Have them complete the student questions while they have the diagrams to refer to and to manipulate. Remember the manipulatives have the colored species, the students’ hand-written reports will not. If you wish, have the students color their hand-written reports: gray or silver (Mg & Al), brown (Cu), and blue (Cu2+).
- Extend/Elaborate: Questions on the lab report address the concept of oxidation/reduction more generally.
- Students poured some of the solution from Procedure 2 onto a watch glass. Question 16 asks them what they expect to find on the watch glass once the water evaporates. Remember to have students check the watch glasses in a few days.
- How to prevent Metals from Corroding is a good resource for the “how metals can be protected” research for students.
- The Writing prompts give a new scenario for students to evaluate in terms of particle locations, charges, oxidation/reduction and colors. This is intended to be given a day or two after this activity. See Collins Writing Approach for more information.
- Additional Information: These kits can also be used when you discuss stoichiometry. Use them as above to demonstrate stoichiometric amounts. Add one extra formula for the excess reactant than the number called for stoichiometrically. For example, if 50.0 g of Mg reacts with 50.0 g of HNO3, then Mg is in excess. Place a second Mg card in the Before diagram – the balanced equation calls for one Mg. In the After diagram, place one Mg2+ card to represent the magnesium that reacted and also one Mg card representing the excess Mg that did not react. If 5.00 g of Mg reacts with 50.0 g of HNO3, then HNO3 is in excess. Place three H1+ cards and three NO31- cards in the Before diagram. In the After diagram, place one H2 card, one H1+ card and one NO31- card. You can show the H1+ moving about in the After diagram “looking” for a Mg to react with. But there is no Mg, only Mg2+.
- See photo below for Student Kit Contents for Reaction #1: Mg + HNO3
- See photo below for Student Kit Contents for Reaction #2: Al + CuCl2