Understanding the Discrepant Reactivity of Copper in the Presence of Strong Acids Mark as Favorite (8 Favorites)
In this demonstration, students practice their observation skills during the additions of different acids to two test tubes containing copper. The activity is structured to allow students to make thoughtful remarks about what they observe, using rich indicators of both chemical and physical properties and changes. In subsequent lessons on new concepts, students can reflect back on their observations to rationalize the discrepant results of the reactions in the demonstration.
This demonstration will help prepare your students to meet the performance expectations in the following standards:
- HS-PS1-2: Construct and revise an explanation for the outcome of a simple chemical reaction based on the outermost electron states of atoms, trends in the periodic table, and knowledge of the patterns of chemical properties.
- Scientific and Engineering Practices:
- Constructing Explanations and Designing Solutions
- Engaging in Argument from Evidence
By the end of this demonstration, students should be able to
- Identify chemical and physical changes from a chemical reaction.
- Record written evidence for a chemical reaction.
- Reproduce, in written form, an experiment that examines discrepant chemical events.
This demonstration supports students’ understanding of
- Chemical Reactions
- Chemical Change
- Acids & Bases
- Activity Series
- Single Replacement Reactions
Teacher Preparation: 20 minutes
Lesson: 10 minutes
- 5 ml of 12 Molar HCl
- 5 ml of 16 Molar HNO3
- Granulated Copper (~0.2 g)
- 2 Test Tubes
- 2 Test Tube Clamps
- Ring Stand
- Always wear safety goggles and lab coat when handling chemicals in the lab.
- Students should wear proper safety gear during chemistry demonstrations. Safety goggles and lab apron are required.
- Concentrated hydrochloric acid is an extremely corrosive substance. Teachers should wear gloves, lab coat and goggles while handling. It should be contained in the fume hood as its vapors are also hazardous.
- Concentrated nitric acid is a strong oxidizer. Teachers should wear gloves, lab coat and goggles while handling. It should be kept away from any heat source, including sparks, flames and hot surfaces. It should be contained in the fume hood as its vapors are also hazardous.
- It is suggested that this demonstration is used at the beginning of a unit on chemical reactions.
- Teachers should practice this demonstration prior to carrying it out in front of students.
- Demonstration Procedure:
- Attach clamps to ring stand, and place a test tube in each clamp inside of the fume hood (see photograph of set-up).
- Be intentional about talking aloud through the demonstration. Describe what you are doing throughout the event, and emphasize examples of chemical and physical properties/changes you observe.
- Place a small amount (~0.2 g) of granulated copper in each of the two test tubes.
- Explain that a different acid will be added to each test tube.
- Add approximately 5 mL of “Acid A” (concentrated hydrochloric acid, HCl) to the first test tube.
- Students should record their observations in the data table.
- Add approximately 5 mL of “Acid B” (concentrated nitric acid, HNO3) to the second test tube.
- Be sure to handle concentrated acids with care, and to perform the demonstration in the fume hood due to the evolution of noxious fumes from both the acids and subsequent nitrogen dioxide, NO2, gas formation.
- Students again record observations in the data table.
- Expected Results:
- In the first test tube, containing hydrochloric acid and copper, no reaction will occur. The acid will simply rest atop the solid copper granules.
- In the second test tube, containing nitric acid and copper, a vigorous reaction will be observed. Brown nitrogen dioxide, NO2, will escape from the test tube and a vivid blue copper (II) nitrate, Cu(NO3)2, solution will rest at the bottom of the test tube.
- The differences in the two potential reactions engages the students in a scientifically oriented question “Why is there is a difference in addition of Acid A to the copper versus the addition of Acid B to the copper?”
- After the demonstration:
- In a subsequent lesson, students will use their observational data and reflection to write a reproducible procedure of the demonstration. This task helps students understand the sequencing and mechanics of an experiment, and further allows them to systematically compare the two acids in the presence of the copper.
- The students will attempt to explain the discrepant results during lessons later in the unit on chemical reactions. For example, the activity series can be used to explore why copper will not react with hydrogen from either acid. This is sufficient to explain the supposed non-reaction between Cu and HCl, but is not sufficient to explain the vigorous reaction between Cu and HNO 3.
- Students will need to study concepts of oxidation and reduction to fully explain the reaction between Cu and HNO3. Referring back to the demonstration at this point in the unit provides the students with the continued visual cues.
- Additional Information for Subsequent Lessons
- The activity series for metals displays a list of elements that adopt a positive charge when bonding. In terms of a single replacement reaction, the element has to be more reactive than the positive ion it is attempting to replace within a compound. Using the activity series, this means the element has to be above the potentially replaced ion on the list. Copper lower than hydrogen, which means copper cannot replace hydrogen ions within acids.
- Redox concepts can explain why copper will oxidize from 0 to 2+ in the presence of nitric acid. The N5+ within the nitrate ion of nitric acid is reduced to N2+ as it eventually forms NO gas, a precursor to the brown NO2 gas evolved in the demonstration. In truth, redox can also explain that Cu and HCl does indeed elicit a reaction, albeit a very subtle one compared to Cu and HNO3. Copper in the presence of concentrated HCl will produce a faint yellow [CuCl2]- complex that can barely be detected visually during the demonstration. Even so, concepts connected to the formation of this complex are likely beyond the scope of an introductory chemistry course. Students are better served if they focus on using redox concepts to explain just the Cu and HNO3 reaction.
For the Student
Your instructor will perform a demonstration for you today. Your task to observe the reactions and take detailed notes of the events. You will reflect on this demonstration through this unit, so be sure to ask questions if you need clarification.
What questions do you have about the demonstration today?