Edufy

Activity Details
Edufy

Details

Objective

  • Formulate questions about what they think might have happened. 
  • Make observations using both qualitative and quantitative statements. 
  • Observe 4 signs of a chemical change: 
            color change- the solution goes from blue to almost colorless 
            energy change- the reaction is highly exothermic- the temperature changes more than 50 degrees C 
            a gas is evolved- hydrogen gas is liberated 
            a precipitate forms- there is solid copper as a brownish red precipitate 
 

Materials

Materials for each group of students:

  • 2 clear plastic solo cups, to hold the reaction (these will slightly melt from the heat from the reaction)
  • large test tube, in which to perform a flame test
  • ~5cm x ~5cm square of aluminum foil, to be used during the reaction
  • 180 mL bottle of copper chloride solution (see below for details)
  • thermometer, for observing temperature change during the reaction
  • something to stir the solution (a spoon or the thermometer work well)
  • water, for rinsing copper precipitate
  • something to hold copper precipitate during flame test (metal spatula, tweezers, or thongs work well)
  • graduated cylinder, to measure fluids
 
To make copper chloride solution (50,000 PPM Cu ions), dissolve 134 grams of CuCl2-2H2O in 886 mL of water to make 1 liter of solution 
 
optional:
  • rust, to show what rust is for students who don't know (steel wool that has rusted works well)
  • turquoise, to show what this looks like for students who don't know
 

Procedure

Procedure 

1. Fold the aluminum foil in half, then in half again to make a 2.5cm square.      
 
2. Fold the corners so that the foil fits in the bottom of the cup. 
 
3. Pour 25 mL of the copper chloride solution onto the aluminum foil. 
 
4. Use the thermometer to stir the solution and record the highest temperature reached during the reaction.
 
5. Use the spoon to push the corners down of the foil so that it is completely covered. 
 
6. As the reaction proceeds, record as many observations as you can.
 
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7. What evidence did you observe that a chemical reaction took place? Give three examples. 
 
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PART 1 - GAS: 
1. If you saw bubbles, what did you think was happening?
 
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2. How would you go about proving or disproving that the gas you generated was water vapor (steam)? 
 
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3. What was your highest temperature observed during the reaction? ________
 
4. Since water boils at 100*C, that couldn't be boiling water. What else could this gas be?
 
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5. Our atmosphere is made up of 78% nitrogen (N2) and 21% oxygen (O2), which are clear gasses, so they are hard to measure. How can we measure the gasses coming from the cup in this investigation?
 
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6. Using a new method discussed by your teacher to determine what gas you might have. Record your observations:
 
 
7. We started with CuCl2 (copper chloride), H2O (water), and Al (aluminum). The result was H2 (hydrogen), Cl2 (chlorine), and Al (aluminum). Which of these might burn?
 
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PART 2 - PRECIPITATE: (in literature means 'falling', in science means 'what sinks after a reaction takes place' )
 
1. Record your observations of the precipitate that formed after the reaction. ("That's rust" is not an observation, but "That looks like rust" is.)
 
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2. The precipitate isn't rust. Please write down everything you know about rust and how it forms.
 
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3. Take some precipitate from your cup.
 
4. Wash it thoroughly in water, and examine the precipitate more closely, recording your observations.
 
5. Light your burner, and perform a flame test on a small amount of precipitate, recording your observations.
 
6. Copper compounds usually have a blue or green color. Turquoise is a compound of copper. How effective does the aluminum appear to be in removing thee copper from the copper chloride solution?  What is your evidence? 
 
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7. Perform a flame test on a small amount of the blue copper chloride solution, recording your observations.
 
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Teacher Notes

Teachers Notes:

This is a great lab to pursue some misconceptions with simple measurements and tests.  I also like to use plastic solo cups for this since the cup will slightly melt from the heat and it gives an added dimension to the ooh and ahh effect! See questions to use before you begin the class discussion which follows. 
 
"What did you think the bubbles were?" They inevitably claim it was the solution's water boiling. I then ask them to recall the boiling temperature of water- 100 degrees C.  From their measurements with the thermometer they see that the highest temperature reached is around 70, too low for water to boil. I emphasize the importance of this quantitative measurement and then ask how we might find out what the gas was. This leads to me or them suggesting a flame test. We test the gas(you have ready a large test tube with the copper chloride solution and a cylinder shaped- rolled piece of foil which you insert in the tube), The gas burns- I ask what it might be by examining the reactants: 
                        we have           CuCl2           H2O   and    Al 
                        we've eliminated H2O.....   so the gas could be    H2     Cl2  or    Al 
 
Since aluminum melts at a very high temperature, we can rule it out,and chlorine is a yellow green gas, not flammable, we are left with hydrogen. This process stimulates the use of physical properties and their emerging inductive capacity to arrive at this conclusion. 
 
Next we turn to the precipitate. Again they will invariable say it is rust. Ask them what is rust exactly? Most will know its formed from iron. Again refer to the reactants- there is no iron there. This is a very powerful realization at an early point in their chemical career in your class. They realize and understand that you can't produce something from nothing- and will come to understand that in a balanced chemical reaction, the reactants must equal the products, matter is conserved. The law of conservation of mass will soon follow. 
 
I suggest that we examine the precipitate more closely and take some from one of their cups, wash it thoroughly in water, and then take a small amount and place it on the end of a metal spatula. I light the burner, and perform a flame test. The bluish-green flame helps us to identify copper, I then take the blue copper chloride solution and perform a flame test on it. Aha! There again is the blue color. I suggest that we will soon discover that this type of reaction is called replacement where the aluminum has replaced the copper. We will study this more later in the class. 
 
The importance here is for the teacher to use the student's own experience to discover how valuable observations can be and through them how the essential principles of science unfold. This is the key to the learning cycle, and is what forms the basis for discovery and excitement in students. They will "own" this knowledge and it will make their learning experience a key for motivation in your class and in their lives. 
The actual chemical reaction is actually very complex. The overall reaction is:  
 
6Al  +  6CuCl2  +  6H2O  -->  2Al(OH)3 + 4AlCl3 + 6Cu + 3H2 
 
I also like to point out that aluminum metal foil has now disappeared. The aluminum must be somewhere in the solution. If we were able to reverse this reaction and obtain aluminum metal, it would take a great deal of energy. I like to point out that this is the best reason to recycle aluminum- most of its costs-95%- comes from the energy (electricity) needed to refine it. You may be studying this later in the year- but again it's a great opportunity to make this connection- especially when a national goal of our energy policy is energy efficiency! 

Information

The overall reaction is: 

     4Al(s)  +  3CuCl2(aq) +  6H2O (l)  --> 2Al(OH)3(aq)  + 2AlCl3(aq)  + 3Cu(s)  +  3H2(g) 
 
Reaction1 Single replacement/oxidation of Aluminum and reduction of copper(II) ions: 
3Cu2+   +   6e-  -->  3Cu(s) 
          2Al(s)       --> 2Al3+  +  6e- 
3Cu2+   + 2Al(s) -->  3Cu(s) +   2Al3+ 
 
Reaction2  Reaction of Aluminum with water (after the acidic CuCl2 solution removes the oxide coating of the aluminum foil): 
 
6H-OH  + 6e-  --> 3H2(g)  +  6OH- 
         2 Al(s)    -->  2 Al3+   +  6e- 
6H-OH  +  2Al(s) --> 3H2(g)+  2Al3+  +  6OH- 
 
CombiningReactions 1 and 2 we get: 
3Cu2+   + 2Al(s)                 --> 3Cu(s)  +   2Al3+ 
6H-OH  +   2Al(s)                -->  3H2(g)+  2Al3+  +  6OH- 
3Cu2+   + 6H-OH +   4Al(s) --> 3Cu(s) + 3H2(g) +  4Al3+  +  6OH- 
 
Adding the spectator ions 6Cl-, and combining them with Al ions we get 2AlCl3 
Adding the aluminum ions to the hydroxide ions we get the overall balanced equation: 
3CuCl2(aq)+  4Al(s) +6H2O(l) --> 3Cu(s) +  3H2(g) +  2AlCl3(aq)  +  2Al(OH)3(s) 
 

References

Shared on the NSTA list serv by Mike Reeske, Valley Center, CA, USA

Standards

Virginia CH.1  More Info 

Virginia CH.2  More Info 

Virginia CH.5  More Info 

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Mark Moverman on September 05, 2013

Have you ever done this lab quantitatively as a stoichiometry exercise? If so, how well did it work? Also, what separation procedure did you use?