1 CONSERVATION OF MATTER AND CHEMICAL PROPERTIES I. OBJECTIVES AND BACKGROUND The object of this experiment is to demonstrate the conservation of matter- or more particularly, the conservation of "atoms" in chemical reactions and how this conservation is represented using balanced chemical equations. Beginning with copper metal, you will perform a series of chemical reactions; the last of which will result in the formation of copper metal. The Law of Conservation of Matter suggests that the mass of copper you start with will be the same as the copper you recover at the end--how close can you get to this? II. CHEMICALS AND EQUIPMENT Balance Glass stirring rod Graduated cylinder 2-50 ml beakers 150 ml beaker Büchner funnel Vacuum filter flask Filter paper Red litmus paper Disposable plastic pipet Cu (copper) foil Mg (magnesium) ribbon 6M HNO3 solution, nitric acid 6 M NaOH solution, sodium hydroxide 6 M HCl solution, hydrochloric acid Celite acetone
2 III. PROCEDURE Preparation of Cu(NO3)2 from Cu metal: copper (solid) + nitric acid (aqueous solution) copper(ii) nitrate (aqueous solution) + nitrogen dioxide (gas) + water (liquid) 1. While one lab partner is obtaining the copper and performing the first reaction step, the other partner can set up the Büchner funnel apparatus in Step 4. Place a 50 ml beaker on a lab balance and press the tare button to zero the balance. Add copper foil to the beaker until the mass reading is between 0.28 and 0.32g. (Caution: do not try to tear the copper foil with your hands; you may end up cutting yourself. Use the scissors provided. Record the mass to as many decimal places as the balance gives you. Place the beaker under a hood and use a plastic pipet to slowly add 5mL of 6M HNO3 one ml at a time (CAUTION!! NITRIC ACID IS HIGHLY CORROSIVE. AVOID ALLOWING IT TO COME IN CONTACT WITH YOUR SKIN, EYES OR CLOTHING. IF THIS SHOULD HAPPEN, WASH THE AFFECTED AREA WITH COLD WATER AND ASK YOUR TA FOR HELP). The brownish red gas which is evolved is nitrogen dioxide and the bluish color in the solution is due to the copper(ii) nitrate produced. Keep the beaker in the hood until no more of the gas is visible. 2. Take the beaker back to your bench area. Once all of the copper metal has dissolved, very slowly and carefully dilute the solution by adding about 5 ml of distilled water.(the addition of water to concentrated acid results in the generation of heat; if the water is added too quickly, the solution will begin to boil and spatter acid.) The resulting light blue solution contains the copper(ii) nitrate product. We could recover this product as a blue crystalline solid by evaporating off the water, if we wished, but we will leave it dissolved in water for the next reaction. Preparation of Cu(OH)2 from Cu(NO3)2: copper(ii) nitrate (aqueous solution) + sodium hydroxide (aqueous solution) copper(ii) hydroxide (solid)+ sodium nitrate (aqueous solution)
3 3. Obtain ~4mL (it doesn't have to be exact!) of 6M NaOH solution (CAUTION: THIS SOLUTION IS CAUSTIC). Using a plastic pipet, add the NaOH solution to to the Cu(NO3)2 solution from Step 2 one ml at a time, stirring after each addition, until 3 ml have been added. The light blue solid (called a precipitate) which forms is the copper(ii) hydroxide. Continue to add NaOH drop by drop until new formation of the blue solid can no longer be observed, then begin testing the solution after each drop with red litmus paper to determine whether there is any unreacted (excess) NaOH present. This can be done by touching a drop of the solution to the litmus paper using the glass stirring rod. You can use each strip of litmus paper several times. Stop adding NaOH as soon as you obtain a positive litmus test (i.e., a blue spot). Do not add a large excess of NaOH. 4. When all of the copper(ii) hydroxide is precipitated (as indicated by a positive litmus test), we can separate it from the soluble sodium nitrate byproduct by filtration. Add one scoopula of Celite to the beaker. Celite is an inert material which makes it easier to filter off the copper hydroxide product without clogging the filter paper. Stir the contents of the beaker thoroughly and before the slurry has a chance to resettle, filter it using a Büchner funnel and vacuum filter flask (Your instructor will demonstrate the use of this equipment). Rinse the beaker two or three times with ~2 ml aliquots of distilled water and pour each of the washings into the funnel. This is done to remove the last traces of solid from the beaker. Allow the vacuum to run until the precipitate has formed a solid matte in the funnel. 5. Turn off the vacuum and using a spatula, remove the filter paper full of precipitate. Place the filter and precipitate in a clean 150 ml beaker, taking care not to lose any of the precipitate. Scrape out as much of the solid remaining in the filter funnel as possible and add it to the beaker. Now examine the solution in the filter flask. If it is nearly colorless, it can be discarded. If it still has a distinct blue color, not all of the copper(ii) nitrate has reacted. If this is the case, pour the solution into a 250 ml beaker and add more NaOH solution. If more precipitate forms, treat it according to steps 3 and 4 and add the additional filter and precipitate to the first filter in the 150 ml beaker.
4 Preparation of CuCl2 from Cu(OH)2: copper(ii) hydroxide (solid) + hydrochloric acid (aqueous solution) copper(ii) chloride (aqueous solution) + water 6. Add about 2 ml of 6M HCl solution (CAUTION: THIS SOLUTION IS CORROSIVE) to the beaker in step 5 containing the precipitate and filter paper and-stir the mixture until the blue copper(ii) hydroxide has dissolved and a pale green milky mixture remains. The filter paper and inert Celite remain unaffected. Clean the Büchner funnel and filter flask by rinsing them two or three times with distilled water. Place a clean piece of filter paper in the funnel and filter the mixture in the beaker. Rinse the residue in the beaker two or three times with 2 ml portions of distilled water, and add each washing to the Büchner funnel. At this point, the filter paper and the Celite in the Büchner funnel should be colorless. The Celite residue remaining in the Büchner funnel and the filter paper may be discarded. The filtrate (the bluish green solution in the filter flask) contains the copper(ii) chloride, which, if we so wished, could be further isolated as a green solid by evaporating off the water. If this liquid appears cloudy due to residual Celite, transfer it to a 50 ml beaker, clean out the funnel and filter flask, and, using a fresh piece of filter paper, repeat the filtration. Preparation of Cu metal from CuCl2: copper(ii) chloride (aqueous solution) + magnesium (solid) copper (solid)+magnesium dichloride (aqueous solution) 7. Pour the liquid in the filter flask into a clean 50 ml beaker and place the beaker under a hood. Obtain ~0.2 g of magnesium. Add ~10 pieces of Mg metal to the solution, taking care to prevent excess foaming. The foaming is due to a side reaction between the Mg metal and the excess HCl which leads to the evolution of dihydrogen gas: Mg(s) + 2 HCl(aq) MgCl2(ag) + H2(g) Use the stirring rod, from time to time, to break up any pieces of Mg metal that have become coated with Cu. As soon as all of the Mg disappears, add more, and repeat this
5 process until the blue-green color of the copper dichloride disappears. (Note: it may not be necessary to add all of the magnesium metal to completely react with the copper(ii) chloride. If you add too much magnesium, it will be time-consuming to get rid of the excess magnesium.) Then add 1 ml of 6M HCl (CAUTION: THIS SOLUTION IS HIGHLY CORROSIVE) to destroy any excess Mg present according to the above equation. The precipitated Cu metal is unaffected by the hydrochloric acid. Wait until the bubbling of dihydrogen gas is completed and no more of the magnesium metal can be observed in the flask. 8. Clean the Büchner funnel and filter flask, place a piece of filter paper in the funnel, and filter off the precipitated Cu metal prepared in step 7. Turn the vacuum off and pour 2-3 ml of acetone onto the Cu (CAUTION!! ACETONE IS FLAMMABLE. THERE SHOULD BE NO OPEN FLAMES PRESENT IN THE LABORATORY). Turn the vacuum back on and, when all of the acetone has passed through the filter, add a second 2-3 ml of acetone, this time keeping the vacuum running. When the filtration is complete, continue drawing air through the filter paper for at least five minutes to dry the Cu precipitate. While your copper is drying, measure the mass of a clean watch glass. 9. Carefully scrape the precipitate off of the filter paper and onto to the watch glass, and measure the mass of the watch glass containing the Cu product. Obtain the mass of the Cu produced as the difference between this mass and the mass of the watch glass alone. IV. DATA AND CALCULATIONS Fill out the Report Sheet on the next page.
6 Report Sheet: Conservation of Matter Name Date Partner's Name Instructor's Initials A. Original mass of Cu used in step 1 B. Mass of Cu and watch glass in step 9 C. Mass of watch glass in step 8 D. Mass of Cu recovered E. Percent of Cu recovered
7 V. QUESTIONS 1. Based on the word description given at the beginning of each section, write a balanced chemical equation for each of the transformations of Cu in this experiment. Be sure to include the states of each of the reactants and products as well. Hint: for the first reaction, use 2 for the coefficient for NO2(g) 2. Classify each of the reactions in question 1. 3. For each of the reactions in question 1, indicate what observations were used to determine when the reactions were completed (e.g., appearance or disappearance of a color, an external chemical test, etc.). 4. Comment on your percentage of recovered Cu. How would you account for the fact that you did not get 100% recovery?