Chemical Reactions of Copper and Percent Yield

CHEMICAL REACTIONS OF COPPER AND PERCENT YIELD KEY Objective To gain familiarity with basic laboratory procedures, some chemistry of a typical transition element, and the concept of percent yield. Apparatus and Chemicals 0.5 g piece of no. 16 or no. 18 copper wire evaporating dish 250 mL beaker (2) weighing paper concentrated HNO3 (4 – 6 mL) 6.0 M H2SO4 (15 mL) graduated cylinder granular zinc 3.0 M NaOH (30 mL) methanol carborundum boiling chips acetone stirring rod towel iron ring and ring stand balance wire gauze aluminum foil cut in 1-inch squares Bunsen burner concentrated HCl (drops) Discussion Most chemical synthesis involves separation and purification of the desired product from unwanted side products. Some methods of separation, such as filtration, sedimentation, decantation, extraction, and sublimation were discussed earlier. This experiment is designed as a quantitative evaluation of your individual laboratory skills in carrying out some of these operations. At the same time you will become more acquainted with two fundamental types of chemical reactions - redox reactions and metathesis (double-displacement) reactions. By means of these reactions, you will finally recover the copper sample with maximum efficiency. The chemical reactions involved are the following. Cu(s) + 4 HNO3(aq) Cu(NO3)2(aq) + 2 NO 2(g) + 2 H2O(l) Cu(NO3)2(aq) + 2 NaOH(aq) Cu(OH)2(s) CuO(s) + CuSO4(aq) CuO(s) H2SO4(aq) + Zn(s) Cu(OH)2(s) + + 2 NaNO3(aq) H2O(g) CuSO4(aq) ZnSO 4(aq) + + H2O(l) Cu(s) Redox [1] Metathesis [2] Dehydration [3] Metathesis [4] Redox [5] Each of these reactions proceeds to completion. Metathesis reactions proceed to completion whenever one of the components is removed from the solution, such as in the formation of a gas or an insoluble precipitate (driving forces). This is the case for reaction [1], [2], and [3], where in reactions [1] and [3] a gas and in reaction [2] an insoluble precipitate are formed. Reaction [5] proceeds to completion because zinc has a lower ionization energy or oxidation potential that copper. Reactions of Copper and Percent Yield Page 1 of 8 The objective in this experiment is to recover all of the copper you begin with in analytically pure form. This is the test of your laboratory skills. The percent yield of the copper can be expressed as the ratio of the recovered weight to initial weight, multiplied by 100: % yield = recovered weight of Cu initial weight of Cu x 100 Procedure Weight approximately 0.500 g of no. 16 or no. 18 copper wire (1) to the nearest 0.0001 g and place it in a 250 mL beaker. Add 4-5 mL of concentrated HNO3 to the beaker, IN THE HOOD. After the reaction is complete, add 100 mL distilled H2O. Describe the reaction (6) as to color change, evolution of gas, and change in temperature (exothermic or endothermic) in the report sheet. Add 30 mL of 3.0 M NaOH to the solution in your beaker and describe the reaction (7). Add two or three boiling chips and carefully heat the solution -- while stirring with a glass stirring rod -- just to the boiling point. Describe the reaction on your report sheet (8). Remove the boiling chips. Allow the black CuO to settle; then decant the supernantant liquid. Add about 200 mL of very hot distilled water and allow the CuO to settle. Decant once more. What are you removing by washing and decanting (9)? Add 15 mL of 6.0 M H2SO4. What copper compound is present in the beaker now (10)? Your instructor will tell you whether you should use Zn or Al for the reduction of Cu (II) in the following step. A. Zinc In the hood, add 2.0 g of 30-mesh zinc metal all at once and stir until the supernatant liquid is colorless. Describe the reaction on your report sheet (11). What is present in solution (12)? When gas evolution has become very slow, heat the solution gently (but do not boil) and allow it to cool. What gas is formed in this reaction (13)? How do you know (14)? B. Aluminum In the hood, add several 1-inch squares of aluminum foil and a few drops of concentrated HCl. Continue to add pieces of aluminum until the supernatant liquid is colorless. Describe the reaction on your report sheet (11). What is present in solution (12)? What gas is formed in this reaction (13)? How do you know (14)? When gas evolution has ceased, decant the solution and transfer the precipitate to a preweighed porcelain evaporating dish (3). Wash the precipitated copper with about 5 mL of distilled water, allow to settle, decant the solution, and repeat the process. What are you removing by washing (15)? Wash the precipitate with about 5 mL of methanol (KEEP THE METHANOL AWAY FROM FLAMES _ IT IS FLAMMABLE!) Allow the precipitate to settle, and decant the methanol. (METHANOL IS ALSO EXTREMELY TOXIC: AVOID BREATHING THE VAPORS AS MUCH AS POSSIBLE.) Finally, wash the precipitate with about 5 mL of acetone (KEEP THE ACETONE AWAY FROM FLAMES - IT IS EXTREMELY FLAMMABLE!), allow the precipitate to settle, and decant the acetone from the precipitate. Prepare a steam bath as illustrated and dry the product on your steam bath for at least 5 minutes. Wipe the bottom of the evaporating dish with a towel, remove the boiling chips and weigh the evaporating dish plus copper (2). Calculate the final a\weight of copper (4). Compare the weight with your initial weight and calculate the percent yield (5). What color is your copper sample (16)? Is it uniform in appearance (17)? Suggest possible sources of error in this experiment (18). Reactions of Copper and Percent Yield Page 2 of 8 KEY Chemical Reactions of Copper and Percent Yield Pre-lab (Review Questions) 1. Give an example, other than the ones listed in this experiment, of redox and metathesis reactions. Redox: Zn + MgCl2 ZnCl2 + Mg Zinc is oxidized: Zn0 Zn2+ + 2e- Magnesium is reduced: Metathesis: Pb(NO3)2 + H2S PbS + HNO3 Mg2+ + 2e- Mg0 (metathesis is also known as a double-replacement reaction) 2. When will reactions proceed to completion? Driving forces for double rreplacement eplacement reactio reaction n is formation of w water, ater, gas or a sso o lid. Single re replacement placement reactions we use an act activity ivity series to pr predict edict if they will oc occur. cur. For a react reaction ion to proceed to completion all of the reactants must m mix: ix: they may need to be stirred, or heated to assist in the process o off them reacting. 3. Define percent yield in general terms. Percent yield is a measure of how well the reaction proceeded to completion. The formula for percent yield is the experimental yield divided by the calculated (theoretical yield). 4. Name six methods of separating materials. a) filtration b) magnetism c) centrifugation d) decantation e) color f) distillation 5. Give criteria in terms of temperature changes for exothermic and endothermic reactions. Exothermic reactions - release heat and feel “hot” to the touch Endothermic reaction - gain heat and feel “cold” to the touch 6. If 1.65 g of Cu(NO 3)2 are obtained from allowing 0.93 g of Cu to react with excess HNO 3, what is the percent yield of the reaction? % = 1.65 g Cu 0.93 g + 2 HNO3 Cu(NO3)2 excess xg x g Cu(NO3)2 = 0.93 g Cu 1 mol Cu 63.5 g Cu Reactions of Copper and Percent Yield + H2 1 mol Cu(NO3)2 187.5 g Cu(NO3)2 1 mol Cu 1 mol Cu(NO3)2 1.65 g 2.75 g % = 60% yield = 2.75 g Cu(NO 3)2 Page 3 of 8 KEY Prelab – Chemical Reactions of Copper and Percent Yield 7. What is the maximum percent yield in any reaction? 100%; any value higher would be impurities in product (e.g. water, by-product) 8. What is meant by the terms decantation and filtration? Decantation - pour off solvent leaving behind precipitate Filtration - pass through filter that separates our components of a mixture by differences in particle size 9. When Cu(OH) 2(s) is heated, Copper (II) oxide and water are formed. Write a balanced equation for the reaction. Cu(OH)2(s) CuO(s) + H2O(g) 10. When sulfuric acid and copper (II) oxide are allowed to react, copper (II) sulfate and water are formed. Write a balanced equation for this reaction. H2SO4(aq) + CuO(s) CuSO4(aq) + H2O(l) 11. When copper (II) sulfate and aluminum are allowed to react, aluminum sulfate and copper are formed. What kind of reaction is this? Write a balanced equation for this reaction. 3 CuSO4(aq) + 2 Al(s) Al2(SO4)3(aq) + 3 Cu(s) This reaction is an example of a redox reaction: where aluminum is oxidized and copper is reduced. The copper is the oxidizing agent and the aluminum is the reducing agent. Aluminum is oxidized: Al0 Al3+ + 3e- Copper is reduced: Cu2+ + 2e- Cu0 It could also be called a single replacement reaction – where aluminum is chemically more active than copper. Reactions of Copper and Percent Yield Page 4 of 8 KEY REPORT SHEET Chemical Reactions of Copper and Percent Yield 1. Weight copper initial _______________ 2. Weight of copper and evaporating dish _______________ 3. Weight of evaporating dish _______________ 4. Weight of copper final _______________ 5. % Yield (show calculations) _______________ 6. Describe the reaction Cu(s) + HNO3(aq) Cu(NO3)2(aq) + 2 NO2(g) + 2 H2O(l) The he ad addi dititio ion of ni nitri ric ic aci cid id ca caus use sed ed the he co cop oppe per er metal al to sl slo low owl wly di dis iss ssol olv lve ve. e. A re red ed-br bro row own wn ga gas as (N (NO 2) wa was as pr pro rod odu duc uced e ed. d. The he od odo dor or wa was as si sim imi mila lar ar to the sm sme mel ellll of ch chl hlo lori rrin ine ne. The nititri ric ic ac aci cid id liq iqu iq quid id ch cha han ang nged ed co col olor or fro rom co col olo lor orl rle less ss to a bl blu luee e-g -gr gre ree een co col olo lor or. r. Cu(s) + 4 HNO3(aq) Cu(NO 3)2(aq) + 2 NO2(g) + 2 H2O(l) 7. Describe the reaction Cu(NO 3)2(aq) + NaOH(aq) Cu(OH) 2(s) + 2 NaNO3(aq) The he add ddi dd dititio ion on of so sodi diu ium um hy hyd ydr dro rox oxid iid de e so sol olu lut utio ion on wi witith the he co cop opp ppe per er ni nititra rat ate sol olu lut utio ion on pr pro rodu duc uce ced ed a br bri rig igh ght ht bl blu lue ue co col olo lor ored ed so sol olid id (g (ge gell-l -lilik ike ke)) mat ater eri erria ial al [Cu Cu( u(O (OH) H)2]]. Cu(NO3)2(aq) + 2 NaOH(aq) Cu(OH)2(s) 8. Describe the reaction Cu(OH) 2(s) CuO(s) + + 2 NaNO3(aq) H2O(g) When the When he be beak ake akker er co con ont ntai ain ini nin ing ng the co cop opp ppe per er (I (II) hy hyd ydr drox oxid ide de an and nd wa water er wa was hea eat ated ed the he bl blu lue sol oli ollid id cha han ha ang nge ged ed co col olo lor or in int nto a bl bla lac ack ck fin ine in ne po powd wde der er. r. Up Upon o on co coo oolillin ing ng the he bl bla lack cck po powd wde der er (Cu CuO Cu uO) cou oul ou uld ld be se sep epa par arat ated ated fro rom om the wat ater er by de dec eca can ant ntat atio ion on. n. Cu(OH)2(s) CuO(s) + H 2O(g) 9. What are you removing by this washing? Unreacted (impurities) and excess NaNO 3 that wasn’t removed in the previous step. 10. What copper compound is present in the beaker? Cu CuS uSO4 co cop opp ppe per er (I (II) sul ulf ullfat ate 11. Describe the reaction CuSO4(aq) + Zn(s), or CuSO4(aq) + Al(s) When al When alu lum umi minum um foi oilil is ad add dde ded ed to the he so sol olu lut utio ion on of cop opp ppe p pe err (I (II) su sul ulflfat ate, ate, the he foi oilil di dis iss sso sol olv lve ves es and nd ha has as co cop opp ppe per er sp spo pot ots. s. The so solu llut utio ion on ch cha han ang nge ges es co colo lor or fro rom om pal ale le bl blu lue e to co colo llor orle les le ess ss. s. A ga gas as is re rel ele leas ase sed ed (h (hy hyd ydr dro rog o og gen e en) n). ). 12. What is present in solution? Al3+ io ions n ns an and nd SO42- io ion ons. s. The hey ar are re re remo m mov ove ved ed in the he fin inal al wa was ash shin iing ng to le lea eave ve yo you ou wi witith pur ure urre Co Cop o op ppe p per er! r! Reactions of Copper and Percent Yield Page 5 of 8 KEY REPORT SHEET – page 2 Chemical Reactions of Copper and Percent Yield 13. What is the gas? Hydrogen gas. 14. How do you know? Any time a an n acid is added to a metal hydrog hydrogen en gas is released. We also checked w wii th a glowing wooden sp splint lint that burst into flames when p placed laced in the beak beaker. er. 15. What are you removing by washing? Al3+ ions and SO42- ions. 16. What color is your copper sample? Initial color is red-brown (c (copper opper colored) wh when en wet. Aft After er drying, many samples wi willll have a white residue of a aluminum luminum sulfate cry crystals stals that weren’t removed during w washing. ashing. Small b bits its of excess aluminum m may ay be present g giving iving the silver co color lor of aluminum mix mixed ed in with copper sa sample. mple. Finally, upon standi standing, ng, the copper may oxidize and change to a slightly green co color. lor. 17. Is it uniform in appearance? Yes, with exceptio exceptions ns listed in question #16 (above). 18. Suggest possible sources of error in this experiment. When decan decanting, ting, lost some sa sample mple by pouring it out. It is difficult to have all CuO sett settle le to bottom and remove al alll liquid. Approxim Approximate ate volumes of acids and bases were a added. dded. We assum assumed ed we had excess in all cases an and d removed th the e excess by wash washing. ing. Extremely h hard ard to remove exces excess s Al foil – a addition ddition of [hydroc [hydrochloric hloric acid] to dissolve foil may have caused the co copper pper product tto o react and form cop copper per (II) chloride. Reactions of Copper and Percent Yield Page 6 of 8 KEY POST LAB QUESTIONS 1. If your percent yield of copper was greater than 100%, what are two plausible errors you may have made? Sample w was as not fully dried a and nd still contain contained ed water. Impur Impurities ities were mixed in with copper causing it to weigh more than it should. 2. Consider the combustion of methane, CH4: CH4(g) + 2 O2(g) -----> CO2(g) + 2 H2O(g) Suppose 2 mole of methane is allowed to react with 3 mol of oxygen. a) What is the limiting reagent? (show work) OXYGEN is the limiting reacta reactant. nt. According to the balanced cchemical hemical equation, yyo o u need 2x the amount of oxygen as methane. You would need 4 mole of oxyg oxygen en to react with 2 mol of methane (y (you ou have only 3 mol of oxygen). b) How many moles of CO2 can be made from this mixture? How many grams of CO2? x mol CO2 = 3 mol O2 x g CO2 = 3 mol O2 1 mol CO2 = 1.5 mol CO2 2 mol O2 1 mol CO2 44 g CO2 2 mol O2 1 mol CO2 = 66 g CO2 3. Suppose 8.00 g of CH4 is allowed to burn in the presence of 6.00 g of oxygen. How much (in grams) CH4, O 2, CO2, and H2O remain after the reaction is complete? limiting CH4(g) + 2 O2(g) -----> 8g CO2(g) + 2 H2O(g) 6g / 16 g 0.5 mol CH4 / 32 g 0.1875 mol O2 2 1 0.5 0.09375 [Have 0 g O2 remaining] x g CH4 = 0.1875 mol O2 x g CO2 = 0.1875 mol O2 x g H2O = 0.1875 mol O2 1 mol CH4 16 g CH4 2 mol O2 1 mol CH4 1 mol CO2 44 g CO2 2 mol O2 1 mol CO2 2 mol H2O 18 g H2O 2 mol O2 1 mol H2O Reactions of Copper and Percent Yield = 1.5 g CH4 [Have 6.5 g CH4 remaining] = 4.125 g CO 2 [Have 4.12 4.125 5 g CO2 produced] = 3.375 g H 2O [Have 3.37 3.375 5 g H2O produced] Page 7 of 8 4. How many milliliters of 6.0 M H 2SO4 are required to react with 0.80 g of CuO according to Equation [4]? CuO(s) + H2SO4(aq) 0.80 g CuSO4(aq) + H2O(l) x mol x mol H2SO4 = 0.80 g CuO 1 mol CuO 1 mol H2SO4 79.5 g CuO 1 mol CuO = 0.0100628 mol H2SO4 convert to millimoles (mmol) by dividing by 1000. Molarity (M) = mol solute 6.0 M H2SO4 = L of solution 10.0628 mmol X mL of solution = 1.677 mL 6. 6.0 0 M H2SO4 5. If 2.00 g of Zn is allowed to react with 1.75 g of CuSO4 according to Equation [5], how many grams of Zn will remain after the reaction is complete? (from the question we can assume that Zn in excess reactant) excess CuSO4(aq) + Zn(s) 1.75 g ZnSO4(aq) + Cu(s) 2.00 g x g / 159.5 g x = 0.72 g x 65.4 g/mol Zn 0.01097 mol 0.01097 mol The reaction will consume 0.72 g of Zn. Therefore you will have 2.00 g – 0.72 g or 1.28 g of Zn rem remaining aining aining. 6. What is meant by the term limiting reagent? The limitin limiting g reactant is the start starting ing material use used d in a chemical react reaction ion that is used up first (or that you run out of) of).. When no more o off it remains, no a additiona dditiona dditionall product can be made and the re reaction action stops. The quantity of limiting reactant determines (LIMITS) the amount of pro product duct that is made. Reactions of Copper and Percent Yield Page 8 of 8 TEACHER NOTES: PowerPoint Presentation W a r n i n g : This lab experiment requires a large quantity or reagents. The acids and base are very concentrated and should only be used in a fume hood with proper teacher supervision. For a class of 100 students you will need: 750 mL concentrated nitric acid [HNO3] 650 mL concentrated sulfuric acid [H2SO4] 480 g sodium hydroxide (NaOH) The concentrated nitric acid is not diluted. The concentrated sulfuric acid is diluted to 6 M H2SO4. Concentrated sulfuric acid is 18.1 M. Therefore, take 1 part [H 2SO4] to 2 part H2O. The resulting solution will be approximately 6 M H2SO4. To make 3 M NaOH, begin with 2000 mL of cold distilled water and add 240 g NaOH. You will need to mix two batches of NaOH to yield 4 L of 3 M NaOH. Be sure students have read the lab and completed the pre-lab before going into the lab. The lab requires two full days (in the lab) to complete. Day 1) Students should be able to get through equation (3). They must have added the NaOH to yield Cu(OH)2 Heating the solution with a boiling chip is ideally where they should get to – boiling chips must be removed and not left in beaker over night. If students are rushed for time and can’t heat – don’t worry, the reaction will proceed to completion on its own over night. Day 2) Complete lab Reactions of Copper and Percent Yield Teacher Notes