Analysis of Silver in an Alloy Essay Example

Analysis of Silver in an Alloy Essay Example Analysis of Silver in an Alloy Paper Analysis of Silver in an Alloy Paper Analysis of Silver in an Alloy Introduction In this experiment an alloy of silver will be analyzed to determine its silver content. The silver-copper alloy will be dissolved in nitric acid, the silver will be precipitated as silver chloride, and the silver chloride will be filtered, washed, dried and its mass determined. From the mass of the silver chloride formed and the mass of the original sample, you will be able to calculate the percent of silver in the alloy. Because the results are based on the mass of a product, this procedure is classified as a gravimetric analysis. Silver and copper are very nonreactive metals. Neither will dissolve in hydrochloric acid or sulfuric acid. The oxidizing acid nitric acid, HN03, is required. In acidic solutions the nitrate ion is an excellent oxidizer, and it will oxidize Ag(s) to Ag+(aq) and Cu(s) to Cu2+(aq). The reduction product is the gas NO. As the colorless nitrogen monoxide gas forms, it immediately reacts with the oxygen in the air to produce the orange-brown gas N02. The half-reactions for the oxidation of silver and copper by nitric acid are as follows: Ag(s) > Ag+(aq) + e- Cu(s) > Cu2+(aq) + 2e- 4H+(aq) + NO3- (aq) + 3e- > NO(g) + 2H2O(l) Once the silver and copper ions are in solution, they can be separated from each other by precipitating the silver ions as silver chloride. Silver chloride (AgCl) is very insoluble in water, while copper(II) chloride (CuCl2) is soluble. The addition of chloride ions to the solution will precipitate essentially all of the silver and none of the copper. The silver chloride precipitate is then filtered from the solution. Experimental Methods Follow protocol as listed for Laboratory Experiment #1 in â€Å"Experiments for Advanced Placement Chemistry† by Sally Ann Vonderbrink, Ph. D. With these modifications: instead of using a Gooch Crucible and fiber glass pad, we used a Buchner Funnel and filter paper. Theoretical Methods Balance out the half reactions of Ag and NO3- 3( Ag(s) > Ag+(aq) + e- ) 4H+(aq) + NO3- (aq) + 3e- > NO(g) + 2H2O(l) 3Ag(s) + 4H+(aq) + NO3- (aq) > 3Ag+(aq) + NO(g) + 2H2O(l) Calculate amount of NaCl needed to precipitate NaCl > Na+ + Cl- Ag + Cl > AgCl(s) .3015 g Ag 1 mol Ag 1 mol Cl 1 mol NaCl 58. 44 g NaCl 107. 7g Ag 1 mol Ag 1 mol Cl 1 mol NaCl = . 1635g NaCl multiply needed NaCl x 2 .1635 x 2 = . 3269 g NaCl Calculate the percent of Ag in the Alloy .3555 g AgCl 1 mol AgCl 1 mol Ag 107. 87 g Ag = . 2675 g Ag 143. 23 g AgCl 1 mol AgCl 1 mol Ag % Ag = . 2675 g Ag x 100 = 75. 25% .3555 g AgCl Calculate the percent error between percents of Ag in an Alloy % error = actual – experimental x 100 = actual = 90. 08 – 75. 25 x 100 = 16. 46 % 90. 8 Results Experimental Results: We first weighed our original sample of silver alloy, as shown in table 1. Then added 10 ml of nitric acid to the silver alloy which dissolved, by heating, the alloy into silver and copper ions as nitrite gas escaped in an orange cloud. After the alloy was completely dissolved we added a solution of sodium chloride dissolved in distilled water. When calculating out sodium chloride we doubled the necessary amount to make sure that a full and complete reaction occurred when once again heating. After letting that sit over night covered in para-film to form large precipitate particles of silver chloride we filtered the particles from the solution with the buchner funnel. We used a diluted nitric acid as our wash because it helped to keep the precipitate from forming to small particles. We then heated the sample so we could weigh out the final product as shown in table 1, and then calculated the percent of silver as shown in table 3. Theoretical Results: In table 2 it shows the necessary amount of sodium chloride that was needed for this reaction to four decimal places. While in table 1 it is shown that we had to round to two decimal places lacing a small error. And not only that but also the fact that as table 2 shows that the final amount needed was . 0131 g less to the actual mount that we added from table 1. During the procedure our percent error must of have come from loosing the silver ions. After heating we had to wash the moisture of the watch glass back into the beaker and there could have been a possibility that we didn’t wash of all of the silver back into the beaker. When it precipitated, some of the particles may have peptized causing us to filter out some of the silver chloride precipitate. Also when removing the filter paper from the buchner funnel some of the precipitate may have been lost, which is just due to human error. Which all experiments have. Discussion In the lab you must first dissolve the alloy in nitric acid; if the allot doesnt completely dissolve it may cause some error in your final result because not all of your silver ions were dissolved. You then must make a solution of distilled water added with sodium chloride. Once you calculate the amount of sodium chloride needed you must double the amount for the experiment to push the reaction to a full completion. This essentially should precipitate all of silver and none of the copper. No accurate balance is needed to measure out the sodium chloride because as long as you have an excess of sodium chloride to provide enough chloride ions to precipitate all of the silver ions its is safer than having a need of more chloride ions.. Before adding the sodium chloride solution to the dissolved silver you must rinse any moisture on the bottom of your watch glass back into the beaker of dissolved silver so that way not silver is lost. Thus causing a source of error if not all of the silver ions were still in the beaker upon adding the sodium chloride solution to precipitate the silver as silver chloride. You wash to remove all of the other chemicals from the silver chloride precipitate. There will be copper(II) nitrate, nitric acid and excess sodium chloride. These have to be washed through to make sure only (wet) sodium chloride is left in the filter. The nitric acid in the wash water will not interfere with the weight of the silver chloride because it will be washed through with the filtrate leaving the solid sodium chloride in the filter paper. If the silver chloride is not cool when its mass is determined the calculated percent would be too high, because if we do not dry the precipitate in the oven then the water in the filter paper and solid will affect the weight making it higher. We need for that excess water to be dried out to have a good calculation. We dont use hydrochloric acid to both dissolve and precipitate the silver because the acid cannot dissolve either silver or copper because they are insoluble when mixed. It could work to make a precipitate but not to dissolve the alloy. A special filter crucible rather than plain filter paper is used because it gives rapid filtering and helps pull the soluble material and water through. Also you have to wash several times so this speeds up the process. Observations made throughout the experiment is that when the solution of the alloy when dissolved with the nitric acid, were being heated a yellow orange gas of nitric escaped which smelled like rusted metal. The liquid that was left behind had an aqua color to it. But once the sodium chloride was added it turned into a cloudy blue. When this new solution is heated for 15 minutes the sodium chloride begins to precipitate into white clumps of the bottom of the beaker. When precipitate is filtered and dried in oven it then looks like white dust particles. Conclusion In this lab, the percent of silver in the alloy was calculated by dissolving the alloy in nitric acid to precipitate the silver as silver chloride. By weighing the mass of the precipitate you are able to calculate the percent of silver in the original sample. Anytime you are wanting a quantitative percent of an element in an alloy dissolving it in a solution then precipitating the solution should give you the result s you want as long as it is insoluble in the reaction. Improvements that we could o made during the experiment are that my partner and I should have divided the lab in a better way in which we both could have the job done faster and with more patience. Because one of the sources of error that could be possible is that when we are washing the solution back into the beaker we could have not washed it properly and some silver could have been left behind. Data Tables Mass of Ag alloy, g 0. 3015 g 6M Nitric Acid, ml 10 ml Mass of NaCl added, g 0. 34 g Distilled water, ml 25 ml Mass of filter paper, g 0. 3765 g Distilled water, ml 150 ml 6M Nitric Acid, ml 2 ml Mass of filter paper with the AgCl precipitate, g .7320 g Mass of AgCl precipitate, g .3555 g Table 1 – experimental measurements Needed NaCl, g .1635 g Grams of NaCl * 2 .3269 g Table 2- Calculated values ( theoretical measurements) Percent of Ag in alloy 75. 25% Actual percent of Ag in alloy 90. 08% Percent error 16. 46% Table 3- calculated percent error in percent of Ag in alloy