CHM131 Exp 1 Lab Report PDF

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Experiment 1: The Alkaline Earths and the Halogens: Two Families in the Periodic Table

Introduction: The purpose of this experiment was to be able to develop and put to use a simple, generalized procedure for identifying an unknown alkaline earth halide present in a given solution. This was achieved through analyzing the results and patterns of precipitation reactions (alkaline earth cations) and oxidation/reduction reactions (halogens and halides). Experimental Procedure: A. Alkaline Earth Metals (page 004 in logbook) a. Set up a transparency with a grid comprised of 16 boxes, the columns labeled left to right with 1M Na2CO3, 1M 1M H2SO4, 0.25M (NH4)2C2O4, and 1M K2CrO4/1M HC2H3O2, and the rows labeled with the alkaline earth solutions Ca(NO3)2, Ba(NO3)2, Mg(NO3)2, and Sr(NO3)2. b. Fill the grid with drops of each alkaline earth solution in their corresponding boxes, being careful not to allow the dropper to become contaminated with any drops on the film. c. Add a single drop of 1M Na2CO3 to each alkaline earth cation slot in the Na2CO3 column. Repeat this with 1M H2SO4, 0.25M (NH4)2C2O4, and 1M K2CrO4/1M HC2H3O2 in their respective columns.  d. Record all observations of the reactions (if precipitate was present, color of solution, etc.) B. Halogens and Halides (pages 004-005 in logbook) a. Calibration 1. Under the fume hood, fill a test tube with 2 mL bromine-saturated water and 1 mL hexane, stopper, and shake. 2. Repeat using a test tube of chlorine water and hexane and a test tube of iodine water and hexane. 3. Record any and all color changes of the solutions. b. Oxidizing Ability 1. Add 1 mL bromine water, 1 mL hexane, and 1 mL 0.8M NaCl to a test tube, stopper, and shake. 2. In a new test tube, repeat Step i with the same measurements of bromine water and hexane, and 1 mL 0.8M NaI. 3. Repeat Steps 1 and 2 using chlorine water, hexane, and 0.8M NaBr. 4. Repeat Steps 1 and 2 using chlorine water, hexane, and 0.8M NaI. 5. Repeat Steps 1 and 2 using iodine water, hexane, and 0.8M NaI. 6. Repeat Steps 1 and 2 using iodine water, hexane, and 0.8M NaBr. 7. Record any color changes of the solutions in a data table.

Data and Observations PART A: Precipitation Reactions of Alkaline Earth Metals 1M Na2CO3

1M H2SO4

.25M (NH4)2C2O4

1M K2CrO4 1M HC2H3O2

Ca(NO3)2

Precipitate, white

No precipitate

Precipitate, white

No precipitate, yellow solution

Ba(NO3)2

Precipitate, white

Precipitate, white

Precipitate, white

Precipitate, yellow solution and precipitate

Mg(NO3)2

Precipitate (white, though only a slight amount)

No precipitate

No precipitate

No precipitate, yellow solution

Sr(NO3)2

Precipitate, white

Precipitate, white

Precipitate, white

No significant precipitate, yellow solution

This table shows how different alkaline earth cations reacted with different precipitation reagents. These results were used to show any periodic table trends of solubility. PART B: Oxidation/Reduction Reactions using Halogens Calibration Reactions Chlorine and Hexane

Bromine and Hexane

Iodine and Hexane

Before Shaking

Hexane: clear Cl: clear

Hexane: clear Br: light orange

Hexane: clear I: yellow/orange

After Shaking

Hexane: light yellow Cl: clear

Hexane: dark orange Br: light orange

Hexane: purple I: clear

Results from these tests were used to compare to the oxidation/reduction tests (as a control). If any of the solutions matched the colors produced in the calibration test, it could be concluded that no oxidation/reduction had occurred.

Oxidation/Reduction Reactions Cl2 (Hexane)

Br2 (Hexane)

I2 (Hexane) 

Cl-

X (hexane layer would Top layer (hexane): red be light yellow) Bottom layer: yellow **** NO CHANGE

Top layer (hexane): purple, Bottom layer: clear/has an oily appearance **** NO CHANGE

Br-

Top layer (hexane): red X (hexane layer would be Bottom layer: yellow red)

Top layer (hexane): purple Bottom layer: pale yellow **** NO CHANGE

I-

Top layer (hexane): purple Bottom layer: red/brown

X (hexane layer would be purple)

Top layer (hexane): purple Bottom layer: red

This table shows how halogens and halides reacted compared to the calibration results. The three shaded boxes represent where a halogen would be mixed with its own halide, which would produce a color matching the calibration reactions. PART D Precipitation Reaction Results of Unknown #2 1M Na2CO3

1M H2CO4

.25M (NH4)2C2O4

1M K2CrO4 1M HC2H3O2

N/A

Precipitate

N/A

No Precipitate

This table shows how Unknown #2 reacted to two of the tests. The significance of these results, as well as the method by which they were obtained, are detailed in the discussion. Oxidation/Reduction Results of Unknown #2 Cl2(Hexane) Before: all three components are clear After: hexane layer is red, bottom layer is yellow This table shows how Unknown #2 reacted to the Cl2(Hexane) solution. The significance of these results, as well as the method by which they were obtained, are detailed in the discussion.

Discussion: PART A Net Ionic Equations of Alkaline Earth Metal Precipitation Reactions:* 1. Ca2+(aq) + CO32-(aq) → CaCO3 (s), precipitate formed 2. Ca2+(aq) + SO42-(aq) →Ca2+(aq) + SO42-(aq), no precipitate 2- 3. Ca2+(aq) + C2O  4 (aq) → CaC2O4 (s), precipitate formed 4. Ca2+(aq) + CrO42-(aq) → Ca2+(aq)+ CrO42-(aq), no precipitate 5. Ba2+(aq) + CO32- (aq) → BaCO3 (s), precipitate formed 6. Ba2+(aq) + SO42- (aq) → BaSO4 (s), precipitate formed 2- 7. Ba2+(aq) + C2O  4 (aq) →  BaC2O4 (s), precipitate formed 2- 2+ 8. Ba (aq) + CrO4 (aq) → BaCrO4 (s) , precipitate formed 9. Mg2+(aq) + CO32- (aq) → MgCO3 (s), precipitate formed 2+ 2- 10. Mg2+(aq) + SO42-  (aq) → Mg (aq) + SO4 (aq), no precipitate 2+ 2- 2- 11. Mg2+(aq) + C2O  4 (aq), no precipitate  4 (aq) → Mg (aq) + C2O 12. Mg2+(aq) + CrO42- (aq) → Mg2+(aq) + CrO42- (aq), no precipitate 13. Sr2+(aq) + CO32- (aq)→SrCO3 (s), precipitate formed 14. Sr2+(aq) + SO42-  (aq) → SrSO4 (s), precipitate formed 2+ 2- 15. Sr (aq) + C2O  4 (aq) →  SrC2O4 (s), precipitate formed 2- 2+ 16. Sr (aq) + CrO4 (aq) → Sr2+(aq) + CrO42-  (aq), no precipitate *Numbers in this list correspond with numbers in the table from the logbook pictures PART B Balanced Equations of Halogen/Halide Reactions 1. Cl2(aq) + 2Br-(aq)→2Cl-(aq) + Br2(aq), reaction occurred 2. Cl2(aq) + 2I-(aq)→2Cl-(aq) +  I2(aq), reaction occurred - - 3. Br2(aq) + 2Cl (aq)→2Br (aq) + Cl2(aq), no reaction 4. Br2(aq) + 2I-(aq) → 2Br-(aq) + I2(aq), reaction occurred 5. I2(aq) + Cl-(aq)→2I-(aq) + Cl2(aq), no reaction 6. I2(aq) + Br-(aq) →2I-(aq) + Br2(aq) , no reaction

PART C In this experiment, there were only 12 alkaline earth halides because only four of the six alkaline earth metals (Group 2) three of the halogens (Group 7) were used. This created a total of 12 possible combinations of alkaline earth metals and halogens (alkaline earth halides). Other elements were excluded due to being too reactive to use in this lab setting. Based on the results of the precipitation reactions, it was clear that there was a trend in solubility moving down the alkaline earth metal group. Magnesium, the element nearest to the top (of the four alkaline earth metals worked with in this part of the lab), demonstrated the best results in terms of solubility, producing precipitate with only Na2CO3 . Moving down the group (Calcium to Strontium to Barium) showed evidence of decreasing solubility, meaning that the farther down the element was in the group, the more likely it was to form a precipitate with the precipitating agents (Na2CO3 , H2SO4, (NH4)2C2O4, and 1M K2CrO4/1M HC2H3O2). Based on the results of the oxidation/reduction reactions, it was concluded that the halogen group exhibited a trend of decreasing oxidizing ability moving down from chlorine to bromine to iodine. Evidence of an oxidation reaction occurring was seen through the color changes of the hexane layer after the test tube was shaken. The stronger the oxidizing ability of the halogen, the more varying the color changes were. Using this information, chlorine was found to have the highest oxidizing ability (each of the three solutions in the chlorine column produced a different color for the hexane layer), while bromine and iodine had respectively decreasing oxidizing ability (less variance in colors of the solution). This corresponds to their order from top to bottom in Group 7 of the periodic table, suggesting that elements lower down in the group are weaker and unable to displace the elements that are situated higher. If a halogen was to be mixed with its own halide, as would be the case in the three shaded boxes, essentially no reaction would occur. A halogen and its halide have the same level of reactivity and little to no affinity for each other, so any color change in the hexane would look similar/identical to the calibration reactions.

PART D Schemes for Identifying Cations and Anions Cations When testing for an unknown cation, the first precipitation reagent that it should be mixed with is 1M H2 SO4 because the initial tests showed that two of the four possible cations would produce and precipitate and two would not. Using H2 SO4 first therefore eliminates half of  the pool of unknowns (based on whether a precipitate is formed or not), leaving only two to test between in the next step. The second precipitation reagent that should be used then depends on whether the H2SO4 formed a precipitate with the cation. If a precipitate was formed, then the  unknown solution should be mixed with a drop of 1M K2CrO4 and a drop of 1M HC2H3O2

because the two cations that would have formed a precipitate with H2SO4 have different reactions  2+ 2+ to 1M K2CrO4/1M HC2H3O2 (Ba forms a precipitate, Sr does not). If the reaction with H2SO4 did not form a precipitate, then the second step would be to mix the unknown solution with 0.25M (NH4 )2C2O4. This reagent has different reactions with Mg2+  and Ca2+  (the two cations that wouldn’t form a precipitate with H2SO4). Thus, if a precipitate forms, the cation is Ca2+  . If a 2+ precipitate doesn’t form, then the cation is Mg . In this experiment, we did not immediately recognize this method, and so a less efficient one was used. The less efficient method started with 1M K2CrO4/1M HC2H3O2, which, if there is precipitate, gives an immediate solution (Ba2+  ). However, if there is no precipitate, there are three possible cations left. The next reagent used to test between those three was H2 SO4, which, much like the first test, could either give an immediate answer (if there is precipitate, the unknown cation is Sr2+  ) or could require a third and final test (with .25M (NH4 )2C2O4). In the case of Unknown #2, precipitate was not formed in the first test but was formed in the second. While this method sometimes only requires only one step, it may often require two or three, which is overall less efficient than a test that consistently requires only two steps. Anions Testing to determine the anion (i.e. one of the three halides being used in this lab) in an unknown solution requires only one step because only one of the halogen/hexane solutions must be used to distinguish between the three possible halides. Based on the results of the oxidation/reduction reactions, chlorine was found to be the most reactive and therefore provide the greatest variance in colors amongst the halides, making it the best halogen to use to test for the unknown. So, when chlorine water, hexane, and the unknown are mixed (using the measurements from the original procedure), the color of the solution provides an immediate answer. If the hexane layer is purple, then the unknown in I- ; if the hexane layer is red, then the unknown is Br- ; if the hexane layer is yellow, then the unknown is Cl- . Identification of the Unknown Based on the results of these two methods, it was determined that the cation in Unknown  Solution #2 was Sr2+  and the anion was Br- . Sr2+  was identified because the unknown formed no precipitate with 1M K2CrO4/1M HC2H3O2, but did form a precipitate with H2SO4. The only alkaline earth cation that fit those parameters was Sr2+  . Br- was identified because when the unknown was mixed with chlorine water and hexane, the color of the solution was red on top and yellow on the bottom, fitting the results of the chlorine water, hexane, and Br- from the oxidation/reduction tests. Balanced Equations for Unknown #2 (in the order in which they were tested) 1. Sr2+(aq) + CrO42- (aq) → Sr2+(aq) + CrO42-  (aq), no precipitate 2+ 2- 2. Sr (aq) + SO4 (aq) → SrSO4 (s), precipitate formed

3. Cl2(aq) + 2Br-(aq) → 2Cl-(aq) + Br2(aq) Conclusion: By conducting this experiment, a generalized, efficient procedures for identifying an unknown alkaline earth halide (of the twelve possible in this experiment) were created. This is significant because, if ever given an unknown alkaline earth halide, these procedure can be used to prevent an individual from spending undue time on tests that may not be necessary or may leave more room for error....