Highline Community College, Chemistry 161 Experiment: Trends in Periodic Table Groups Introduction In the lecture portion of this course, properties of atoms and ions of the elements are related to their electron configurations as predicted by elementary quantum mechanics. The textbook provides data for atomic radius, ionization energy, and electron affinity to illustrate the trends among the elements and point to the success of quantum mechanics in explaining the basis of the periodic law. Nevertheless, electron configurations and properties of isolated atoms may still seem somewhat remote or abstract. The objective of this experiment is to make it easier to relate to such topics by demonstrating some trends that are easily observed in the lab, yet rely on the same type of variation in atomic properties that have been discussed before. In part A, solutions containing dissolved cations of the alkaline earth metals (Mg2+, Ca2+, Sr2+, Ba2+) will be mixed with a variety of solutions containing large anions (SO42-, CO32- , C2O4-, and IO3-). The resulting compounds may be insoluble in water, thus forming visible solid particles called precipitate. Two principle factors determining solubility of a compound are the lattice energy of the compound and the hydration energy of the dissolved ions. In the context of solubility, the lattice energy of an ionic solid is the energy required to separate its ions into the gas phase. The hydration energy is the energy change occurring when gaseous ions are added to and hydrated by water. A solution (dissolving) reaction can be idealized as a three-step process involving these energy changes, as shown in the example below. Figure: Qualitative Energy Changes in the Dissolution Process of NaCl Ionic solids tend to be soluble as long as the sum of lattice energy and hydration energy is negative or only slightly positive, since random motion usually favors the dissolving process. The lattice energies (Figure Step 1) and hydration energies of the anions (Figure Step 2) will be relatively small due to the large size and low charge density of the anions chosen for this experiment. Thus the main variable in determining the solubilities will be the hydration energies of the cations (Figure Step 3). Hydration energy is larger when an ion’s charge density is larger. Charge density is the amount of charge per volume of the atom. Here we hope that you will observe a trend from the alkaline earth cation that precipitates with the most anions to the cation that precipitates with the least anions. In part B, we will investigate a trend in the properties of the diatomic halogen (Group VIIA) molecules (Cl2, Br2, I2). This investigation is done by a series of reaction tests in which a molecular halogen is added to a different halogen in the form of halide ions. If the halogen X2 can take away the extra electron from halide Y-, then the following reaction may occur: X2 (aq) + 2 Y- (aq) ----- > Y2 (aq) + 2 X- (aq) If the reaction does not occur, no electron is transferred, indicating that Y has a greater affinity for the electron. You will test the halogens Cl2, Br2, and I2 against one another in this way and note when reactions occur. You will then rank these elements based on their ability to attract an electron. The reactions take place in aqueous solutions; however, the nonpolar halogen molecules are not very soluble in water, so we will mix the reaction solution with nonpolar hexane in which the halogens are more soluble. Hexane is insoluble in water so it will for a separate layer. Halogen molecules will tend to migrate to the hexane layer where each gives a distinctive color. Thus we can readily detect whether Cl2, Br2, or I2 is present at the end of the reaction. This will indicate whether or not a reaction occurred. Materials and Safety The small quantities of ions and precipitates from part A should not represent hazards for either handling or disposal. All solutions and precipitates from part A can be disposed of in the sinks. The iodate in part A and all of the halogen-water solutions in part B have some oxidizing power but are at relatively low concentration in the solutions. Avoid prolonged contact with skin by rinsing if contact occurs. Since the halogens are not particularly soluble in water, the halogen gases will be escaping the solutions during the part B reaction tests. The gases can be irritating when inhaled. For this reason, the operations of part B are performed in the hood area. Since hexane is not water-soluble, we will collect the waste hexane in separate flasks rather than disposing in the sinks. Procedure Part A Clean out the 15 cm test tubes from your drawer and rinse with distilled water. Organize four of them by positive in a rack or by pencil labels on their white marking spots so that you will know what ions have been added to each tube. If you carry test tubes in the rack, be careful that they do not slip out. The rack wells are shallow and hold poorly against sideways motion. Bring the tubes to the class materials cart. Using the dropper bottle reservoirs, add 15-18 drops of one of the 0.1 M alkaline earth metal nitrate solutions to each of the four tubes. Next add 15-18 drops of 1 M H2SO4, 1 M Na2CO3, 0.25 M (NH4)2C2O4, and 0.1 M KIO3, one each to a different one of the four test tubes. Leave the dropper bottles at the cart and take the tubes to your drawer area. (Going back to your drawer area allows your classmates to use the materials.) Mix the contents of each tube well. You may use a glove (found in the stockroom window) and use your thumb to cover the tube while shaking. Record observations as to whether or not a precipitate formed and record any qualitative observations (such as color). A precipitate is indicated by solid particles (it may look milky). After the observations have been recorded, clean and rinse the test tubes and repeat the process starting with another alkaline earth metal nitrate. Continue until all four metal nitrates: Mg(NO3)2, Ca(NO3)2, Sr(NO3)2, and Ba(NO3)2 have been tested for a total of sixteen combinations. Record all your observations in the Data Table. Part B The operations in this part will take place in the hood. To avoid traffic problems, plan ahead and work efficiently. After making a set of test tube additions, step back to mix the tubes and allow others to have access to the materials. Clean and rinse the 10 cm and 15 cm test tubes from your drawer. Bring them to the bench area close to the ventilation hood at the east end of the lab. Use three of the smaller test tubes to observe the colors of the halogen molecules in hexane. Start by adding 10-12 drops of the chlorine-water solution from a dropper bottle to a tube. Add 10-12 drops of hexane. To avoid continual opening and closing of the primary hexane reservoir, there will probably be a small flask or beaker to pour a small amount of hexane in and use as a source for the pipet droppers. Use a gloved finger to cover the tube and shake to mix the contents. Since water is more dense, it will be in the bottom layer. Record in your data table the colors apparent in both the upper hexane layer and the lower water layer. Repeat the process with bromine-water and hexane, then with iodine-water and hexane. Keep the three tubes on hand for comparisons. In the reaction tests that follow, the colors in the hexane layer will be compared to the reference colors to determine which molecular halogen is present and whether a reaction occurred. There are six reaction tests. Do two at once so you can alternate accessing materials with other teams. Choose one of the halogen-water solutions and add 15-18 drops to each of two of the larger test tubes. Add 15-18 drops of hexane without mixing to each tube. Then, to each, add 15-18 drops of the 0.1 M sodium salt solution of one of the other two halides. For example, to two test tubes with Cl2 in water, add hexane to each. Then add 0.1 M NaBr to one tube and 0.1 M NaI to the other tube. Mix the contents by shaking, then record the color seen in the hexane layer. Discard all solutions in the hexane waster collector, a labeled bottle in the hood. Repeat until each halogen has been tested against the other two halides. As soon as the six tests are done, move away from the hood area. Work with your partner on a preliminary interpretation of your data. Prelab Assignment Answer the following questions before lab in your lab notebook. 1. Rank the alkali metal cations (Na+, K+, Rb+, and Cs+) from small to large ionic radius. 2. Consider the charges and atomic radii of each alkali metal. Then rank the alkali metal cations from low to high charge density (i.e., charge per atomic volume). 3. Based on your answers to 1 and 2 above, predict which alkali metal cation exhibits the greatest hydration energy. 4. (a) Write the equation for the reaction that would occur between aqueous bromide ion (Br-) and aqueous chlorine (Cl2). (Hint: See generic equation in the Introduction.) (b) If this reaction occurs, which halogen (Cl or Br) has a greater ability to attract electrons? 5. (a) Write the equation for the reaction that would occur between aqueous chloride ion (Cl-) and aqueous bromine (Br2). (b) Can both of the reactions in questions 4a and 5a be favorable under the same conditions? Data Recording: Part A Prepare a data table like the one below in your notebook before coming to lab. When entering data, write “precipitate” if a solid was formed or “no precipitate” if no solid was formed. You may wish to add observations about color and appearance of precipitates. SO42- CO32- C2O42- IO3- Mg2+ Ca2+ Sr2+ Ba2+ Postlab Questions: Part A (Answer the questions below in your laboratory notebook.) 1. Rank the alkaline earth cations (Mg2+, Ca2+, Sr2+, Ba2+) from the one forming the most precipitates to the one forming the least precipitates in the tests. Indicate ties, if any. 2. How does the solubility of alkaline earth cations (i.e., tendency to precipitate or not) change with position in the periodic table? The cation higher in the table has LOWER / HIGHER solubility. The cation lower in the table has LOWER / HIGHER solubility. 3. What fundamental atomic property of the cations provides an explanation of the above trend? (choose from below) Atomic Radius Ionization Energy Electron Affinity Explain your answer above: Data Recording: Part B Prepare a data table like the one below in your notebook. Record the color in the hexane and water layers that were observed after shaking each halogen-water solution with hexane. Reference Colors Cl2 Br2 I2 Hexane layer Water layer For each reaction test, simply record the color you see in the hexane layer. Compare the final hexane layer color to the reference colors in the first table. A reaction has occurred if the final hexane color shows the presence of a halogen molecule other than the initial reactant. Write R in the box if a reaction occurred, or NR for no reaction. Reaction Tests Cl2 Br2 I2 Cl- Br- I- Postlab Questions: Part B (Answer the questions neatly in your lab notebook). 1. Examine your data for the reaction tests of Cl2 with Br- (Cl2/Br-) versus Br2 with Cl- (Br2/Cl-). (a) Which reaction test results in no change in the color of the hexane layer? (Cl2/Br-) or (Br2/Cl-) (b) Which reaction test results in a change in the color of the hexane layer? (Cl2/Br-) or (Br2/Cl-) (c) Which of these two reactions actually occurred? (d) Based on your answers above, which element has a greater affinity for an electron? Circle your answer: Cl2 or Br2 2. Rank the species Cl2, Br2, and I2 in order of weakest to strongest electron affinity. (Note: Strongest here means most negative.) Weakest electron affinity Strongest electron affinity 3. How does the electron affinity change with position in the periodic table? Circle your answers below. The halogen higher in the table has a WEAKER / STRONGER electron affinity. The halogen lower in the table has a WEAKER / STRONGER electron affinity. Discussion (Write in your lab notebook. Use complete sentences!) 1. Problems and Errors – Discuss any problems you encountered during the lab, including any place where you committed a human error that may have affected your results. 2. Learning: Discuss what you learned from the experiment. This should agree with your purpose.
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