PERIODIC RELATIONSHIPS

Transcription

1 PERIODIC RELATIONSHIPS OBJECTIVES: Gaining insight into property and reactivity trends within families and across periods for the chemical elements through experimental observation. SKILLS: Observations, manipulation and preparation of solutions EQUIPMENT: Clean test tubes SAFETY AND DISPOSAL: The vigorous exothermic reactions of Li, Na, and K with water will be demonstrated. Everyone must wear safety goggles during the demonstration. Small pieces of unreactive metals (EXCEPT Na, Li, K) may be rinsed with water and placed in the trash can. Acid solutions should be diluted with water and can be flushed down the sinks. Solid elemental oxides should be combined with water and diluted before disposal down the sinks. Organic solvents (i.e., hexane) containing halogens should be placed in the waste jar in the hoods. All halide salts can be disposed of in the trash. INTRODUCTION: The periodic table provides a useful theoretical and experimental summary of the behavior of the chemical elements. It was developed from observations of reactivity similar to those that will be made during this experiment. In particular, observations of similar behavior permits grouping of different elements into chemical families, while noting trends within families allows ordering of elements in each column. These groupings, based on experimental observations, are in agreement with theoretical descriptions of the atom. From a theoretical view, atoms have protons and neutrons in the nucleus, surrounded by electrons grouped into shells. Filled shells are stable, and atoms may react by gaining or losing electrons to complete their outer shells and become ions. The periodic table arranges elements with electrons in different outer shells into different rows, and puts elements with similar outer shell electron configurations into the same column. Thus going down columns of the periodic chart, elements behave similarly because of their similar electron configurations; going across rows, elements show changing properties because of different electron configurations. Trends in each column, or Group as they are known, are governed by the ability of an atom to gain or lose electrons. Smaller elements, near the top of the chart, have electrons (negatively charged) closer to the nucleus (positively charged). Thus electrons will be attracted more strongly and in metals electrons will be lost with more difficulty than elements lower in each column. Conversely, for non-metals, which gain electrons to from ions, the elements near the top of the group will be more active since fewer electrons screen the positive nucleus. This theoretical prediction can be used to understand and explain the experimentally observed behavior.

2 Trends in the behavior of elements in each group are governed by the preference to have a complete outer shell. Elements closer to complete shells tend to be more reactive than elements with electron configurations farther from completion. The noble gas elements, which are generally unreactive, all have complete electron shells. This experiment begins with observations of the reactivity of some metals and simple oxides (to gain insight into trends). It then focuses on the halogen family, showing the behavior of three of these elements plus their salts and solutions in water and hexane. Reactions between them are used to illustrate trends and to permit identification of unknown samples. I. Reactivity of Metals The reactivity of Na, Li, and K from Group IA, of Mg and Ca from Group IIA, and of Al from Group IIIA will be compared by observing some of their reactions with water and hydrochloric acid. From the observations, comparisons between Groups and within Groups can be made. II. Halogens The properties of chlorine (Cl 2 ), bromine (Br 2 ) and iodine (I 2 ) will be observed. Chlorine is a greenish gas at room temperature and is commonly used in the laboratory. Aqueous solutions of chlorine will be used to illustrate some of the properties of chlorine. Bromine will be observed in a closed tube. Iodine will be sublimed (converted directly from the solid to the gaseous phase) and recrystallized to study its behavior. Solutions of each of these elements in water and in hexane will be examined. III. Halide Salts Halide salts are compounds formed from metals and halogen elements. The potassium halide salts will be studied by comparing their colors, solubilities, and reactivities, both alone and in combination with water, hexane, and aqueous solutions of chlorine, bromine, and iodine. Halide ions are formed when neutral halogen molecules are reduced (gain electrons from some other species, which loses them). Halide ions may lose their electrons to a more active halogen. In this experiment, chloride (Cl - ), bromide (Br - ), and iodide (I - ) ions will be combined with aqueous (water) solutions of Cl 2, Br 2, and I 2 molecules to observe when these reactions will occur. Hexane will be added to provide evidence of reaction, since the halogen molecule which remains after reaction (or lack of reaction) can be inferred from observing the color of the hexane layer. There is a clear trend in halogen reactivity that is related to its position on the periodic chart. This trend can be interpreted in terms of the distance between the nucleus and the outer shell electrons for each atom. The experimental observations can also be used to identify an unknown halide by comparing its behavior to that

3 of a known sample. IV. Determination of an Unknown Halide Salt An unknown solution of either KCl, KBr, or KI in water will be assigned to each student. Students will be able to determine which of the unknown salts they have by applying the tests used in Part III D. Since the focus of this experiment is observation, be careful to write down what you see in your lab notebook as you do each step. Some of the observations in early sections will be needed to answer questions in later sections of this experiment. EXPERIMENTAL PROCEDURES I. Reactivity of Metals A. (This section will be demonstrated by your instructor.) Observe freshly cut pieces of sodium, lithium and potassium. Record the physical state, color and consistency of each element. Observe the reactivity of each metal when your instructor places a small piece of each metal in separate beakers containing water and phenolphthalein. (NOTE: This is a demonstration of the reactivity of the metals with water. This is NOT an appropriate method of disposal because of the explosion potential related to the very high reactivity of these elements compared to Ca, Mg and Al.) B. Place one small piece of Ca, Mg and Al into separate test tubes. Slowly add 3-4 ml of distilled water and a drop of phenolphthalein to each test tube. Phenolphthalein will identify one of the products formed; it changes to a pink or red in the presence of a base (OH - ) and remains colorless in an acidic (H + ) solution. Note that some reactions happen very quickly while others take much more time. If you do not observe any reaction initially, check the test tube after 15 minutes to see if a slow reaction occurred. Did any of the solutions become basic? Observe the reactivity of each metal. Which is more reactive? C. Place a small piece of Mg and Al into separate test tubes. Carefully add 3 ml of 6 M HCl (hydrochloric acid) drop by drop. Observe the reactivity of each metal. Which is the most reactive? II. Halogens In order to recognize the characteristic color of each halogen (Cl 2, Br 2, and I 2 ) you must first complete a series of control experiments where an aqueous solution of each halogen is independently mixed with

4 hexane. Since non-polar halogens are more soluble in the non-polar hexane layer, the color of the hexane layer will indicate the identity of the halogen present. Place 2 ml of aqueous solutions of Cl 2, Br 2, and I 2 into separate test tubes. Observe the characteristic colors of each of the aqueous solutions. To each tube add 1 ml of hexane. (note which layer is the hexane layer. Is it on top or bottom?) Water and hexane are immiscible (not mutually soluble). Stopper the tube (do not use your fingers) and shake vigorously. Observe the color of each halogen in the hexane layer. Halogens are more soluble in hexane than in water, so the halogen (Cl 2, Br 2, or I 2 ) will move to the hexane layer. The colors in hexane are characteristic of the halogen and can be used to identify which halogen is present in a sample. Prepare a table of observations with three columns (halogen, color of water layer, and color of hexane layer) with rows for Cl 2, Br 2 and I 2. III. Halide Salts A. Observe the appearance of three halide salts - KCl, KBr, and KI - as solids and in solution (if any). Put a few crystals of each one into separate test tubes, add water, and shake to see if they dissolve. Put a few crystals of each one into separate test tubes, add hexane, and shake to see if they dissolve. Prepare a table of observations with three columns (solid, with water, and with hexane) and with rows for KCl, KBr and KI. Your observations should include color (distinguishing the color white from colorless), whether or not the solid dissolves, and the color of liquids. B. Using clean test tubes, prepare the following set of nine systems: 3 test tubes, each containing 2 ml of 0.1 M KCl and 1 ml of hexane. 3 test tubes, each containing 2 ml of 0.1 M KBr and 1 ml of hexane. 3 test tubes, each containing 2 ml of 0.1 M KI and 1 ml of hexane. To study the reactivity of the halide salts, mix each of their solutions with 1 ml of aqueous solutions of Cl 2, Br 2, and I 2. (Put 1 ml of Cl 2 solution into a test tube with KCl solution, 1 ml of Cl 2 solution into a test tube with KBr, and 1 ml of Cl 2 solution into a test tube with KI solution. Then do the same for Br 2 and I 2 solutions.) Each box in the table represents one of your nine test tubes.

5 Cl - Br - I - Cl 2 Cl 2 + Cl - Cl 2 + Br - Cl 2 + I - Br 2 Br 2 + Cl - Br 2 + Br - Br 2 + I - I 2 I 2 + Cl - I 2 + Br - I 2 + I - Observe the color of the hexane layer after mixing, comparing it to the results in Part IIC. If the color is the same as would be expected for the added halogen, then no reaction has occurred. If it is not, then this is evidence of a reaction. Prepare a table of observations with three columns (for Cl 2, Br 2 and I 2 ) and with rows for KCl, KBr and KI (similar to the table shown above). In each of the 9 boxes, put your observations of the colors of the (bottom) water layer and the (top) hexane layer. The hexane layer color will be crucial in determining what halogen is present after any possible reaction has taken place. At this point you should be able to write the chemical reactions for samples that exhibited a reaction. For example, when chlorine (Cl 2 ) is mixed with the iodide ion (I - ) a reaction should be observed (after shaking, the hexane layer is the characteristic color if iodine, I 2 ). Since you began with the iodide ion (KI) and ended up with the iodine molecule (I 2 ), this is evidence of a reaction: The potassium ion is a spectator ion and is omitted from the net ionic reaction above. In many cases you will not observe any reaction (the color of the hexane layer exhibits the characteristic color of the halogen you began with). In this case, no reaction took place between the halogen and the halide. IV. Determination of an unknown halide Take one of the unknown solutions, which will contain KCl, KBr, or KI, and determine which it is by applying a test used in Part IIIB. To do this, it will be necessary to plan a procedure to distinguish between the solutions. Write down the unknown number of your salt solution. Include a clear description of the procedure you have chosen in your lab notebook. Write down your observation of the outcome of this procedure and your conclusion about the identity of your unknown.