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2512 Periodic Properties

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    440578
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    PERIODIC PROPERTIES

    1.0 INTRODUCTION

    Elements in the periodic table are arranged in order of increasing atomic number into horizontal rows whose lengths are such that elements with similar properties recur periodically. The elements within a given column of the periodic table, while having similar electron configuration, tend to have similar chemical properties. Because of this, columns of elements are often referred to as “groups” or “families” of elements. These families include the most familiar alkali metals, alkaline earth metals, halogens, and noble gases. Other families are the pnictogens (nitrogen family) and the chalcogens (oxygen family).

    The physical and chemical properties of the elements within a given family tend to change gradually as one goes from one element in the column down to the next. In this experiment, the some of the properties of the alkali and alkaline earth metals will be observed and compared. The solubility properties of various ionic compounds containing alkaline earth metal cations will be observed. The properties of the halogen family, as neutral molecules and as anions, will be studied including the relative reactivity between halogens and halides.

    1.1 Group 1 (or 1A) – The Alkali Metals

    Alkali metals are some of the most reactive metals in the periodic table. Extreme caution should be observed in handling these metals. Lithium, sodium, potassium, rubidium, and cesium all react vigorously or even violently in cold water. The reaction produces alkali metal hydroxide and hydrogen gas.

    2 M(s) + 2 H2O(l) ­čí¬ 2 MOH(aq) + H2(g)

    Alkali metals lose an electron to make ions with a +1 charge. They are easily oxidized and typically used in organic reactions as reducing agents, species that tend to reduce (give electrons to) other species.

    1.2 Group 2 (or 2A) – The Alkaline Earth Metals

    The alkaline earth metals— beryllium, magnesium, calcium, strontium, barium, and radium—are all moderately reactive. They are less metallic in character than the alkali metals. Alkaline earth metals lose two electrons to make ions with a +2 charge. Their reaction with acids (H+), produces the metal cation and generates hydrogen gas.

    M’(s) + 2 H+(aq) ­čí¬ M2+(aq) + H2(g)

    When solutions of Group 2 metal cations are mixed with solutions containing anions such as CO32-or SO42-, ionic compounds of the general form MX will precipitate if the compound MX is insoluble under the reaction conditions used, as shown in the net ionic equation below.

    M2+(aq) + X2-(aq) ­čí¬ MX(s) if MX is insoluble

    No precipitate will be observed if the compound MX is soluble.

    When the solubilities of compounds containing various cations combined with a given anion are compared, a solubility trend that follows the order in the periodic table is expected. For example, for the solubilities of the sulfate salts, the solubility is expected either to increase or decrease as we go down the alkaline earth family.

    1.3 Group 17 (or 7A) – The Halogens

    The halogens include fluorine, chlorine, bromine, iodine, and astatine. They are also relatively reactive. Unlike the alkali and alkaline earth metals, the halogens tend to gain electrons to form anions, such as Cl-, Br-, and I-. Since they are reduced when this occurs, the halogens are oxidizing agents, species that tend to oxidize (remove electrons from) other species. Thus, it is possible for some halogens (Cl2, Br2, I2) to react with halide ions (Cl-, Br-, I) in a single replacement reaction. Taking X2 to be a halogen, and Y- to be a halide ion, the reaction would be as follows:

    X2 + 2 Y- ­čí¬ 2 X- + Y2

    The reaction will only occur if X2 is a better oxidizing agent than Y2, since X2 has to remove electrons from the Y- ions. If Y2 is a better oxidizing agent than X2 then no reaction will occur. Solutions of halogens and halide ions will be combined to determine the relative oxidizing abilities of the halogens. A trend can be shown as one goes from one halogen to the next in the Periodic Table.

    Halogens have characteristic colors in non-polar organic solvents, such as cyclohexane, while the halide ions are all colorless in polar solvents like water. In this case, we can use the color changes that occur (or don’t occur) to determine whether or not one halogen has displaced another in the above reaction. The relative oxidizing abilities of the halogens will be determined from the reactivity patterns.

    Example. Suppose that an aqueous solution of bromine or bromine water (Br2H2O) is mixed with cyclohexane. Cyclohexane is less dense than and insoluble in water. The cyclohexane layer will be the top layer and water will be the bottom layer. The bromine is much more soluble in cyclohexane than in water and goes into the cyclohexane layer if you shake the mixture well, giving the cyclohexane layer an orange color. Now suppose we add a solution containing chloride ion (Cl-), to the bromine mixture and mix well. There are two possible results:

    a. If bromine (Br2) is a better oxidizing agent (more easily reduced) than chlorine (Cl2), it will take electrons from the chloride (Cl-) ions to be converted to bromide (Br-) ions:

    Br2 + 2 Cl- ­čí¬ 2 Br- + Cl2

    If this reaction occurs the color of the cyclohexane layer will change from orange (the color of Br2 in cyclohexane) to that of a solution of Cl2 in cyclohexane.

    b. On the other hand, if no reaction occurs:

    Br2 + 2 Cl- ­čí¬ No Reaction

    There will be no color change; the cyclohexane layer will remain orange. This implies that bromine is not a better oxidizing agent than chlorine or, in other words, that chlorine is a better oxidizing agent than bromine.

    In summary, no color change means that no reaction occurred, and a color change means that a reaction occurred. In either event, the halogen that remains after the halogen and halide are mixed is the less active halogen.

    NOTE: It is important to keep in mind the difference between the halogen elements and the halide ions:

    Halogens Halide Ions

    Bromine, Br2 Bromide ion, Br-

    Chlorine, Cl2 Chloride ion, Cl-

    Iodine, I2 Iodide ion, I-

    The halogens are molecular substances, oxidizing agents, have odors, and are distinct colors. They are only slightly soluble in water and much more soluble in cyclohexane.

    The halide ions are soluble only in water, have no color or odor, and are not oxidizing agents. They do not dissolve in cyclohexane.

    2.0 SAFETY PRECAUTIONS AND WASTE DISPOSAL

    !!Wear your safety goggles!!

    Handle all metals with forceps, tongs, or a spatula, not with your fingers.

    Use plastic containers when adding small samples of Na and K to water, along with all normal safety precautions.

    Na and K metals must be kept separate. They form an air-reactive liquid metal alloy at room temperature and require special precautions if they are to be used. NaK is much more inherently dangerous and requires special techniques to be safely used.

    Cyclohexane is flammable. Do not use it anywhere near an open flame.

    Avoid breathing the halogen vapors.

    Don’t use your finger to stopper the tubes. Notify your instructor if you spill any bromine solution; it can be cleaned up with sodium thiosulfate solution.

    All reaction mixtures should be placed in the waste container when you are done with them.

    3.0 CHEMICALS AND SolutionS

    Chemical

    Concentration

    Approximate amount

    Notes

    Na metal

    N/A

    One small piece

    These metals are soaked/stored in mineral oil. Consult your instructor for proper handling.

    K metal

    N/A

    One small piece

    Ca metal

    N/A

    One small piece

     

    Mg metal

    N/A

    One small piece

     

    Phenolphthalein

    N/A

    A few drops

     

    HCl

    6 M

    10 mL

     

    Ba(NO3)2

    0.1 M

    10 mL

     

    Ca(NO3)2

    0.1 M

    10 mL

     

    Mg(NO3)2

    0.1 M

    10 mL

     

    Sr(NO3)2

    0.1 M

    10 mL

     

    H2SO4

    1 M

    10 mL

     

    Na2CO3

    1 M

    10 mL

     

    (NH4)2C2O4

    0.25 M

    10 mL

     

    K2CrO4

    1 M

    10 mL

     

    CH3COOH

    1 M

    10 mL

     

    Cl2•H2O

    N/A

    5 mL

     

    Br2•H2O

    N/A

    5 mL

     

    I2•H2O

    N/A

    5 mL

     

    NaCl(aq)

    0.1 M

    5 mL

     

    NaBr(aq)

    0.1 M

    5 mL

     

    NaI(aq)

    0.1 M

    5 mL

     

    Cyclohexane

    N/A

    10 mL

     

    4.0 GLASSWARE AND APPARATUS

    Item

    Use

    Notes

    Test tubes

    To hold reaction mixtures for observation

     

    Cork or rubber stopper

    To contain liquid sample in a test tube

     

    Test tube rack

    To place test tube in while observing reaction

     

    Stirring rod

    To stir reaction mixture

     

    250-mL beaker

    To hold reaction mixtures for observation

     

    Watch glass

    To cover beaker

     

    Crucible tong or forceps

    To pick up or grasp small metal pieces

     

    5.0 PROCEDURE

    Part A. Reactivities of Some Alkali and Alkaline Earth Metals

    1. Metallic sodium and potassium (Optional Instructor Demo)

    a) For safety, these reactions must be conducted under the fume hood and performed separately.

    1. Fill a small, plastic, rectangular bin about halfway with laboratory water. Place 2 to 3 drops of phenolphthalein solution into the water.
    2. Using large forceps, obtain a small piece of Na metal from the reagent bottle and soak in the rinsing solution provided. Dry the metal piece completely in a paper towel.
    3. Carefully place the metal into the bin. Lower the glass shield of the fume hood and allow the reaction to proceed. Record your observations.
    4. Make sure that the metal has completely reacted before discarding the solution into the sink.
    5. Repeat steps 2-5, this time using K metal.
    1. Metallic magnesium and calcium

    Place two clean, dry medium test tubes in a test tube rack. Using crucible tongs, put a small piece of Mg in one test tube and a small piece of Ca into another. Add 5 mL of laboratory water into each test tube. Record your observations in the data recording sheet.

    Add 5 drops of 6M HCl to each test tube. If no change is evident, add an additional 5 mL of 6 M HCl. Record your observations.

    Part B. Solubilities of Some Salts of Alkaline Earth Metal Cations

    Four tests will be performed on solutions containing these ions: Mg2+, Ca2+, Sr2+, and Ba2+. You will be given aqueous solutions of Mg(NO3)2, Ca(NO3)2, Sr(NO3)2, and Ba(NO3)2. Nitrate is a spectator ion and will not interfere with any of the tests.

    Between tests, the test tubes should be washed, rinsed with tap water, and then rinsed at least twice with small amounts of laboratory water. The test tubes do not have to be dried. The stirring rod, if you must use one, should be rinsed in a beaker of laboratory water between uses.

    1. Add about 1 mL (approximately 12 to 15 drops) of the 0.1 M solutions of Mg(NO3)2, Ca(NO3)2, Sr(NO3)2, and Ba(NO3)2 to separate test tubes. To each test tube, add 1 mL of 1 M H2SO4. Swirl the test tube vigorously or use a stirring rod to mix the contents. Observe whether or not an insoluble precipitate forms.
    2. Record your observations in the data recording sheet, noting whether precipitate forms, and any characteristics—as color, odor, amount, size of particles, and settling tendencies—that might distinguish it.
    3. Discard reaction mixture into a waste beaker.
    4. Clean the test tubes as directed above and add 1 mL of each of the metal nitrate solution once again. To each test tube, add 1 mL of 1 M Na2CO3. Mix the contents as before and record observations.
    5. Repeat the procedure two more times, once with 1 mL of 0.25 M (NH4)2C2O4 added to each of the four metal nitrate solutions, and finally with a mixture of 1 mL of 1 M K2CrO4 and 1 mL of 1 M CH3COOH added to the four metal nitrate solutions.

    Part C. Reactivities of Halogens and Halides

    Obtain solutions containing Cl2, Br2, I2, Cl-, Br-, and I-. For the halogens, you will be given halogen water solutions: Cl2•H2O, Br2•H2O, and I2•H2O. For the halides, you will be given aqueous solutions of NaCl, NaBr, and NaI. The sodium ion (Na+) is a spectator ion and will not interfere with any of the tests.

    1. Determine the color of the bromine in cyclohexane by placing 1 mL of bromine water (Br2•H2O) in a small test tube, adding 1 mL of cyclohexane, and mixing the contents well until the bromine color is mostly in the cyclohexane layer. Note the color of the cyclohexane layer. Although the colors of the two layers may be different, it is the color in the cyclohexane layer that will be observed throughout this experiment.
    2. Determine the colors of chlorine and iodine in cyclohexane the same way as for bromine. Combine chlorine water (Cl2•H2O) with cyclohexane, and then iodine water (I2•H2O) with cyclohexane. Record the color of the cyclohexane layer in each case.
    3. Prepare each of the following mixtures in a small test tube by combining 1 mL of cyclohexane with 1 mL of each of the two reagents listed. Cover the test tube with a cork stopper and shake the contents vigorously. Note and record the color of the (upper) cyclohexane layer in your data recording sheet. The test tubes must be washed and then rinsed at least twice with laboratory water between tests.

    Mixture #

    1 mL of halogen + 1 mL of cyclohexane; shake

    Then add 1 mL of halide; shake

    1

    Br2

    NaCl

    2

    Br2

    NaI

    3

    Cl2

    NaBr

    4

    Cl2

    NaI

    5

    I2

    NaCl

    6

    I2

    NaBr


    6.0 DATA RECORDING SHEET

    Last Name

    First Name

     

    Partner Name(s)

    Date

    1. Reactivities of Some Alkali and Alkaline Earth Metals

    Metal

    Behavior in Cold Water

    Sodium

     

    Potassium

     

    Write balanced chemical equations consistent with your observations above.






    Metal

    Behavior in Cold Water

    6 M HCl Added

    Magnesium

       

    Calcium

       

    Write balanced chemical equations consistent with your observations above.









    1. Solubilities of Some Salts of Alkaline Earth Metal Cations

    Indicate whether precipitate forms (“ppt”) or there is no precipitate (“no ppt”). Note any distinguishing characteristics of the precipitates.

     

    1 M H2SO4

    1 M Na2CO3

    0.25 M (NH4)2C2O4

    1 M K2CrO4 +

    1 M acetic acid

    Ba(NO3)2

           

    Ca(NO3)2

           

    Mg(NO3)2

           

    Sr(NO3)2

           
    1. Reactivities of Halogens and Halides

    Indicate the color of the cyclohexane layer after mixing cyclohexane and each halogen.

    Halogen

    Color of Cyclohexane Layer

    Br2

     

    Cl2

     

    I2

     

    Indicate (a) the color of the cyclohexane layer after addition of each halide reagent and (b) the corresponding halogen that is present. If a reaction occurred, write a net ionic equation. If no reaction occurred, write “NR”.

    Mixture

    Color of cyclohexane layer after mixing

    Halogen present in the cyclohexane layer

    Reaction

    Br2 + Cl-

         

    Br2 + I-

         

    Cl2 + Br-

         

    Cl2 + I-

         

    I2 + Cl-

         

    I2 + Br-

         

    7.0 POST-LAB QUESTIONS AND CONCLUSIONS

    1. Which is more reactive with water, Na or K? Explain your answer based on your observations.





    1. Based on your answer in #1, what would be the trend for reactivity with water for alkali metals as atomic number increases.



    1. Which is more reactive, Mg or Ca? Explain your answer based on your observations.





    1. Will you expect a similar reactivity trend for alkaline earth metals as you observed for alkali metals? Why or why not?





    1. Consider the relative solubilities of the Group 2 cations in the various precipitating reagents. On the basis of the trends you observed, list the four alkaline earth metal ions in order based on the solubility of the salts formed—the one that forms the greatest number of soluble compounds is the most soluble and the one that forms the greatest number of precipitates is the least soluble

    ________ > ________ > ________ > ________

    most soluble least soluble

    1. Is this order consistent with the data from all of the precipitation reagents? If not, why did you order them as you did?



    1. Is this order consistent with a periodic trend? If no, how is it different? If yes, does solubility increase or decrease down the column?



    1. Rank the halogens in order of their decreasing oxidizing ability:

    ___________ > ___________ > ___________

    most reactive least reactive

    1. Does the order above follow a periodic trend? If no, how is it different? If yes, does oxidizing ability (“activity’) increase or decrease down the column?




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