Chem 2115
Experiment # 6
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
REFERENCE: Chemistry: The Molecular Nature of Matter, Jespersen et al., 6th edition, Sections 5.1–5.2, 8.7–8.10
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 trashcan. 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
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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.
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 (Cl2), bromine (Br2) and iodine (I2) will be observed. At room temperature
chlorine is a greenish gas, bromine is a reddish/orange liquid, and iodine is a purple solid. Solutions of
each of these elements in water and in hexane will be examined to determine the characteristic color of
each of the halogens in solution.
III.
Halide Salts
Halide salts are ionic 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 Cl2, Br2, and I2 molecules to observe when these reactions will occur. Hexane will be added to
provide evidence of reaction, since the halogen molecule that remains after reaction (or lack of reaction)
can be inferred from observing the color of the hexane layer.
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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
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
BE SURE TO WEIGH YOUR CRUCIBLE AND PRODUCT FROM EXPERIMENT 5 THAT HAS DRIED
OVER THE WEEK IN YOUR LOCKER. RECORD THE MASS AND ENTER THE DATA ON THE LAB
COMPUTER.
I. Reactivity of Metals - Observations for this section should include color changes, bubbling (gas formation),
temperature changes, formation/disappearance of a precipitate, and color when phenolphthalein is added.
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. Observe the
reactivity of each metal. Did any of the solutions become basic? Which is more reactive?
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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 (Cl2, Br2, and I2) you must first complete a
series of control experiments where an aqueous solution of each halogen is independently mixed with
hexane. Since halogens (which are non-polar) are more soluble in hexane (also non-polar), the color of the
hexane layer will indicate the identity of the halogen present.
Place 2 mL of aqueous solutions of Cl2, Br2, and I2 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 (Cl2, Br2, or I2) 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 Cl2, Br2 and I2 (Note: Since water is denser than hexane, the aqueous layer is on the
bottom of the test tube.) You will need to use this table in part IIIB to identify the halogen present after
mixing each sample.
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.
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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 Cl2, Br2, and I2. (Put 1 mL of Cl2 solution into a test tube with KCl solution, 1 mL of
Cl2 solution into a test tube with KBr, and 1 mL of Cl2 solution into a test tube with KI solution.
Then do the same for Br2 and I2 solutions.) Each box in the table below represents one of your
nine test tubes. (Note: In each case the potassium ion (K+) is a spectator ion and is omitted from
the table)
Cl2
Br2
I2
Cl-
Cl2 + Cl-
Br2 + Cl-
I2 + Cl-
Br-
Cl2 + Br-
Br2 + Br-
I2 + Br-
I-
Cl2 + I-
Br2 + I-
I2 + I-
Observe the color of the hexane layer after mixing, comparing it to the results in Part II. 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 Cl2, Br2 and I2) 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. Your
table from Part II should enable you to identify the halogen present in the hexane layer after
mixing. (Note: Five of these systems should have I2 in the hexane layer after the halogen addition,
three should have Br2 and one should have Cl2. If your results do not agree with this, check with
your instructor.)
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At this point you should write the chemical reactions for samples that exhibited a reaction.
For example, when chlorine (Cl2) is mixed with the iodide ion (I-, from KI) a reaction should be
observed (after shaking, the hexane layer is the characteristic color if iodine, I2). Since you began
with the iodide ion (KI) and ended up with the iodine molecule (I2), this is evidence of a reaction:
Note: The potassium ion (K+) is a spectator ion and is omitted from the net ionic reaction written
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 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.
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