05. Periodic Properties of
the Halogens

Ionization energy, electronegativity, and electron affinity also vary in similar manners (see the sections in your textbook).

These periodic properties often influence the chemical reactivities of an atom. This experiment is designed to allow you to determine the relative reactivities of three of the Group 17 elements (i.e. the halogens Cl, Br, and I) in reduction - oxidation reactions (referred to as redox reactions). Specifically, you will be exploring the reactivity of the Group 17 elements as a reducing agent and as an oxidizing agent.

Oxidation-Reduction Reactions

Redox reactions involve the exchange of electrons. In the example below, 'X' represents a halogen (e.g. F, Cl, Br, or I) and 'Y¯' denotes a halide ion. A redox reaction of halogen X₂with halide Y¯ would be as follows:

X₂ + 2 Y¯ → 2 X¯ + Y₂

Redox reactions are best understood when written in terms of two half reactions.

Redox reactions are best understood when written in terms of two half reactions.

Viewed as a half reaction the reduction of X₂ would be:

X₂ + 2 e¯ → 2 X¯ a gain of electrons

And the oxidation half reaction of Y¯ would be:

2 Y¯ → Y₂ + 2 e¯ a loss of electrons

In this reaction, X₂ is being reduced; it is called the oxidizing agent because it is taking electrons away from Y¯, causing Y¯ to be oxidized. Conversely, Y¯ is being oxidized; it is called the reducing agent because it is causing X₂ to be reduced.

Upon successful completion of the exercise, you will be able to arrange the three halogens in order of their relative reactivity as oxidizing agents. Likewise, you will be able to arrange the three halides in order of their relative reactivity as reducing agents.

Qualitative Analysis to determine if a reaction has occurred

A chemical reaction may be analyzed quantitatively or qualitatively. In quantitative analysis, the amounts of reactants and products are carefully measured and those data are used to make precise interpretations about the reaction. In qualitative analysis, changes in a reaction mixture are observed and that information is used to determine whether or not a reaction occurred. Qualitative changes include:

• formation of a precipitate

• evolution of a gas

• release of energy as heat

• dissolution of a solid

• and changes in the color or changes in clarity (clear vs. cloudy) of a solution.

In this experiment, we will be using qualitative analysis to observe color changes and in some cases the formation of a precipitate to assess that a redox reaction has occurred.

For this lab, you will be adding prepared solutions of the halogens and halides in water to a test tube, then observing the color. Remember, the reaction you will be performing is:

X₂ + 2 Y¯ → 2 X¯ + Y₂

If a reaction has occurred, you will see the color of halogen Y₂ (product) in your test tube. If no reaction has occurred, you will see the color of halogen X₂ (reactant) in your test tube. Unfortunately, halogens dissolved in water appear very similar in color. Therefore, we need to find a way to differentiate the halogens in the reaction in order to determine if a reaction as occurred or not.

Halogens dissolve in mineral oil much more easily than they dissolve in water. Mineral oil is composed of long, non-polar hydrocarbon molecules. Since halogens are also non-polar, they will have a high affinity for dissolving in the non-polar mineral oil. Interestingly, when halogens dissolve in mineral oil, they appear colored! Meanwhile, the ionic halides (X^-) will not dissolve in mineral oil due to their charged state which makes them very polar and only soluble in polar water. Therefore, for each reaction performed, mineral oil will be added to separate out the halogen that is present in the solution so its color can be observed.

Spectator Ions and Net Ionic Equations

Many ionic compounds dissociate in water to form solvated ions, as can be seen in Common Ion Chart found in the Week 1 Moles Galore Lab. With rare exceptions, salts containing Group 1 elements are water soluble. For example, NaCl dissociates to form Na⁺_(aq) and Cl¯_(aq), where (aq) means the ions are in an aqueous (water) solution. In addition to atomic ions, many polyatomic ions exist that remain as a unit when dissociated, e.g. potassium permanganate, KMnO₄ in water forms K⁺_(aq) and MnO₄¯_(aq). When we write a reaction, it can be written in terms of ionic compounds:

NaOH + HCl → H₂O + NaCl

or in terms of solvated ions:

Na⁺_(aq) + OH¯ _(aq) + H⁺_(aq) + Cl¯_(aq) → H₂O_(l) + Na⁺_(aq) + Cl¯_(aq)

You may notice that there are some species in the above equation that appear on both sides of the equation. These species do not participate in the reaction and are known as spectator ions. When a reaction is written in terms of solvated ions, the spectator ions should be removed, leaving the net ionic equation:

OH^-_(aq) + H⁺_(aq) → H₂O_(l)

This reaction is not a redox reaction. No electrons are exchanged between the OH^- and the H⁺.

Synopsis of the Experiment

Part I Qualitative Identification: In Part I of this experiment, you will develop a qualitative identification scheme which you can use to identify the presence of each halogen/halide used in the lab focusing mostly on the mineral oil.

Part II Halogens as Oxidizing Agents: In Part II, you will mix each halide with each halogen to see which halogens are capable of oxidizing which halides. Using your identification scheme from Part I, you will be able to identify products if they form. In addition, you will combine each halogen with solid copper to see whether it can oxidize the copper. Using your results, you will be able to draw conclusions about the relative oxidizing strengths of the halogens.

Part III Halides as Reducing Agents: In Part III, you will mix each halide with CuSO_4(aq) and with KMnO_4(aq). Again, using your qualitative identification scheme from Part I, you will be able to identify products if the halides are successful as reducing agents. You will be able to draw conclusions about the relative reducing strengths of the halides.

Experiment

Materials and Equipment

• • Test tubes (13 mm)

  • Mineral oil

  • Aqueous solutions of Cl₂, Br₂, and I₂

  • 0.25 M aqueous solution of NaCl

  • 1 M aqueous solution of NaBr

  • 0.1 M aqueous solution of NaI

  • Cu turnings

  • 0.25 M CuSO₄

  • 0.01 M KMnO₄ in 1 M H₂SO₄

  • Parafilm

Instructions

Students will work in groups of 2 unless otherwise specified by your instructor.

Prepare for Part II

The reactions of solid copper and aqueous halogens tend to react slowly. Since it is best to have these solutions sit for about an hour, you will need to create these reactions before proceeding with the rest of the experiment.

1. Place approximately 1 mL of each of the aqueous halogen solutions into three separate test tubes.

2. To each of the test tubes, add a small piece of copper turning (approximately 2 to 3 cm in length).

3. Shake to mix and let sit until you get to the rest of Part II.

Your instructor may have also set these reactions up ahead of time. If so, make sure to head to the front bench to make observations of these reactions.

Part I: Qualitative Identification

Before we can perform any reactions between various halides and halogens, we need a qualitative identification scheme which we can use to confirm the presence of the halogens and halides that will be reactants and products in our reactions.

When preparing to perform an experiment, it is advantageous to construct a table in your notebook for recording observations and results. This not only guides your work while you are conducting the experiment but also makes the review of the results easier after the work is complete.

1. Create a table similar to that shown below:

Table 1: Qualitative Identification of halogens and halides in water and mineral oil

2. Place approximately 1 mL of each of the aqueous halogen and halide solutions in six separate test tubes and record your observations in the table in the column “Aqueous Solution Only”.

3. Add approximately 1 mL of mineral oil to each of the test tubes and mix well.

4. Again, record your observations in the table under “Aqueous Solution with Mineral Oil”. Note: mineral oil has a density of approximately 0.8g/mL while water has a density of approximately 1.0 g/mL.

5. Save these test tube solutions to use them as your "controls" when trying to identify which halogen/halide are present in your reactions in Parts II and III.

Part 1—Questions for thought:

• Are the solvents (water and mineral oil) miscible (capable of being mixed)? If not, which solvent forms the top layer and which solvent the lower layer?

• What can you conclude about the solubilities of the halogens and halides in mineral oil? Explain the preference for one solvent or the other.

• Do halides or halogens possess a unique color defining their presence? Which solvent accentuates the halogen's presence in a solution?

Part II. Halogens as Oxidizing Agents

In Part II, you will be ranking the three halogens (Cl₂, Br₂ and I₂) in terms of their strength as oxidizing agents. You will do this by reacting each of the halogens with each of the aqueous halides and with solid copper. When a halogen and halide from different elements are combined, (i.e.-Br₂ & Cl^-) two outcomes can occur: (1) nothing, (2) the reaction will occur to produce the opposite halogen and halides (i.e.Br^- & Cl₂).

You and your partner will combine various aqueous solutions of halogens and halides, then add mineral oil and look for indications of the presence of reactant or products (new halogens and halides from your starting materials) to determine whether or not a reaction occurred. In this way, the relative oxidizing strength of halogens can be determined.

Another way of testing the strength of halogens is to look for products when they are combined with metals, in this case, solid copper. When a halogen reacts with elemental copper, Cu_(s), the elemental copper dissolves and the Cu²⁺_(aq) ion forms in the test tube (indicated by blue-green color).

1. Create a table similar to Table 2 below.

Table 2: Observations of halogen reactions with and halides and solid copper in water and mineral oil

Note the grayed out portion of the table. These combinations do not need to be performed as the halogen and the halide are the same element.

2. Prior to performing any reactions in this part of the lab, predict whether you think a reaction will take place. Record your predictions in your data table under “Predict.” as either a ‘yes’ if you think a reaction will occur, or ‘no’ if you think no reaction will occur.

3. Add approximately 1 mL of each halogen to approximately 1 mL of each halide in separate test tubes.

4. Add ~1 cm of mineral oil to each test tube and mix vigorously.

5. Allow the contents of the test tubes to settle a moment

6. Record your observations of both aqueous and oil layers in your data table.

7. Add approximately 1 mL of mineral oil to your previously created Cu/halogen test tubes and mix well.

8. Record the observations of both aqueous and oil layers in your data table.

Use the "controls" from the Qualitative Identification Scheme from Part 1 to interpret the results and determine if a reaction occurred or not. Record your conclusions in your data table under “Rxn”. Were your predictions correct? Remember that just because a color appears it does not mean that a reaction has occurred. A reaction has occurred if you observe the disappearance of the reactants and/or evidence of the products.

Please check your results with your instructor.

Part II—Questions for thought:

• What physical change(s) did you observe which indicated that a reaction had occurred with the copper metal?

• Write the balanced net ionic chemical equation for each reaction that occurred. Do not write an equation if no reaction occurred. Note which chemical species is being oxidized and which is being reduced.

• What statements can you make about the relative oxidizing strengths of the halogens? Support your statements by referring to experimental results. Arrange the halogens in order of increasing strengths as oxidizing agents.

Part III. Halides as Reducing Reagents

In Part III, you will be ranking the three halides (Cl^-, Br^- and I^-) in terms of their strength as reducing agents. You will do this by reacting each of the halides with aqueous copper sulfate (CuSO_4(aq)) and potassium permanganate (KMnO_4(aq)).

One way of testing the reacting strength of a halide is to look for the reaction products when it is combined with an oxidizing agent that has a distinctive aqueous color that identifies its presence. The loss of the oxidizing agent's characteristic aqueous color indicates its disappearance upon reaction to product.

If an excess of halide is mixed with a blue solution of copper sulfate (CuSO_4(aq)) or a purple solution of potassium permanganate (KMnO_4(aq)), the halide may act as a reducing agent to form solid copper halide (CuX_(s)) or manganese dioxide (MnO_2(s)), respectively and its corresponding halogen.

1. Create a data table similar to those created in Parts I and II that will allow you to collect predictions, observations, and reaction conclusions for the reactions of each of the halides with both copper sulfate (CuSO_4(aq)) and potassium permanganate (KMnO_4(aq)).

2. Perform the reactions of each of the halides with both copper sulfate (CuSO_4(aq)) and potassium permanganate in a similar manner as you did in Part II. Use the "controls" from the Qualitative Identification Scheme from Part 1 to interpret the results and determine if a reaction occurred or not.

3. Record your conclusions in your data table. Were your predictions correct? Remember that just because a color appears it does not mean that a reaction has occurred. A reaction has occurred if you observe the disappearance of the reactants and/or evidence of the products.

Please check your results with your instructor.

Part III—Questions for thought:

• Why were the copper ion and permanganate ions wise choices to serve as the oxidizing agents?

• What statements can you make about the relative reducing strengths of the halides? Support your statements by referring to experimental results. Arrange the halides in order of increasing strengths as reducing agents.

• Write the balanced net ionic chemical redox reactions for each reaction that occurred (net ionic means that spectator ions [ions that are not involved in the reaction] are not listed). Do not write an equation if no reaction occurred. For each reaction written, note which chemical species is being oxidized and which is being reduced. See below for the general net ionic balanced redox reactions you will need to write the redox reactions in this part of the experiment.

General Net Ionic Balanced Redox Reactions

Here are the generic net ionic balanced reactions for the reactions that react in Part II and III

Part II:

Reaction of halogens with halides:

X₂ + 2Y^- → 2X^- + Y₂

Reaction of halogens with solid copper:

X₂ + Cu_(s) → 2X^-_(aq) + Cu²⁺_(aq)

Part III:

Reaction of halide with copper sulfate:

2Cu⁺²_(aq) + 4X^-_(aq) → 2CuX_(s) + X_2(aq)

Reaction of halide with potassium permanganate:

8H⁺_(aq) + 6X^-_(aq) + 2MnO₄^-_(aq) → 3X_2(aq) +2MnO_2(s) + 4H₂O_(l)

For more information:

Group 17 – The Halogens:

http://www.rsc.org/periodic-table

Royal Society of Chemistry web site which has a visual elements section and other chem. stuff

Introduction to the Halogens:

http://www.docbrown.info/page03/The_Halogens.htm

“Doc Brown Chemistry Clinic” by a chemistry teacher in the UK

very colorful, with some cute graphics (can “cute” and “chemistry” be used on the same page?!)

Redox Balancing Practice Problems:

http://wc.pima.edu/~skolchens/C152OL/Ch21/REDOX.htm

https://www.chem.wisc.edu/deptfiles/genchem/netorial/rottosen/tutorial/modules/electrochemistry/02half_reactions/18_21.htm