Anion Analysis

1. Anion Analysis Background Anions - Elements or molecules that have gained electrons. They have negative charge and have been reduced. They can be oxidizing agents in a ReDox reaction Cations - Elements or molecules that have lost electrons. They have positive charge and have been oxidized. They can be reducing agents in a Redox reaction Families of Elements Elements of similar electron structure located in vertical columns (Groups) in the Periodic Table. Group I - Alkali Metals (Hydrogen, Lithium, Sodium, Potassium) Group II - Alkaline Earth Metals (Beryllium, Magnesium, Calcium, Barium) Group VII - Halogens (Fluorine, Chlorine, Bromine, Iodine, Astatine)

2. Anion Analysis Oxidation • Atom or Molecule Loses Electrons • Increase Oxidation State (+4  +5) • Gain Oxygen (in Organic Molecules) • Lose Protons (in Organic Molecules) • Oxidized Species is a Reducing Agent Reduction • Atom or Molecule Gains Electrons • Decrease Oxidation State (+5 +4) • Lose Oxygen (in Organic Molecules) • Gain Protons (in Organic Molecules) • Reduced Species is an Oxidizing Agent An Oxidation-Reduction Reaction involves one species gaining electrons (it is reduced) and another species loosing electrons (it is oxidized).

3. Anion Analysis Group II Elements – The Alkaline Earth Elements Beryllium, Magnesium, Calcium, Strontium, Barium These elements have two (2) electrons in the outer ‘s’ atomic orbital. Removing these two electrons results in a more stable noble gas configuration, i.e., the outer shell is complete. Therefore, these elements tend to lose electrons in aqueous solution forming positively charged divalent cations. M: → M+2 + 2 e- Group VII Elements – The Halogens Fluorine, Chlorine, Bromine, Iodine, Astatine These elements lack a single electron in the outer ‘p’ orbital. Adding an electron completes the shell resulting in a stable noble gas configuration. Therefore, these elements tend to gain an electron in aqueous solution forming negatively charged mono-valent anions. :X: + e- → :X:- ¨ ¨ ¨ ¨

4. Anion Analysis Group Properties • Atomic and Ionic Radii Increase with Increasing Atomic Number within a family (group). • Ionization Energy and Electronegativity decrease with Increasing Atomic Number within a family (group). Electronegativity The relative attraction of an atom for a pair of electrons. • Halogens are more electronegative than alkaline earth metals, thus have a tendency to gain electrons and form negative ions. • In a reaction between a halogen and an element from the left side of the periodic table – hydrogen, alkali metal, alkaline earth metal – the halogen has a stronger affinity for the shared electron pair. The electron pair is not shared equally, thus, a polar covalent bond is formed. • Electron pairs in diatomic molecules such as Hydrogen (H2) and Chlorine (CL2) are shared equally, thus forming a nonpolar covalent bond.

5. Anion Analysis Ionic Equations Chemical reactions between cations (positively charged ions) and anions (negatively charged ions) in aqueous solution can result in dissociated species - they retain ionic charges in solution - and undissociated species. Undissociated species are composed of ions still confined to a crystal lattice and are often insoluble precipitates. When writing the equations for these reactions, undissociated species are represented without ionic charge designation and an indication of the form, such (s) for solid.

6. Anion Analysis Anions Halogens exist in their elemental form as diatomic molecules(Cl2, Br2, I2, At2). Their relatively high electronegativity gives them the ability to easily gain an electron - they are reduced - forming an anion. Thus, they are relatively good oxidizing agents – easily reduced. Since electronegativity decreases with increasing atom number within a group or family, Chlorine is a better oxidizing agent (stronger affinity for accepting electrons) than Bromine, which in turn, is a better oxidizing agent that Iodine. In water – a polar solvent - the ionic halides are more soluble than are the halogens. In a relatively non-polar organic solvent, the non-polar halogens are more soluble than the halides. Note that polar water and non-polar organic solvents are generally immiscible. i.e., not soluble in each other. Halides are colorless in aqueous solution, while the halogens have unique colors in an organic solvent.

7. Anion Analysis The reaction for the reduction of a halogen diatomic molecule to two halide anions is: : X – X : + 2e- → 2 : X :- The diatomic Halogen X2 has 14 valence electrons and the two halide anions had a total of 16 valence electrons; thus the halogen atoms have been reduced – gained electrons. Powerful oxidation agents can oxidize the halide to halogen. 2 :X:- → :X – X: + 2e- Halides (X-) and Halogens (X2) have different solubilities in water and in non-polar organic solvents. ¨ ¨ ¨ ¨ ¨ ¨ ¨ ¨ ¨ ¨ ¨ ¨

8. Anion Analysis The basis of the anion analysis part of this experiment deals with what happens when two different halogen/halide pairs are mixed together. Consider the following mixtures: Chlorine (Cl2) and Bromide (Br-) Cl2 + 2 Br-→ 2 Cl- + Br2 (1) Or Chloride (Cl-) and Bromine (Br2) 2 Cl- + Br2 → Cl2 + Br- (2) If reaction (1) occurs, it means that Chlorine (reduced to Chloride) is a stronger oxidizing agent than Bromine. If reaction (2) occurs, then Bromine is a stronger oxidizing agent than Chloride. By observing the color produced in the organic solvent when various Halide / Halogen mixtures are prepared, the relative oxidizing power of the Halogen can be determined.

9. Anion Analysis Anion Analysis For this experiment, the Halogens will be pre-prepared for you in aqueous form to which you will add an organic solvent (Cyclohexane) in which the Halogen will preferentially dissolve producing the appropriate color. You will prepare nine (9) Halogen / Halide mixtures. To each Halogen solution (after the organic solvent has been added) you will add a Halide solution.

10. Anion Analysis The Procedure • Set up 9 clean, distilled water washed test tubes. • Add 1 mL of a halogen solution to 3 test tubes. • Repeat for the next Halogen solution. • Repeat for the next Halogen solution. • Label each test tube for its Halogen content. • Add 2 mL of the organic solvent to each test tube. • Cork each test tube and shake thoroughly. • Note the color of the organic layer – color of halogen X2 • Add 1 mL of a Halide solution (X-) drop by drop to a halogen test tube. • Shake gently during the addition; then cork and shake thoroughly.

11. Anion Analysis Anion Analysis (con’t) Procedure (con’t) • Observe the color of the organic layer – the top layer • Repeat using the same Halide solution on another halogen test tube. • Repeat for the third Halogen test tube. • Repeat steps 9-13 for the second Halide solution. • Repeat steps 9-13 for the third Halide solution. • Obtain an unknown Halide from the instructor and use the following analysis scheme to determine its identity.

12. Anion Analysis Anion Analysis (Con’t) Data Analysis • For each test tube determine which Halogen (X2) is present in the top organic layer. • Cl2 – Pale Yellow • Br2 – Brown or Orange • I2 – Purple • State whether the reaction in each test tube was an oxidation / reduction reaction between the mixed pair. • From your results determine a scheme for analyzing an unknown Halide solution using just one of the halogens.

13. Anion Analysis Anion Analysis (con’t) Data Analysis (con’t) • Discuss the relative oxidizing powers of the Halogens based on your results. • Do you think Astatine (At2) could oxidize Iodide (I-)? • Could Chloride (Cl-) reduce Fluorine (F2)? • Write the appropriate net ionic equations to support your answers.