Name Experiment 12 Section Advance Study Assignment: The Alkaline Earths and the Halogens 1. As pure elements, all of the halogens are diatomic molecular species. Their melting points are: F?, 85 K, Cl?, 239 K, Br?, 332 K, and I?, 457 K. Using the Periodic Table, predict as best you can the molecular formula of elemental astatine, At, the only radioactive element in this family. Also predict whether it will be a solid, liquid, or gas at room temperature. Elemental formula: Phase at room temperature: 2. Substances A, B, and C can all act as oxidizing agents. In solution, A is green, B is yellow, and C is red. In the reactions in which they participate, they are reduced to A?, B?, and C? ions, all of which are colorless. When a solution of C is mixed with one containing B? ions, the color changes from red to yellow. Which species is oxidized? Which is reduced? When a solution of C is mixed with one containing A? ions, the color remains red. Is C a better oxidizing agent than A? Is C a better oxidizing agent than B? Arrange A, B, and C in order of increasing strength as an oxidizing agent. 3. You are given an unknown, colorless, solution that contains one of the following salts: NaA, NaB, NaC. In solution, each salt dissociates completely into the Na? ion and the anion A?, B?, or C?, whose properties are given in Problem 2. The Na? ion is effectively inert. Given the availability of solutions of A, B, and C, develop a simple procedure for identifying the salt that is present in your unknown.
The Correct Answer and Explanation is:
Name: [Your Name]
Experiment 12 – Section: Advanced Study Assignment: The Alkaline Earths and the Halogens
1. Elemental Astatine
- Elemental formula: At₂
- Phase at room temperature: Solid
Explanation:
Like the other halogens (F₂, Cl₂, Br₂, I₂), astatine exists as a diatomic molecule (At₂) in its elemental form. This is consistent with the halogens’ Group 17 chemistry, where they form stable X₂ molecules due to their high reactivity and need to complete their valence shell. Examining the trend in melting points of halogens:
- F₂ = 85 K
- Cl₂ = 239 K
- Br₂ = 332 K
- I₂ = 457 K
Each successive halogen has higher London dispersion forces due to increased atomic size and electron count. Astatine, being below iodine in the periodic table, will be larger and more polarizable, hence should have an even higher melting point than I₂. Therefore, At₂ is expected to be a solid at room temperature.
2. Oxidizing Agent Analysis
- C + B⁻ → B + C⁻
- Oxidized: B⁻ (loses electrons, becomes B)
- Reduced: C (gains electrons, becomes C⁻)
- When C + A⁻ → no change, the color remains red, meaning no redox reaction occurred.
- C is a stronger oxidizing agent than B (it oxidized B⁻),
but not stronger than A (could not oxidize A⁻).
Order of increasing oxidizing strength:
B < C < A
3. Identifying the Unknown Salt
Procedure:
- Divide the unknown solution into three test tubes.
- Add solution A to test tube 1, solution B to test tube 2, and solution C to test tube 3.
- Observe color changes:
- Color change to red: Anion is likely B⁻ (because C oxidizes B⁻).
- Color change to yellow: Anion is likely A⁻ (because C cannot oxidize A⁻, but B can).
- No color change: Anion is C⁻ (already reduced; C, A, or B won’t react).
Explanation
This experiment explores redox behavior in halogens and helps predict trends and reactivity using periodic properties. For question 1, astatine belongs to the halogen family. All halogens exist as diatomic molecules due to their electron configuration and bonding tendencies. The trend in melting points increases down the group due to increasing van der Waals forces. Since I₂ has a melting point of 457 K, and astatine is larger and more polarizable, its melting point would be higher—suggesting it is a solid at room temperature.
In question 2, analyzing redox reactions helps determine relative oxidizing strengths. The green, yellow, and red colors correspond to the oxidized forms of A, B, and C respectively. When C reacts with B⁻ and changes from red to yellow, C is reduced, and B⁻ is oxidized, indicating C is a stronger oxidizing agent than B. However, C doesn’t react with A⁻, so it’s weaker than A. Thus, the order of oxidizing strength is B < C < A.
In question 3, to determine the identity of a colorless unknown salt (NaA, NaB, or NaC), we exploit the redox behavior of known oxidizing agents. By adding A, B, and C to separate aliquots of the unknown and observing the color change (if any), we can identify the anion present. For example, if red color appears after adding C, the anion is B⁻. If yellow appears, it’s A⁻. No change indicates C⁻. This simple yet elegant test uses color and redox chemistry to identify unknowns.
