Triiodide

Key takeaways

Triiodide belongs to physics; place it in that map before memorising details.
Learn the definition first, then one example that makes the definition concrete.
Connect Triiodide to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of Triiodide from memory before moving on to harder problems.

Reference excerpt

In chemistry, triiodide usually refers to the triiodide ion, I−3. This anion, one of the polyhalogen ions, is composed of three iodine atoms. It is formed by combining aqueous solutions of iodide salts and iodine. Some salts of the anion have been isolated, including thallium(I) triiodide (Tl+[I3]−) and ammonium triiodide ([NH4]+[I3]−). Triiodide is observed to be a red colour in solution.

Nomenclature Other chemical compounds with "triiodide" in their name contain three separate or independent iodide centers rather than a single unit that consists of three iodine atoms. The prefix "tri" in that situation is a multiplicative prefix for iodide—a 3:1 stoichiometric ratio—rather than part of the name of the unified three-iodine structure. Examples include nitrogen triiodide (NI3) and phosphorus triiodide (PI3), each of which have a central atom to which each of the three iodine atoms is bonded. To clarify the structural nature of a chemical with three iodides, other naming schemes can be used. Typical options are to use tris as an alternative prefix, such as the chemical thallium tris(iodide), where there three "iodide" units, as distinct from the chemical thallium(I) triiodide, where thallium is oxidation state +1 and therefore "triiodide" is the single monovalent unit.

Preparation The following exergonic equilibrium gives rise to the triiodide ion:

I2 + I− ⇌ I−3 In this reaction, iodide is viewed as a Lewis base, and the iodine is a Lewis acid. The process is analogous to the reaction of S8 with sodium sulfide (which forms polysulfides) except that the higher polyiodides have branched structures.

Structure and bonding The ion is linear and symmetrical. According to valence shell electron pair repulsion theory, the central iodine atom has three equatorial lone pairs, and the terminal iodine atoms are bonded axially in a linear fashion, due to the three lone pairs bonding to the central iodine-atom. In the molecular orbital model, a common explanation for the hypervalent bonding on the central iodine involves a three-center four-electron bond. The I−I bond is longer than in diatomic iodine, I2. In ionic compounds, the bond lengths and angles of triiodide vary depending on the nature of the cation. The triiodide anion is easily polarised and in many salts, one I−I bond becomes shorter than the other. Only in combination with large cations, e.g. a quaternary ammonium such as [N(CH3)4]+, may the triiodide remain roughly symmetrical. In solution phase, the bond lengths and angles of triiodide vary depending on the nature of solvent. The protic solvents tend to localize the triiodide anion's excess charge, resulting in the triiodide anion's asymmetric structure. For example, the triiodide anion in methanol has an asymmetric bent structure with a charge localized on the longer end of the anion. The dimensions of the triiodide [Ia−Ib−Ic]− bonds in a few sample compounds are shown below:

Properties The triiodide ion is the simplest polyiodide; several higher polyiodides exist. In solution, it appears yellow in low concentrations, and brown at higher concentrations. The triiodide ion is responsible for the well-known blue-black color which arises when iodine solutions interact with starch. Iodide does not react with starch; nor do solutions of iodine in nonpolar solvents. Lugol's iodine contains potassium iodide and a stoichiometric amount of elemental iodine, so that significant amounts of triiodide ion exist in this solution. Tincture of iodine, although nominally a solution of elemental iodine in ethanol, also contains significant amounts of triiodide, due to its content of both iodide and water.

Photochemistry Triiodide is a model system in photochemistry. Its reaction mechanism has been studied in gas phase, solution and the solid state. In gas phase, the reaction proceeds in multiple pathways that include iodine molecule, metastable ions and iodine radicals as photoproducts, which are formed by two-body and three-body dissociation. In condensed phases, due to confinement, geminate recombination is more common. In solution, only two-body dissociation of triiodide has been observed. In the protic solvents, an iodine atom at the shorter end of the triiodide anion dissociates upon photoexcitation showing two-body dissociation. In the solid state, the triiodide photochemistry has been studied in compounds involving quaternary ammonium cations, such as tetrabutylammonium triiodide. It has been shown that the solid state photoreaction mechanism depends on the light wavelength, yielding fast recovery in a few picoseconds or going through a two-stage process that involves the formation and break-up of a tetraiodide intermediate on longer timescales. Besides, triiodide photochemistry is an important contributor in the environmental cycle of iodine. Because of the presence of heavy iodine atoms and the well-calibrated chemical pathways, triiodide has also become a computational benchmark system for relativistic quantum chemistry.

Electrochemistry The redox reactions of triiodide and iodide has been proposed as critical steps in dye-sensitized solar cells. and rechargeable batteries.

See also Polyiodide Tribromide Polyhalogen ions Three-center four-electron bond Iodine–starch test Iodometry Povidone-iodine Lugol's iodine Dye-sensitized solar cell Organic superconductor

References

External links Kinetic study of the iodine–persulfate reaction Archived 2011-07-16 at the Wayback Machine

Illustrations

Worked examples

Example 1 — a first encounter with Triiodide

Start with the simplest possible case. Write down what Triiodide claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In physics, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to Triiodide before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about Triiodide ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of Triiodide

In research: Triiodide appears in physics research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry: Engineering practice reuses Triiodide in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom: Triiodide is common in secondary-school and first-year university syllabi. It links to neighbouring topics Anions, Homonuclear ions, Homonuclear triatomic molecules, so understanding it makes those chapters shorter.
In everyday life: Look for Triiodide outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

How to study Triiodide in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what Triiodide means in your own words.
Compare your version with the excerpt and mark what you missed.
Work through the three examples above with pen and paper.
Explain Triiodide out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is Triiodide in simple terms?

In chemistry, triiodide usually refers to the triiodide ion, I−3. This anion, one of the polyhalogen ions, is composed of three iodine atoms.

Why does Triiodide matter?

Because it connects several physics ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study Triiodide?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on Triiodide.