Hydrogen iodide

Key takeaways

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

Reference excerpt

Hydrogen iodide (HI) is a diatomic molecule and hydrogen halide. Aqueous solutions of HI are known as hydroiodic acid or hydriodic acid, a strong acid. Hydrogen iodide and hydroiodic acid are, however, different in that the former is a gas under standard conditions, whereas the latter is an aqueous solution of the gas. They are interconvertible. HI is used in organic and inorganic synthesis as one of the primary sources of iodine and as a reducing agent.

Properties of hydrogen iodide HI is a colorless gas that reacts with oxygen to give water and iodine. With moist air, HI gives a mist (or fumes) of hydroiodic acid. It is exceptionally soluble in water, giving hydroiodic acid. One liter of water will dissolve 425 liters of HI gas, the most concentrated solution having only four water molecules per molecule of HI.

Hydroiodic acid Hydroiodic acid is an aqueous solution of hydrogen iodide. Commercial "concentrated" hydroiodic acid usually contains 48–57% HI by mass. The solution forms an azeotrope boiling at 127 °C with 57% HI, 43% water. The high acidity is caused by the dispersal of the ionic charge over the anion. The iodide ion radius is much larger than the other common halides, which results in the negative charge being dispersed over a large volume. This weaker H+···I− interaction in HI facilitates dissociation of the proton from the anion and is the reason HI is the strongest acid of the hydrohalides.

HI(g) + H2O(l) → H3O+(aq) + I−(aq) Ka ≈ 1010 HBr(g) + H2O(l) → H3O+(aq) + Br−(aq) Ka ≈ 109 HCl(g) + H2O(l) → H3O+(aq) + Cl−(aq) Ka ≈ 106

Synthesis The industrial preparation of HI involves the reaction of I2 with hydrazine, which also yields nitrogen gas:

2 I2 + N2H4 → 4 HI + N2 When the synthesis is performed in water, the HI can be purified by distillation. Anhydrous HI can be prepared by reaction of iodine with tetrahydronaphthalene:

C10H12 + 2 I2 → C10H8 + 4 HI HI can also be distilled from a solution of NaI or other alkali iodide that is treated with the dehydration reagent phosphorus pentoxide (which gives phosphoric acid). Concentrated sulfuric acid is unsuited for acidifying iodides, as it oxidizes the iodide to elemental iodine. An historical route to HI involves oxidation of hydrogen sulfide with aqueous iodine:

H2S + I2 → 2 HI + S Additionally, HI can be prepared by simply combining H2 and I2:

H2 + I2 → 2 HI This method, which can be catalyzed by platinum, is usually employed to generate high-purity samples. For many years, this reaction was considered to involve a simple bimolecular reaction between molecules of H2 and I2. However, when a mixture of the gases is irradiated with the wavelength of light equal to the dissociation energy of I2, about 578 nm, the rate increases significantly. This supports a mechanism whereby I2 first dissociates into 2 iodine atoms, which each attach themselves to a side of an H2 molecule and break the H−H bond:

H2 + I2 + (578 nm radiation) → H2 + 2I → I···H···H···I → 2HI In the laboratory, yet another method involves hydrolysis of PI3, the iodine analog of PBr3. In this method, I2 reacts with phosphorus to create phosphorus triiodide, which then reacts with water to form HI and phosphorous acid:

3 I2 + 2 P + 6 H2O → 6 HI + 2 H3PO3

Reactions Solutions of hydrogen iodide are easily oxidized by air:

4 HI + O2 → 2 H2O + 2 I2 HI + I2 ⇌ HI3 HI3 is brown in color, which makes aged solutions of HI often appear dark. Like HBr and HCl, HI adds to alkenes, in a reaction that is subject to the same Markovnikov and anti-Markovnikov guidelines as HCl and HBr.

HI + RCH=CH2 → RCH(I)−CH3 HI is also used in organic chemistry to convert primary alcohols into alkyl iodides. This reaction is an SN2 substitution, in which the iodide ion replaces the "activated" hydroxyl group (water):

HI + RCH2OH → RCH2I + H2O HI is sometimes preferred over other hydrogen halides. HI (or HBr) can also be used to cleave ethers. Commonly, it is applied to the cleavage of aryl-alkyl ethers to give phenols and the alkyl iodide. In the following idealized equation diethyl ether is split two equivalents of ethyl iodide:

2 HI + (CH3CH2)2O → 2CH3CH2I + H2O The reaction is regioselective, as iodide tends to attack the less sterically hindered ether carbon. HI was commonly employed as a reducing agent early on in the history of organic chemistry. Chemists in the 19th century attempted to prepare cyclohexane by HI reduction of benzene at high temperatures, but instead isolated the rearranged product, methylcyclopentane (see the article on cyclohexane). As first reported by Kiliani, hydroiodic acid reduction of sugars and other polyols results in the reductive cleavage of several or even all hydroxy groups, although often with poor yield and/or reproducibility. In the case of benzyl alcohols and alcohols with α-carbonyl groups, reduction by HI can provide synthetically useful yields of the corresponding hydrocarbon product (ROH + 2HI → RH + H2O + I2). This process can be made catalytic in HI using red phosphorus to reduce the formed I2.

Applications Commercial processes for obtaining iodine all focus on iodide-rich brines. The purification begins by converting iodide to hydroiodic acid, which is then oxidized to iodine. The iodine is then separated by evaporation or adsorption.

References

External links

International Chemical Safety Card 1326 - Hydrogen Iodide

Illustrations

Worked examples

Example 1 — a first encounter with Hydrogen iodide

Start with the simplest possible case. Write down what Hydrogen iodide claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In chemistry, 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 Hydrogen iodide 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 Hydrogen iodide 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 Hydrogen iodide

In research: Hydrogen iodide appears in chemistry 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 Hydrogen iodide 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: Hydrogen iodide is common in secondary-school and first-year university syllabi. It links to neighbouring topics Diatomic molecules, Hydrogen compounds, Iodides, so understanding it makes those chapters shorter.
In everyday life: Look for Hydrogen iodide 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 Hydrogen iodide in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what Hydrogen iodide 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 Hydrogen iodide out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is Hydrogen iodide in simple terms?

Hydrogen iodide (HI) is a diatomic molecule and hydrogen halide. Aqueous solutions of HI are known as hydroiodic acid or hydriodic acid, a strong acid.

Why does Hydrogen iodide matter?

Because it connects several chemistry 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 Hydrogen iodide?

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 Hydrogen iodide.