chemistry

Hydrogen selenide

Hydrogen selenide is a chemistry topic covered in the lgStudy science library. This page brings together a partial reference excerpt, illustrations, worked examples, real-world applications and a short study plan, so you can understand Hydrogen selenide rather than just read about it. In short: Hydrogen selenide is an inorganic compound with the formula H2Se. This hydrogen chalcogenide is the simplest and most commonly encountered hydride of selenium.

Key takeaways

Hydrogen selenide 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 selenide to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of Hydrogen selenide from memory before moving on to harder problems.

Reference excerpt

Hydrogen selenide is an inorganic compound with the formula H2Se. This hydrogen chalcogenide is the simplest and most commonly encountered hydride of selenium. H2Se is a colorless, flammable gas under standard conditions. It is the most toxic selenium compound with an exposure limit of 0.05 ppm over an 8-hour period. Even at extremely low concentrations, this compound has a very irritating smell resembling that of decayed horseradish or "leaking gas", but smells of rotten eggs at higher concentrations.

Structure and properties H2Se adopts a bent structure with a H−Se−H bond angle of 91°. Consistent with this structure, three IR-active vibrational bands are observed: 2358, 2345, and 1034 cm−1. The properties of H2S and H2Se are similar, although the selenide is more acidic with pKa = 3.89 and the second pKa = 11, or 15.05 ± 0.02 at 25 °C.

Preparation Industrially, it is produced by treating elemental selenium at T > 300 °C with hydrogen gas. A number of routes to H2Se have been reported, which are suitable for both large and small scale preparations. In the laboratory, H2Se is usually prepared by the action of water on Al2Se3, concomitant with formation of hydrated alumina. A related reaction involves the acid hydrolysis of FeSe.

Al2Se3 + 6 H2O ⇌ 2 Al(OH)3 + 3 H2Se H2Se can also be prepared by means of different methods based on the in situ generation in aqueous solution using boron hydride, Marsh test and Devarda's alloy. According to the Sonoda method, H2Se is generated from the reaction of H2O and CO on Se in the presence of Et3N. H2Se can be purchased in cylinders.

Reactions Elemental selenium can be recovered from H2Se through a reaction with aqueous sulfur dioxide (SO2).

2 H2Se + SO2 ⇌ 2 H2O + 2 Se + S Its decomposition is used to prepare the highly pure element.

Applications H2Se is commonly used in the synthesis of Se-containing compounds. It adds across alkenes. Illustrative is the synthesis of selenoureas from cyanamides:

H2Se gas is used to dope semiconductors with selenium.

Safety Hydrogen selenide is hazardous, being the most toxic selenium compound and far more toxic than its congener hydrogen sulfide. The threshold limit value is 0.05 ppm. The gas acts as an irritant at concentrations higher than 0.3 ppm, which is the main warning sign of exposure; below 1 ppm, this is "insufficient to prevent exposure", while at 1.5 ppm the irritation is "intolerable". Exposure at high concentrations, even for less than a minute, causes the gas to attack the eyes and mucous membranes; this causes cold-like symptoms for at least a few days afterwards. In Germany, the limit in drinking water is 0.008 mg/L, and the US EPA recommends a maximum contamination of 0.01 mg/L. Despite being extremely toxic, no human fatalities have yet been reported. It is suspected that this is due to the gas' tendency to oxidise to form red selenium in mucous membranes; elemental selenium is less toxic than selenides are.

Illustrations

Worked examples

Example 1 — a first encounter with Hydrogen selenide

Start with the simplest possible case. Write down what Hydrogen selenide 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 selenide 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 selenide 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 selenide

In research: Hydrogen selenide 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 selenide 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 selenide is common in secondary-school and first-year university syllabi. It links to neighbouring topics Foul-smelling chemicals, Highly-toxic chemical substances, Hydrogen compounds, so understanding it makes those chapters shorter.
In everyday life: Look for Hydrogen selenide 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.

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How to study Hydrogen selenide in 20 minutes

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

Frequently asked questions

What is Hydrogen selenide in simple terms?

Hydrogen selenide is an inorganic compound with the formula H2Se. This hydrogen chalcogenide is the simplest and most commonly encountered hydride of selenium.

Why does Hydrogen selenide 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 selenide?

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 selenide.