chemistry

Hydrogen halide

Hydrogen halide 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 halide rather than just read about it. In short: In chemistry, hydrogen halides (hydrohalic acids when in the aqueous phase) are diatomic, inorganic compounds that function as Arrhenius acids. The formula is HX where X is one of the halogens: fluorine, chlorine, bromine, iodine, astatine, or tennessine.

Key takeaways

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

Reference excerpt

In chemistry, hydrogen halides (hydrohalic acids when in the aqueous phase) are diatomic, inorganic compounds that function as Arrhenius acids. The formula is HX where X is one of the halogens: fluorine, chlorine, bromine, iodine, astatine, or tennessine. All known hydrogen halides are gases at standard temperature and pressure.

Comparison to hydrohalic acids The hydrogen halides are diatomic molecules with no tendency to ionize in the gas phase (although liquified hydrogen fluoride is a polar solvent somewhat similar to water). Thus, chemists distinguish hydrogen chloride from hydrochloric acid. Hydrogen chloride is a gas at room temperature that reacts with water to give hydrochloric acid; once the acid has formed, the hydrogen chloride can be regenerated, but only with difficulty and not by normal distillation. Often, the names of the acid and the molecules are not clearly distinguished, and in lab jargon, "HCl" often means hydrochloric acid, not the gaseous hydrogen chloride.

Occurrence and production Hydrogen fluoride, chloride, and bromide are volcanic gases. The hydrogen halides can be produced by many routes industrially and in the laboratory. Focusing on the most abundant compound, hydrogen chloride is mainly produced as a side product in production of chlorocarbons. Hydrogen fluoride is a byproduct of the production of phosphoric acid. Fluorine, chlorine, and bromine react with hydrogen gas to give HF, HCl, and HBr. These gases can also be produced by treatment of halide salts with sulfuric acid. The least stable hydrogen halide, HI, is produced less directly, by the reaction of iodine with hydrogen sulfide or with hydrazine.

Physical properties

The hydrogen halides are colourless gases at standard conditions for temperature and pressure (STP) except for hydrogen fluoride, which boils at 19 °C. Alone of the hydrogen halides, hydrogen fluoride exhibits hydrogen bonding between molecules, and therefore has the highest melting and boiling points of the HX series. From HCl to HI the boiling point rises. This trend is attributed to the increasing strength of intermolecular van der Waals forces, which correlates with numbers of electrons in the molecules. Concentrated hydrohalic acid solutions produce visible white fumes. This mist arises from the formation of tiny droplets of their concentrated aqueous solutions of the hydrohalic acid.

Reactions Upon dissolution in water, which is highly exothermic, the hydrogen halides give the corresponding acids. These acids are very strong, reflecting their tendency to ionize in aqueous solution, yielding hydronium ions (H3O+). With the exception of hydrofluoric acid, the hydrogen halides are strong acids, with acid strength increasing down the group. Hydrofluoric acid is complicated because its strength depends on the concentration, owing to the effects of homoconjugation. However, as solutions in non-aqueous solvents, such as acetonitrile, the hydrogen halides are only moderately acidic. Similarly, the hydrogen halides react with ammonia (and other bases), forming ammonium halides:

HX + NH3 → NH4X In organic chemistry, the hydrohalogenation reaction is used to prepare halocarbons. For example, chloroethane is produced by hydrochlorination of ethylene:

C2H4 + HCl → CH3CH2Cl

Illustrations

Worked examples

Example 1 — a first encounter with Hydrogen halide

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

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

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

Frequently asked questions

What is Hydrogen halide in simple terms?

Why does Hydrogen halide 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 halide?

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 halide.