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a substance that forms hydronium ions in water

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In chemistry, hydronium is the common name for the aqueouscation H|3|O|+, the type of oxonium ion, produced by protonation of water. It is the positive ion present when an Arrhenius acid is dissolved in water, as Arrhenius acid molecules in solution give up a proton (a positive hydrogen ion, H+) to the surrounding water molecules (H2O).

Determination of pH

It is the presence of hydronium ion relative to hydroxide that determines a solution's pH. The molecules in pure water auto-dissociate into hydronium and hydroxide ions in the following equilibrium:

2 H|2|O OH|- + H|3|O|+

In pure water, there is an equal number of hydroxide and hydronium ions, so it has a neutral pH of 7. A pH value less than 7 indicates an acidic solution, and a pH value more than 7 indicates a basic solution.


According to IUPAC nomenclature of organic chemistry, the hydronium ion should be referred to as oxonium. Hydroxonium may also be used unambiguously to identify it. A draft IUPAC proposal also recommends the use of oxonium and oxidanium in organic and inorganic chemistry contexts, respectively.

An oxonium ion is any ion with a trivalent oxygen cation. For example, a protonated hydroxyl group is an oxonium ion, but not a hydronium.


Since O|+ and N have the same number of electrons, H|3|O|+ is isoelectronic with ammonia. As shown in the images above, H|3|O|+ has a trigonal pyramid geometry with the oxygen atom at its apex. The H-O-H bond angle is approximately 113°, and the center of mass is very close to the oxygen atom. Because the base of the pyramid is made up of three identical hydrogen atoms, the H|3|O|+ molecule's symmetric top configuration is such that it belongs to the C3v point group. Because of this symmetry and the fact that it has a dipole moment, the rotational selection rules are ΔJ = Â±1 and ΔK = 0. The transition dipole lies along the c axis and, because the negative charge is localized near the oxygen atom, the dipole moment points to the apex, perpendicular to the base plane.

Acids and acidity

Hydronium is the cation that forms from water in the presence of hydrogen ions. These hydrons do not exist in a free state: they are extremely reactive and are solvated by water. An acidic solute is generally the source of these hydrons; however, hydroniums exist even in pure water. This special case of water reacting with water to produce hydronium (and hydroxide) ions is commonly known as the self-ionization of water. The resulting hydronium ions are few and short-lived. pH is a measure of the relative activity of hydronium and hydroxide ions in aqueous solutions. In acidic solutions, hydronium is the more active, its excess proton being readily available for reaction with basic species.

Hydronium is very acidic: at 25°C, its pKa is -1.7. It is also the most acidic species that can exist in water (assuming sufficient water for dissolution): any stronger acid will ionize and protonate a water molecule to form hydronium. The acidity of hydronium is the implicit standard used to judge the strength of an acid in water: strong acids must be better proton donors than hydronium, otherwise a significant portion of acid will exist in a non-ionized state. Unlike hydronium in neutral solutions that result from water's autodissociation, hydronium ions in acidic solutions are long-lasting and concentrated, in proportion to the strength of the dissolved acid.

pH was originally conceived to be a measure of the hydrogen ion concentration of aqueous solution. We now know that virtually all such free protons quickly react with water to form hydronium; acidity of an aqueous solution is therefore more accurately characterized by its hydronium concentration. In organic syntheses, such as acid catalyzed reactions, the hydronium ion (H|3|O|+) can be used interchangeably with the H+ ion; choosing one over the other has no significant effect on the mechanism of reaction.


Researchers have yet to fully characterize the solvation of hydronium ion in water, in part because many different meanings of solvation exist. A freezing-point depression study determined that the mean hydration ion in cold water is approximately H|3|O|+|(H|2|O)|6: on average, each hydronium ion is solvated by 6 water molecules which are unable to solvate other solute molecules!.

Some hydration structures are quite large: the H|3|O|+|(H|2|O)|20 magic ion number structure (called magic because of its increased stability with respect to hydration structures involving a comparable number of water molecules) might place the hydronium inside a dodecahedral cage. However, more recent ab initio method molecular dynamics simulations have shown that, on average, the hydrated proton resides on the surface of the H|3|O|+|(H|2|O)|20 cluster. Further, several disparate features of these simulations agree with their experimental counterparts suggesting an alternative interpretation of the experimental results.

Two other well-known structures are the Zundel cations and Eigen cations. The Eigen solvation structure has the hydronium ion at the center of an H|9|O|4|+ complex in which the hydronium is strongly hydrogen-bonded to three neighbouring water molecules. In the Zundel H|5|O|2|+ complex the proton is shared equally by

From Yahoo Answers


Answers:Take a monoprotic acid such as HF. this will dissociate in water to form H3O+ and F-: HF + H2O --> H3O+ + F- water, H2O has 2 lone pair of electrons on the oxygen atom: --:O: -/----\ H----H the bond between H and F in HF is a polar covalent bond. The F is more electronegative and pulls the electrons in the single covalent bond toward itself leaving a partial positive charge on H and a partial negative charge on F: H--F (the single bond is actually more like this: +H --F-) one of the lone pairs of electrons in water is attracted to the partially positive H in HF thus pulling the H off of the molecule and leaving fluorine as an anion, F-: -:O:---> +H --F- -> :O---H + F- -/----\-------------------/----\ H----H----------------H-----H The added hydrogen to the water molecule gives hydronium, (H3O^+). This gives the molecule a 1+ charge. Therefore the molecule really looks like this: ----.. H--O--H...+ ---- | ----H I can't draw brackets around the hydronium ion but if you imagine there are brackets around the molecule, the + charge would be outside of the brackets. Here is a picture of how the hydronium ion really looks: http://en.wikipedia.org/wiki/Hydronium So basically, the lone pair of electrons in water pulls the hydrogen off of the HF leaving the anion, F- and the water now has an added H making the overall charge of the newly hydronium molecule a 1+ charge thus calling it a hydronium ion, H3O+. Hope that helped at least some. *** had to edit the structures - even though they look fine when I type them in, they somehow get screwed up when they're submitted. Hopefully now they'll look the way they're supposed to.

Question:In the Bronsted-Lowry definition, an acid is a substance that donates protons accepts protons neutralizes hydronium ions forms a salt forms a water

Answers:According to the Bronsted-Lowry definition, an acid is a substance that donates a proton. A Lewis acid and base would create water. The B-L definition makes up part of the Lewis definition, but does not directly define a B-L acid.

Question:I'm doing a crossword puzzle and I really need help 7 Across: Substance that forms hydronium ions in water (Arrhenius) [4 letters] 17 Across: reaction of an ion with H2O to produce H+(aq) + OH-(aq) [10 letters]

Answers:7 Across: Substance that forms hydronium ions in water (Arrhenius) (4) ACID 17 Across: reaction of an ion with H2O to produce H+(aq) + OH-(aq) (10) HYDROLYSIS Look in notes Google

Question:A. water. B. a weaker base. C. a stronger acid. D. salt.

Answers:H2O Whats up with the Mr. Wizard questions Einstein?