Hydronium

Is water an acid or a base according to the arrhenius theory?

water can be both an acid and a base.
H₂O + H⁺ = H₃O⁺
H₂O + HCO₃^-= H₂CO₃ +OH^-

Why water is called an amphoteric substance?

According to Bronsted concept

Water can act as an acid by losing a proton as
H₂O-------->OH^- + H⁺
Water can act as a base by gaining a proton as
H₂O+ H⁺---------------->H₃O⁺

Why/How is Hydronium (H3O) positively charged?

H₂O + H⁺ --> H₃O⁺

I see that due to the conversation of charge, both sides should have a net charge of +1, but I don't see how is the hydronium positively charged.

The H⁺ has a dative covalent bond with the H₂O so hydrogen's s-orbital should be complete, hence the charge should be zero.

Is this due to the electronegativity of the H₂O molecule?

I will appreciate it if you could explain to me what I am missing here

well H20 has a charge of 0

Two H's give you a +2 Charge and then you have one O (oxide has a charge of -2) therefore H₂0 has a zero charge

your adding something that has a charge of 0 to something that has a charge of 1(H)

thats why the result is H₃O⁺

Hydronium

In chemistry, a hydronium ion is the cation H₃O⁺, a type of oxonium ion produced by protonation of water and isoelectronic with ammonia. This cation is often used to represent the nature of the proton in aqueous solution, where the proton is highly solvated (bound to a solvent). The reality is far more complicated, and a proton is bound to several molecules of water, such that other descriptions such as H₅O₂⁺, H₇O₃⁺ and H₉O₄⁺ are increasingly accurate descriptions of the environment of a proton in water.^[3] The ion H₃O⁺ has been detected in the gas phase.

Determination of pH

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

2 H₂O OH⁻ + H₃O⁺

In pure water, there is an equal number of hydroxide and hydronium ions. At 25 °C and atmospheric pressure their concentrations are approximately equal to 1.0 × 10⁻⁷ mol∙dm⁻³. For these conditions, [H₃O⁺] = 10^−pH so pH = 7 is defined as neutral. A pH value less than 7 indicates an acidic solution, and a pH value more than 7 indicates a basic solution. Note that [H₃O⁺]×[OH⁻], the ionic product of water, strongly increases with temperature so [H₃O⁺] is not equal to 10^−pH for temperatures other than 25 °C.

Structure

Since O⁺ and N have the same number of electrons, H₃O⁺ is isoelectronic with ammonia. As shown in the images above, H₃O⁺ has a trigonal pyramid geometry with the oxygen atom at its apex. The H-O-H bond angle is approximately 113°,^[6] 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 H₃O⁺ molecule's symmetric top configuration is such that it belongs to the C_3v point group. Because of this symmetry and the fact that it has a dipole moment, the rotational selection rules are ΔJ = ±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 -1.74. It is also the most acidic species that can exist in water (assuming sufficient water for dissolution)(see leveling effect): 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.^[7] 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 (H₃O⁺) can be used interchangeably with the H⁺ ion; choosing one over the other has no significant effect on the mechanism of reaction.

Solvation

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 H₃O⁺(H₂O)₆:^[8] 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 H₃O⁺(H₂O)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.^[9] However, more recent ab initio method molecular dynamics simulations have shown that, on average, the hydrated proton resides on the surface of the H₃O⁺(H₂O)₂₀ cluster.^[10] Further, several disparate features of these simulations agree with their experimental counterparts suggesting an alternative interpretation of the experimental results.

picture

Zundel cation

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 H₉O+4 complex in which the hydronium is strongly hydrogen-bonded to three neighbouring water molecules. In the Zundel H₅O+ complex the proton is shared equally by two water molecules in a symmetric hydrogen bond. Recent work indicates that both of these complexes represent ideal structures in a more general hydrogen bond network defect.

Isolation of the hydronium ion monomer in liquid phase was achieved in a nonaqueous, low nucleophilicity superacid solution (HF-SbF₅SO₂). The ion was characterized by high resolution O-17 nuclear magnetic resonance.

A 2007 calculation of the enthalpies and free energies of the various hydrogen bonds around the hydronium cation in liquid protonated water at room temperature and a study of the proton hopping mechanism using molecular dynamics showed that the hydrogen-bonds around the hydronium ion (formed with the three water ligands in the first solvation shell of the hydronium) are quite strong compared to those of bulk water.

A new model was proposed by Stoyanov based on infrared spectroscopy in which the proton exists as an H₁₃O+6 ion. The positive charge is thus delocalized over six water molecules.

Solid hydronium salts

For many strong acids, it is possible to form crystals of their hydronium salt that are relatively stable. Sometimes these salts are called acid monohydrates. As a rule, any acid with an ionization constant of 10⁹ or higher may do this. Acids whose ionization constant is below 10⁹ generally cannot form stable H₃O⁺ salts. For example, hydrochloric acid has an ionization constant of 10⁷, and mixtures with water at all proportions are liquid at room temperature. However, perchloric acid has an ionization constant of 10¹⁰, and if liquid anhydrous perchloric acid and water are combined in a 1:1 molar ratio, solid hydronium perchlorate forms.

The hydronium ion also forms stable compounds with the carborane superacid H(CB₁₁H(CH₃)₅B₆). X-ray crystallography shows a C_3v symmetry for the hydronium ion with each proton interacting with a bromine atom each from three carborane anions 320 pm apart on average. The [H₃O][H(CB₁₁HCl)]11 salt is also soluble in benzene. In crystals grown from a benzene solution the solvent co-crystallizes and a H₃O·(benzene)₃ cation is completely separated from the anion. In the cation three benzene molecules surround hydronium forming pi-cation interactions with the hydrogen atoms. The closest (non-bonding) approach of the anion at chlorine to the cation at oxygen is 348 pm.

There are also many examples of hydrated hydronium ions known, such as the H₅O+2 ion in HCl·2H₂O, the H₇O+3 and H₉O+4 ions both found in HBr·4H₂O.