Aqueous solutions hydrogen ions

The first scientific definition in this new field was described by a Nobel laureate, Swedish chemist Svante Arrhenius (1859-1927). His theory, which is now called the Arrhenius acid-base concept, states that acids are substances that produce hydrogen ions in an aqueous solution. Hydrogen ions (H+) do not exist in solutions because they are always attached to at least one water molecule- often written as hydroxonium ion (H3O+). More detailed studies show that H3O+ ions are not very common, either, and hydrogen ions are often attached to more than one water molecules. But this is only a matter of notation and different chemists use H+, H3O+, Hj02 or H,03+ to mean essentially the same thing. 

Redox reactions of metals with acids are described in Chapter 4. Oxidation of the metal generates hydrogen gas and an aqueous solution of ions. Suppose that 3.50 g of magnesium metal is dropped into 0.150 L of 6.00 M HCl in a 5.00-L cylinder at 25.0 °C whose initial gas pressure is 1.00 atm, and the cylinder is immediately sealed. Find the final partial pressure of hydrogen, the total pressure in the container, and the concentrations of all ions in solution. 

In an aqueous solution, hydrogen chloride (HCI) breaks apart into hydrogen ions (Hq and chloride ions (Cl ). 

In reactions in aqueous solutions water, hydrogen ion, and hydroxide ion may come into action as reactants or products. For example, in an acid solution hydrogen ion may be either a reactant or a product, and water may also be either a reactant or a product in the same reaction. In acid solutions hydroxide ion exists only in extremely low concentration, and would hardly be expected to enter into the reaction. Hence water and hydrogen ion may enter into the reaction now under consideration. 

In aqueous solutions, two ions have dominant roles. These ions, the hydronium ion, H30 (or hydrogen ion, H" ), and the hydroxide ion, (OH ), are available in any aqueous solution as a result of the self-ionization of water, a reaction of water with itself, which we will describe in the next section. This will also give us some background to acid-base equilibrium calculations, which we will discuss in Chapter 17. 

Hydrogen chloride (HCl) is a covalent compound and does not contain ions. However, in water it ionizes completely to form U aq) ions and Cr(aq) ions. The H+ ions are highly reactive, hi aqueous solution, the ions bond to water to form H30 ... 

In aqueous solutions, hydrogen peroxide can oxidize or reduce a variety of inorganic ions. When it acts as a reducing agent, oxygen gas is also produced. 

When a substance dissolves in water, a solution forms. A solution is a homogeneous mixtme because it has a constant composition throughout. A solution contains one or more substances called solutes dissolved in the solvent. The solvent is the most plentiful substance in a solution. An aqueous solution is a solution in which the solvent is water. When dissolved to form aqueous solutions, the ions of ionic compounds separate. Some molecular compounds also produce dissolved ions in water. If hydrogen ions are produced, the substance is called an acid. For example, the gaseous molecular compound hydrogen chloride (HCl) forms H and Cl ions in aqueous solution, which is called hydrochloric acid. 

Schematic polarization curves for a mixed electrode (Fe in an aqueous solution in tire presence of hydrogen ions),...

The alkali metal tetrahydridoborates are salts those of sodium and potassium are stable in aqueous solution, but yield hydrogen in the presence of a catalyst. They are excellent reducing agents, reducing for example ion(III) to iron(II). and silver ions to the metal their reducing power is used in organic chemistry, for example to reduce aldehydes to alcohols. They can undergo metathetic reactions to produce other borohydrides, for example... 

The bond dissociation energy of the hydrogen-fluorine bond in HF is so great that the above equilibrium lies to the left and hydrogen fluoride is a weak acid in dilute aqueous solution. In more concentrated solution, however, a second equilibrium reaction becomes important with the fluoride ion forming the complex ion HFJ. The relevant equilibria are ... 

The oxidising properties of the aqueous solutions of chloric(VII) acid change dramatically with temperature and the concentration of the acid. Cold dilute solutions have very weak oxidising properties and these solutions will react, for example, with metals, producing hydrogen without reduction of the chlorate(VII) ion occurring ... 

Copper(II) ions in aqueous solution are readily obtained from any copper-containing material. The reactions with (a) alkali (p. 430), (b) concentrated ammonia (p 413) and (c) hydrogen sulphide (p. 413) provide satisfactory tests for aqueous copper(II) ions. A further test is to add a hexacyanoferrate(II) (usually as the potassium salt) when a chocolate-brown precipitate of copper(II) hexacyanoferrate(II) is obtained ... 

When treated with bromine or chlorine in aqueous solution alkenes are con verted to vicinal halohydrins A haloni um ion IS an intermediate The halogen adds to the carbon that has the greater number of hydrogens Addition is anti... 

Hence, exists only when hydrogen is bonded to the most electronegative atoms. In aqueous solutions, hydrates to form H O" ion. 

Sa.lts Salting out metal chlorides from aqueous solutions by the common ion effect upon addition of HCl is utilized in many practical apphcations. Typical data for ferrous chloride [13478-10-9] FeCl2, potassium chloride [7447-40-7] KCl, and NaCl are shown in Table 9. The properties of the FeCl2-HCL-H2 0 system are important to the steel-pickling industry (see Metal SURFACE TREATMENTS Steel). Other metal chlorides that are salted out by the addition of hydrogen chloride to aqueous solutions include those of magnesium, strontium, and barium. 

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