Binary hydrogen compounds, acidity

Describe the trend in acidity of the binary hydrogen compounds of the Group 16/VI elements and account for the trend in terms of bond strength. 

If the acid is a binary hydrogen compound, including HF, HCl, HBr, HI, and H2S, the pure compound is named as if hydrogen were an alkali metal. For example, pure HCl is named hydrogen chloride, and H2S is named hydrogen sulfide, with no prefixes for either element in either case. When the hydrogen halides are dissolved in water, they are usually named as acids. Thus HCl in... 

Binary hydrogen compounds that are acids have names in which hydrogen is named as if it were an alkali metal. 

Chlorine does not usually react in this manner however. More frequently the mode of reaction is of a different type. Thus with binary hydrogen compounds it reacts combining with the hydrogen and freeing the other element. For example, bromine and iodine are respectively liberated from hydrobromic and hydriodic acids. 

Table 4 Approximate acid strengths for some binary hydrogen compounds...

The same general acidity trends are observed in aqueous solution. The three heaviest hydrohalic acids—HCl, HBr, and HI— are equally strong in water because of the leveling effect. All the other binary hydrogen compounds are weaker adds, their strength decreasing toward the left in the periodic table. Methane and ammonia exhibit no acidic behavior in aqneous solution, nor do silane (SiH4) and phosphine (PH3). 

Certain binary hydrogen compounds, when dissolved in water, form solutions that have acid properties. Because of this property, these compounds are given acid names in addition to their regular -ide names. For example, HCl is a gas and is called hydrogen chloride, but its water solution is known as hydrochloric acid. Binary acids are composed of hydrogen and one other nonmetallic element. However, not all binary hydrogen compounds are acids. To express the formula of a binary acid, it s customary to write the symbol of hydrogen first, followed by the symbol of the second element (e.g., HCl, HBr, or H2S). When we see formulas such as CH4 or NH3, we understand that these compounds are not normally considered to be acids. 

Acids are hydrogen-containing substances that liberate hydrogen ions when dissolved in water. The same formula is often used to express binary hydrogen compounds, such as HCl, regardless of whether or not they are dissolved in water. Table 6.5 shows several examples of binary acids. 

Figure 1-3. Electron pair cloud models of binary hydrogen compounds arranged according to the position of the kernel element in the periodic table. The inner sphere is the kernel with its net positive charge shown. The other spheres are the tetrahedral (sp ) clouds of spin-paired electrons. The dots represent the protons embedded within electron clouds. These protons are actually too small to put to scale. Features to note (1) The protons move farther from the nucleus as the kernel charge increases. The molecule becomes more acidic toward water. (2) The cloud size (from covalent radii) becomes smaller as the kernel charge increases, and larger as distance from the nucleus increases while kernel charge is held constant from HF to HI. (3) Two factors affect acidity, kernel charge, from CH4 to HF, and cloud size, from HF to HI. The compact cloud of HF can bind the proton better than the diffuse cloud of HI.

A few binary molecular compounds containing H atoms ionize in water to form H+ ions. These are called acids. One such compound is hydrogen chloride, HC1 in water solution it exists as aqueous H+ and Cl- ions. The water solution of hydrogen chloride is given a special name It is referred to as hydrochloric acid. A similar situation applies with HBr and HI ... 

Many hydrogen compounds are acids. Acids require a different system of nomenclature than other compounds. There are two types of acids binary acids and ternary acids. All acids include the word acid in their name. Binary acids contain only two elements one of which is hydrogen and the other a nonmetal. When naming binary acids, the prefix hydro- appears before the root name of the nonmetal, and there will be an -ic suffix. As an acid, H2S would be hydrosulfuric acid. We used H2S to illustrate the fact that there can be two possible names, hydrogen sulfide or hydrosulfuric acid. If your instructor does not specify which name to use, either is correct. Technically, the name hydrosulfuric acid only applies to aqueous solutions of H2S, H2S(aq). 

Some binary molecular compounds of hydrogen possess acidic... 

Binary compounds of hydrogen that are not acids are given special names. Two very important examples are water, H2O, and ammonia, NH3 (Figure 6.1). (Other much less important hydrogen-containing binary compounds are also known by common names. These include phosphine, PH3, and arsine, ASH3.) Hydrogen compounds that are acids in aqueous solution are named, and their formulas are written in special ways (see Section 6.3). 

The systematic names presented for binary nonmetal-nonmetal compounds are not used for the hydrogen compounds of group III, TV, and V elements or for water. These compounds have common names which are used instead. Water and ammonia (NH3) are the most important compounds in this class. See Sec. 6.4 for acid names. 

You probably recognized that HCl is a binary covalent compound that can be named by the rules given earlier in Section 4.10. According to those rules, HCl should be named hydrogen chloride. In fact, that is the correct name for the compound HCl that has not been dissolved in water and is represented in reaction equations by the notation HCl(g). Such compounds that have not been dissolved in water are said to be anhydrous (without water). However, when the gas is dissolved in water and represented in equations by the notation HCl(aq), it behaves as an acid and is given another name. The following rules are used to name acidic water solutions of such compounds ... 

Two types of acids are named differently. Water solutions of binary covalent compounds containing hydrogen and a nonmetal are named following the pattern hydro(stem)ic acid, where (stem) is the stem of the name of the nonmetal bonded to hydrogen. Acids in which hydrogen is bonded to polyatomic ions have names based on the name of the polyatomic ion to which hydrogen is bonded. 

Hydrogen cyanide, HCN, behaves in a water solution very much hke the binary covalent compounds of hydrogen, but it liberates the cyanide ion, CN". Name the acidic water solution by following the rules for binary covalent compounds of hydrogen. 

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