Strong acids dissolving in water

A. When a strong acid dissolves in water, it dissociates completely, yielding HjO (aq) and A" (aq) ions virtually no HA molecules are present. 

If a salt contains the cation of a strong base and the anion of a strong acid, neither ion reacts with water. Therefore, the solution has a pH of 7. Sodium chloride is an example of such a salt. It is formed by the reaction of sodium hydroxide (a strong base) and hydrochloric acid (a strong acid). Salts of strong bases and strong acids dissolve in water and form neutral solutions. 

What happens when a strong acid dissolves in water We can picture the transfer of a proton from the acid to a water molecule as the outcome of a tug-of-war between the proton-accepting powers of HzO and A-, the conjugate base of the strong acid. If the acid is HC1, then A- is Cl and... 

Now, let s explore some of the common acids and bases. Acids donate protons that s their job. Stronger acids are really determined to foist their proton off onto some base. When a strong acid dissolves in water, it is essentially 100% ionized. That means essentially all of the molecular species dissociates into ions. The reaction is not an equilibrating process, so all of the starting materials are converted into products. For example, when HC1 dissolves in water, the following reaction occurs ... 

FIGURE 10.4 An illustration of a strong acid dissolving in water. The acid is a polar covalent compound, like sugar, but when it dissolves in water, it ionizes—that is, the hydrogen atoms break away from the molecule, forming hydrogen ions (+ ions) and - ions. It is a total, complete ionization. 

The catalyst acid sites are both Bronsted and Lewis type. The catalyst can have either strong or weak Bronsted sites or, strong i)i weak Lewis sites. A Bronsted-type acid is a substance capable of donating a proton. Hydrochloric and sulfuric acids are typical Bronsted acids. A Lewis-type acid is a substance that accepts a pair of electrons. Lewis acids may not have hydrogen in them but they are still acids. Aluminum chloride is the classic example of a Lewis acid. Dissolved in water, it will react with hydroxyl, causing a drop in solution pH. 

As pointed out in Chapter 13, strong acids ionize completely in water to form H30+ ions strong bases dissolve in water to form OH- ions. The neutralization reaction that takes place when any strong acid reacts with any strong base can be represented by a net ionic equation of the Bronsted- Lowry type ... 

Nitric acid is one of the six common strong acids. This means that when nitric acid dissolves in water, each acid molecule transfers a proton to a water molecule, generating a hydronium ion and the appropriate anion. Both the reaction and its molecular representation must show this proton transfer. 

The chemical structures of I and VI reveal the strong similarities between ethanoic and methanoic acids, yet the smaller molecule is considerably nastier to the skin. Why Methanoic acid dissociates in water to form the solvated methanoate anion HCOO-(aq) and a solvated proton in a directly analogous fashion to ethanoic acid dissolving in water Equation (6.1). In methanoic acid of concentration 0.01 mol dm-3, about 0.14 per cent of the molecules have dissociated to yield a solvated proton. By contrast, in ethanoic acid of the same concentration, only 0.04 per cent of the molecules have dissociated. We say the methanoic acid is a stronger acid than ethanoic since it yields more protons per mole. Conversely, ethanoic acid is weaker. 

A soluble salt of a strong acid and a strong base dissolves in water to form a neutral solution. A good example is sodium chloride (common salt). The parent acid is hydrochloric acid, a strong acid. The parent base is sodium hydroxide, a strong base. 

A salt formed from a strong base and a weak acid dissolves in water to give a basic solution. 

Thus. HF does not behave as a strong acid, because F and H30+ remain associated with each other. Dissolving one mole of the strong acid HCI in water creates one mole of free H30+. Dissolving one mole of the weak acid HF in water creates little free... 

Figures 10.7 and 10.8 show the submicroscopic behavior of strong and weak acids in water. However, molecules and ions are too small to see. How then does a chemist measure the strength of an acid One way is by measuring a solution s ability to conduct an electric current, as Figure 10.9 illustrates. In pure warer there are practically no ions to conduct electricity. When a strong acid is dissolved in water many ions are generated, as indicated in Figure 10.7. The presence of these ions allows for the flow of a large electric current. A weak acid dissolved in water generates only a few ions, as indicated in Figure 10.8. The presence of fewer ions means there can be only a small electric current.

Ion-exchange Separations. Because amino acids are amphoteric, they behave as adds or bases, depending upon the pH of die solution. This makes it possible to adsorb amino acids dissolved in water on either a strong-acid cation exchange resin or a strong-base anion exchange resin. The affinity varies with the amino acid and the solution pH. Ion-exchange resins are widely used in amino acid separations. 

Weak acids are able to donate hydrogen ions to bases, but they are less determined to do so than strong acids. So when a weak acid dissolves in water, it establishes a dynamic equilibrium between the molecular form of the acid and the ionized form. For example, acetic acid is a very common weak acid. In water, acetic acid establishes the following equilibrium ... 

This reaction illustrates a very important general principle The hydroxide ion is such a strong base that for purposes of stoichiometry it is assumed to react completely with any weak acid dissolved in water. Of course, OH-ions also react completely with the H+ ions in the solutions of strong acids. 

The solvent used to dissolve the precurser salt can also have a significant effect on the degree of impregnation and the metal distribution with specific supports. Chloroplatinic acid dissolved in water is adsorbed more strongly on alumina than on earbon, while in acetone the reverse is true. As shown in... 

Since phenoxymethyl penicillin is a strong acid (pKa in water is 2.74), Mohoric found a nonaqueous titration to be more accurate and faster than the iodometric method. A 100-mg sample dissolved in 20 ml of dimethylformamide is titrated with 0.1N sodium methoxide to a thymol blue end point. 

The earliest advantages recognized arose from the use of amphiprotic solvents, those that have both acidic and basic properties. The prototype is water. Significant differences in acid-base properties are seen in the case of either protogenic solvents (more acidic than water), for example acetic acid, or protophilic solvents (more basic than water), for example ethylenediamine. In the protogenic cases it was found that bases too weak to be titrated in water could be successfully titrated with a strong acid dissolved in the same solvent. For example, primary, secondary, and tertiary amines can be titrated in acetic acid with perchloric acid in acetic add as titrant. Medicinal sulfonamides, which have a primary amino group, can be titrated... 

S When an acid dissolves in water, the water molecules attack the acid molecules and abstract from them a proton, H+. When the acid is strong, almost all the... 

In contrast to the strong acids, such as HCI and H2SO4, acetic acid is a much weaker acid. When acetic acid dissolves in water, the following reaction does not proceed to completion ... 

Figure 8-3 Calculated pH as a function of the concentration of a strong acid or strong base dissolved in water. 

A few polar covalent solutes, namely acids, dissolve in water and also form ions in the process. The strength of the interaction with the water molecule is sufficient to break a covalent bond in these solutes and to form ions. Inorganic acids, such as HCl, H2SO4, and HNO3, completely ionize and are called strong acids. This ionization can be written as follows for hydrochloric acid (HCl), for example ... 

It is the nitronium ion which is responsible for nitrating actions in organic chemistry which are carried out in a mixture of nitric and sulphuric acids. When nitric acid is dissolved in water, its behaviour is that of a strong acid, i.e. ... 

Arsenic(V) acid, H3ASO4 (strictly, tetraoxoarsenic(V) acid) is obtained when arsenic is oxidised with concentrated nitric acid or when arsenic(V) oxide is dissolved in water. It is a moderately strong acid which, like phosphoric V) acid, is tribasic arsenates V) in general resemble phosphates(V) and are often isomorphous with them. 

A base is any material that produces hydroxide ions when it is dissolved in water. The words alkaline, basic, and caustic are often used synonymously. Common bases include sodium hydroxide (lye), potassium hydroxide (potash lye), and calcium hydroxide (slaked lime). The concepts of strong versus weak bases, and concentrated versus dilute bases are exactly analogous to those for acids. Strong bases such as sodium hydroxide dissociate completely while weak bases such as the amines dissociate only partially. As with acids, bases can be either inorganic or organic. Typical reactions of bases include neutralization of acids, reaction with metals, and reaction with salts ... 

Inorganic Chlorides/Halides — These metallic salts are formed from the reaction of a weak base with the strong acid HCl. Salts such as these dissolve in water to produce a markedly acidic solution. This is exemplified by aluminum chloride, which is corrosive due to the acidity resulting from the hydrolysis that produces aluminum and chlorine ions. Anhydrous AICI3 hydrolyzes violently when contacted by water. 

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