Reactions That Form Water Acids and Bases

For each of the following reactions, write the molecular equation, the complete ionic equation, and the net ionic equation. 

The Nobel Prize in chemistry was awarded to Arrhenius in 1903 for his studies of solution conductivity. 

I AIM To learn the key characteristics of the reactions between strong acids and strong bases. 

In this section we encounter two very important classes of compounds acids and bases. Acids were first associated with the sour taste of citrus fruits. In fact, the word acid comes from the Latin word acidus, which means sour. Vinegar tastes sour because it is a dilute solution of acetic acid citric acid is responsible for the sour taste of a lemon. Bases, sometimes called alkalis, are characterized by their bitter taste and slippery feel, like wet soap. Most commercial preparations for unclogging drains are highly basic. 

Acids have been known for hundreds of years. For example, the mineral acids sulfuric acid, H2SO4, and nitric acid, HNO3, so named because they were originally obtained by the treatment of minerals, were discovered around 1300. However, it was not until the late 1800s that the essential nature of acids was discovered by Svante Arrhenius, then a Swedish graduate student in physics. 

Reactions That Form Water Acids and Bases 

Arrhenius Acids and Bases Arrhenius, who was trying to discover why only certain solutions could conduct an electric current, found that conductivity arose from fhe presence of ions. In his studies of solutions, Arrhenius observed that when the substances HCl, HNO3, and H2SO4 were dissolved in water, they behaved as strong electrolytes. He suggested that this was the result of ionization reactions in water. 

The Arrhenius definition of an acid a substance that produces H ions in aqueous solution. 

3KOH(ag) + e(N03hiaq) Fe(OH)3(5) + SKNOsiaq) Complete ionic equation  

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A. Reactions that Form Water Acids and Bases... 

You will note that Table 14-1 also contains standard enthalpies of formation for ions in aqueous solution. It is worth noting here that calorimetry was a strong argument favoring the view that all strong acids and bases exist in dilute water solution only as ions and not as molecules. No matter which combination of strong acid and base is used in a neutralization reaction, the heat value obtained is always very close to the value of AH° = - 13.36 kcal per mole of water formed. The implication of course is that, although the chemicals are different in each case, the reaction is the same it must be... 

If you remember from Chapter 11, we said that if an acid and base are mixed together, a neutralization reaction occurs that produces water and a salt. In this section, we are going to look a bit more closely at the salts that form in neutralization reactions (and salts in general). If you remember, a neutralization reaction takes the form ... 

Indicators Indicators are molecules that react with acids and bases and give products that have different colors, which depend on the pH. The phenolphthalein shown in Figure 8.R.1 is such a molecule. At pH < 7, it has one form, which is colorless in water, and at pH > 7, it assumes another molecular form, which has a bright red color. During acid-base neutralization, it is added to the acid solution and the base is added in a dropwise fashion when the base consumes all the protons, the next drop raises the pH and the color changes all of a sudden. This is how we know that the reaction has reached its endpoint. This is also a method to determine how much of an unknown quantity of acid is in solution. 

Two other kinds of reactions that occur in solution are acid-base reactions—reactions that form water upon mixing of an acid and a base— and gas evolution reactions— reactions that evolve a gas. Like precipitation reactions, these reactions occur when the cation of one reactant combines with the anion of another. As we will see in the next section, many gas evolution reactions also happen to be acid-base reactions. 

Neutralization means that relatively stronger acids and bases are combining to becoming relatively weaker acids and bases. In neutralization reactions, water-which is both a weak acid and a weak base-is often formed. 

The reaction between an acid and a base is called a neutralization reaction, and the ionic compound produced in the reaction is called a salt. The general form of a neutralization reaction of a strong acid and a metal hydroxide that provides the hydroxide ion, a strong base, in water is... 

The first substantial constitutive concept of acid and bases came only in 1887 when Arrhenius applied the theory of electrolytic dissociation to acids and bases. An acid was defined as a substance that dissociated to hydrogen ions and anions in water (Day Selbin, 1969). For the first time, a base was defined in terms other than that of an antiacid and was regarded as a substance that dissociated in water into hydroxyl ions and cations. The reaction between an acid and a base was simply the combination of hydrogen and hydroxyl ions to form water. 

Remember that an acid-base reaction is a double displacement reaction. Therefore, if sulfuric acid and potassium hydroxide are mixed, the positive ions trade places. The hydrogen ions from the sulfuric acid will react with the negative hydroxide ions to form water. Because a hydrogen ion has a charge of + 1 and a hydroxide ion has a charge of -1, they bond in a 1 1 ratio ... 

Generally, acids and bases react according to the rules for replacement and double-replacement reactions given above. They are so important, however, that a special nomenclature has developed for acids and their reactions. Acids were introduced in Sec. 6.4. They may be identified in their formulas by having the H representing hydrogen written first, and in their names by the presence of the word acid. An acid will react with a base to form a salt and water. The process is called neutralization. Neutralization reactions will be used as examples in Sec. 10.5, on titration. 

The Br0nsted theory expands the definition of acids and bases to allow us to explain much more of solution chemistry. For example, the Brpnsted theory allows us to explain why a solution of ammonium chloride tests acidic and a solution of sodium acetate tests basic. Most of the substances that we consider acids in the Arrhenius theory are also acids in the Bronsted theory, and the same is true of bases. In both theories, strong acids are those that react completely with water to form ions. Weak acids ionize only slightly. We can now explain this partial ionization as an equilibrium reaction of the ions, the weak acid, and the water. A similar statement can be made about weak bases ... 

Acids and bases are extremely common substances, as are their reactions with each other. At the macroscopic level, acids taste sour (that is, lemon juice) and react with bases to yield salts. Bases taste bitter (that is, tonic water) and react with acids to form salts. 

H (aq) Cl (aq) hJa (aq) + OH (aq) Fla (aq) "t" Cl (aq) "t" H20(r) AH = —56 kJ Subtracting spectator ions from both sides, the net ionic equation is H+(aq) -I- OH (aq) H20(f) AH = -56 kJ Different combinations of strong Arrhenius acids and bases react with the same exothermic result. Measurements always show the release of 56 kJ of energy per mole of water formed. This makes sense, because the net ionic equation is the same regardless of the specific neutralization reaction that occurs. 

However, this simple picture only applies to gases that do not undergo reactions in the boundary layers. For gases that do react, for example through hydration and acid-base reactions, the net flux depends on the simultaneous movement of all the solutes involved, and the flux will not be the simple function of concentration expressed in Equation (3.25). An example is CO2, which reacts with water to form carbonic acid and carbonate species-H2C03, HCOs and COs . The situation is complicated because the exchange of H+ ions in the carbonate equilibria results in a pH gradient across the still layer, and it is therefore necessary to account for the movement of H+ ions across the still layer as well as the movement of carbonate species. The situation is further complicated in the case of CO2 by the kinetics of hydration and dehydration, which may be slow in comparison with transport. 

According to the Arrhenius definition of acids and bases, acids are substances that produce hydrogen ions (H+) in solution, and bases are substances that produce hydroxide ions (OH ) in solution. When an acid and a base combine, the hydrogen ions from the acid react with the hydroxide ions from the base to form water—a neutralization reaction. 

For complete neutralization to take place, the proper amounts of acid and base must be present. The salt formed in the above reaction is NaCl. If the water were evaporated after completing the reaction, we would be left with common table salt. Sodium chloride is just one of hundreds of salts that form during neutralization reactions. While we commonly think of salt, NaCl, as a seasoning for food, in chemistry a salt is any ionic compound containing a metal cation and a nonmetal anion (excluding hydroxide and oxygen). Some examples of salts that result from neutralization reactions include potassium chloride (KCl), calcium fluoride (CaF ), ammonium nitrate (NH NOj), and sodium acetate (NaC2H302). 

Hydrolysis.—A salt that contains as one of its constituents either a weak acid or a weak base will not dissolve in water to form a neutral solution. Water acts upon such a salt partially to reverse the reaction of neutralization, liberating equivalent parts of both acid and base. To obtain in solid form a normal salt that is subject to hydrolysis, it is necessary to have present in the solution considerable excess of either the acid or the base, depending upon the nature of the salt and the mother liquor from which the salt is crystallized will contain this excess. When the crystals have been drained from this mother liquor, they cannot be washed with the pure solvent, for this would leave them coated with a film of the aqueous solution of the normal salt, which would then undergo hydrolysis. If either the acid or the base of which the salt is composed should be difficultly soluble,... 

NEUTRALIZATION. A chemical reaction in which water is formed by mutual interaction of the ions that characterize acids and bases when... 

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