Weak acids dissolving in water

B. In contrast, when a weak acid dissolves in water, it remains mostly undissociated, yielding relatively... 

Although proton transfer is never complete for a weak acid dissolved in water, some proton transfer always occurs, so the hydronium ion concentration in any solution of a weak acid is greater than that in pure water. Consequently, the pH of an aqueous solution of a weak acid is always less than 7.00. 

Many common foods (such as citrus fruits), pharmaceuticals (such as AspirinT ), and some vitamins (such as niacin, vitamin B3) are weak acids. When a weak acid dissolves in water, it does not completely dissociate. The concentration of the hydronium ions, and the concentration of the conjugate base of the acid that is formed in solution, depend on the initial concentration of the acid and the amount of acid that dissociates. 

If a salt consists of the cation of a strong base and the anion of a weak acid, such as NaCHsCOO, only the anion reacts significantly with water. The reaction produces hydroxide ions. Therefore, the solution will have a pH that is greater than 7. Salts of strong bases and weak acids dissolve in water and form basic solutions. 

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

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.

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. 

Since a weak acid dissolved in water can be viewed as a prototype of almost any equilibrium occurring in aqueous solution, we will proceed carefully and systematically. Although some of the procedures we develop here may seem superfluous, they will become essential as the problems become more complicated. We will develop the necessary strategies by calculating the pH of a 1.00 M solution of HF (Ka = 7.2 X 10-4). 

We can write similar expressions for any weak acid dissolved in water. Using a generalized hypothetical acid (HA), the reaction in water is... 

When a weak acid dissolves in water, it ionizes to give hydro-nium ions and the conjugate base ions. The equilibrium constant for this acid ionization is Ka = [H30" ][A ]/[HA], where HA is the general formula for the acid. The constant can be determined from the pH of an acid solution of known concentration. Once obtained, the acid-ionization constant can be used to find the concentrations of species in any solution of... 

Monoprotic organic acids are usually weak. When a weak acid dissolves in water, only a small percentage of its molecules (typically 1%) react with water molecules to release hydrogen or oxonium ions. An equilibrium is established, with the majority of the acid molecules not undergoing ionization or dissociation. In other words, the equilibrium lies on the left-hand side of the equation. 

FIGURE 10.5 An illustration of a weak acid dissolving in water. The acid is a polar covalent compound, just like a strong acid. It dissolves completely in water, but only partially ionizes. 

Organic acids, such as the carboxylic acids (Chapter 14), can dissolve in water but only partially ionize. The usual exanple of such an acid is acetic acid (HC2H3O2). More details of this partial ionization are discussed in Chapter 11. Acids that partially ionize are called weak acids. The process of a weak acid dissolving in water is depicted in Figure 10.5. 

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. 

However, salts of a weak acid and strong base or of a strong acid and weak base dissolved in water will be alkaline or acidic respectively. Thus, aqueous sodium acetate is basic, whereas aqueous ammonium chloride is acidic. pH values may be calculated from pKa as follows. 

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... 

When a weak acid is dissolved in water, the acid will undergo ionization. The ionization of a weak acid, HA, in water can be expressed as ... 

Weak acids and weak bases dissolve in water principally in the molecular form. Only a small percentage of the molecules dissociate to form the hydronium or hydroxide ion. 

Fine needles from ale mp 219-221 (some decompn). Sol in water at pH 6.5 to the extent of 30-40 mg per 100 ml at 28-30. Freely sol in acetone dioxane 70% ale dil acids. Moderately sol in abs alcohol ethyl acetate and ether- It is a very weak acid dissolves in ]0% alkali (decompn) and forms an alkali metal salt at pH 10. 

It should be clear that these reactions of weak acids and bases are a special case of the equilibrium that we have been exploring throughout this chapter. When any acid dissolves in water, hydronium ions are always formed as one of the products. This species is so common that an additional way to describe its concentration has been devised. To avoid dealing with small numbers in scientific notation, we often use the pH scale, in which pH is defined as the negative logarithm of the hydronium ion concentration ... 

Sodium hydroxide. If the compound is water-insoluble, test its solubility in 1.5 M NaOH solution. Carboxylic acids, which are strong acids, and phenols, which are weak acids, dissolve in sodium hydroxide because they are converted into their water-soluble sodium salts (Eqs. 25.4a and 25.4b). An unknown that is more soluble in NaOH than in water may be either a phenol or a carboxylic acid, and it must be tested for solubility in the weaker base, 0.6 M NaHC03, which may permit distinction between these two functional groups (Part 2, below). If the unknown does not exhibit solubility in NaOH, its solubility in NaHC03 need not be tried rather, it should next be tested for solubility in 1.5 M HCl. 

When a salt derived from a strong acid and a weak base dissolves in water, the solution becomes acidic. For example, consider the process... 

The reversibility of reactions explains why weak acids and bases dissociate into ions to only a small extent the dissociation becomes balanced by a reassociation. For example, when acetic acid dissolves in water, some of the CH3COOH molecules react with water and form H3O+ and CH3COO ions. As more ions form, they react with... 

The acidity ofHA(aq). HF is a weak acid, so the equilibrium position of the acid dissolving in water lies far to the left ... 

The equilibrium law (see page 123) can be applied to aqueous solutions of weak acids and weak bases. For example, when ethanoic acid dissolves in water the following equilibrium results ... 

In Chapter 4, we considered a compound soluble if it dissolved in water and insoluble if it did not. In this section, we look at the dissolution process in the context of an equilibrium existing between the component ions of an ionic solid and the solid itself Consider SrCr04 in water (Figure 15.1). Much like the weak acid HF in water,... 

K2 is called the hydrolysis constant for sodium ethanoate. Hydrolysis occurs when salts involving weak acids or bases are dissolved in water. It is often also found with metal ions in solution. The ion [M(H20) ] dissociates to the hydroxy species [M(H20) , (OH)]f 1. ... 

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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