Weak-acid

Most acidic substances are weak acids and therefore only partially ionized in aqueous solution ( FIGURE 16.9). We can use the equilibrium constant for the ionization reaction to express the extent to which a weak acid ionizes. If we represent a general weak acid as HA, we can write the equation for its ionization in either of the following ways, depending on whether the hydrated proton is represented as H.30 aq) or H aq)  

Because H2O is the solvent, it is omitted from the equilibrium-constant expression. (Section 15.4) Thus, we can write the equilibrium-constant expression as either 

As we did for the ion-product constant for the autoionization of water, we change the subscript on this equihbrium constant to indicate the type of equation to which it corresponds  

The subscript a denotes that is an equilibrium constant for the ionization of an acid, so Kg is called the acid-dissociation constant. 

The magnitude of indicates the tendency of the acid to ionize in water The larger the value of K , the stronger the acid. Chlorous acid (HCIO2), for example, is the strongest acid in Table 16.2, and phenol (HOCgH5) is the weakest. For most weak acids values range from 10 to 10 

It is a common mistake to oonfuse the terms strong and weak acids with the terms concentrated and dilute acids. Can you state the difference between these terms  

In contrast to HCl, HP is a weak acid, one that does not completely ionize in solution. 

When HP dissolves in water, only a fraction of the dissolved molecules ionize into HsO and ions. The solution contains many intact HP molecules. 

I Notice that the strength of a conjugate base is related to its attraction to in solution. 

Solutions such as these are called weak electrolyte solutions. 

These equilibria are in aqueous solution, so we will use equilibrium-constant expressions based on concentrations. Because H2O is the solvent, it is omitted from the equilibrium-constant expression, ogo (Section 15.4) Further, we add a subscript a on the equilibrium constant to indicate that it is an equilibrium constant for the ionization of an acid. Thus, we can write the equilibriimi-constant expression as either  

Based on the entries in Table 16.2, which element is most commonly bonded to the acidic hydrogen  

In order to calculate either the value for a weak acid or the pH of its solutions, we will use many of the skills for solving equdibriiun problems developed in Section 15.5. In memy cases the small magnitude of cJlows us to use approximations to simplify the problem. In doing these calculations, it is important to realize that proton-transfer reactions are generally very rapid. As a result, the measured or calculated pH for a weak acid cdways represents an equilibrium condition. 

A student prepared a 0.10 M solution of formic acid (HCOOH) and found its pH at 25 °C to be 2.38. Calculate for formic acid at this temperature. 

Suppose that you dissolve acetic acid (CH3COOIQ in water. It reacts with the water molecules, donating a proton and forming hydronium ions. It also establishes equilibrium, where you have a significant amount of unionized acetic acid. (In reactions that go to completion, the reactants are completely used up creating the products. But in equilibrium systems, two exactly opposite chemical reactions — one on each side of the reaction arrow — are occurring at the same place, at the same time, with the same speed of reaction. For a discussion of equilibrium systems, see Chapter 8.) 

If you want to see whether a person is a chemist, ask him to pronounce unionized A chemist pronounces it un-ionized, meaning not ionized. Everyone else pronounces it union-ized, meaning being part of a union.  

The acetic acid reaction with water looks like this  

The acetic acid that you add to the water is only partially ionized. In the case of acetic acid, about 5 percent ionizes, while 95 percent remains in the molecular form. The amount of hydronium ion that you get in solutions of acids that don t ionize completely is much less than it is with a strong acid. Acids that only pmUally ionize are called weak acids. 

Calculating the hydronium ion concentration in weak acid solutions isn t as straightforward as it is in strong solutions, because not all of the weak acid that dissolves initially has ionized. In order to calculate the hydronium ion concentration, you must use the equilibrium constant expression for the weak acid. Chapter 8 covers the K, expression that represents the equilibrium system. For weak acid solutions, you use a modified equilibrium constant expression called the K, — the acid ionization constant Take a look at the generalized ionization of some weak acid HA  

When HP dissolves in water, only a fraction of the molecules ionize. 

Notice that two of the weak acids in Table 15.4 are diprotic, meaning that they have two ionizable protons, and one is triprotic (three ionizable protons). We discuss polyprotic acids in more detail in Section 15.9. 

We can also write the formulas for acetic acid and formic acid as CH3COOH and HCOOH, respectively, to indicate that in these compounds the only H that ionizes is the one attached to an oxygen atom. 

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The strength of an acid is measured by the value of its dissociation constant, strong acids, e.g. HCl, HNO3. being substantially fully ionized in solution and weak acids predominately unionized. 

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. ... 

Nitroxylic acid, HiNO . Yellow NajNOa formed by electrolysis NaN02 in NHj. Structure unknown free acid unknown. Nitramide, H2NNO2. A weak acid. 

Examples of the lader include the adsorption or desorption of species participating in the reaction or the participation of chemical reactions before or after the electron transfer step itself One such process occurs in the evolution of hydrogen from a solution of a weak acid, HA in this case, the electron transfer from the electrode to die proton in solution must be preceded by the acid dissociation reaction taking place in solution. 

Values of are small for weak acids and they range very widely (Table 4.1). It is common practice to quote values as the negative logarithm to the base ten, i.e. — logjo K.. since such numbers are less cumbersome and positive when Aj < 1. The symbol for -logio is by convention "p/ fhus -logjo becomes pK,. Table 4.1 shows some typical pAg values. 

Consider first two substances which have very similar molecules. He, hydrogen fluoride and HCl. hydrogen chloride the first is a Weak acid in water, the second is a strong acid. To see the reason consider the enthalpy changes involved when each substance in water dissociates to form an acid ... 

Thus, ihe strongly basic oxide ion attacks the weakly acidic SiOj in a molten salt as solvent (p. 187 ... 

But a solution of carbon dioxide in water behaves as a very weak acid since the effective dissociation constant K is given by ... 

Since carbonic acid is a weak acid, its salts are hydrolysed in aqueous solution ... 

Pure hydrazoic acid is a colourless liquid, b.p. 310 K. It is very ready to detonate violently when subjected to even slight shock, and so is used in aqueous solution. It is a weak acid, reacting with alkali to give azides, which contain the ion Nj. 

Arsenic(IH) acid is an extremely weak acid in fact, the oxide is amphoteric, since the following equilibria occur ... 

Nitrous acid, HNOj, is known as a gas, but otherwise exists only in solution, in which it is a weak acid. Hence addition of a strong acid to a solution of a nitrite produces the free nitrous acid in solution. 

The addition of even a weak acid (such as ethanoic acid) to a nitrite produces nitrous acid which readily decomposes as already indicated. Hence a nitrite is distinguished from a nitrate by the evolution of nitrous fumes when ethanoic acid is added. 

It is an extremely weak acid but does form salts. Two kinds are known, trioxoarsenates(III), for example Na3As03, and dioxo-arsenates(III), for example Cu(As02)2-... 

Hydrogen sulphide is slightly soluble in water, giving an approximately 0.1 M solution under 1 atmosphere pressure it can be removed from the solution by boiling. The solution is weakly acidic and dissolves in alkalis to give sulphides and hydrogensulphides. The equilibrium constants... 

Since the hydrogen-element bond energy decreases from sulphur to tellurium they are stronger acids than hydrogen sulphide in aqueous solution but are still classified as weak acids—similar change in acid strength is observed for Group Vll hydrides. 

These are ionic solids and can exist as the anhydrous salts (prepared by heating together sulphur with excess of the alkali metal) or as hydrates, for example Na2S.9HjO. Since hydrogen sulphide is a weak acid these salts are hydrolysed in water,... 

Tellurium trioxide, TeOa, is an orange yellow powder made by thermal decomposition of telluric(VI) acid Te(OH)g. It is a strong oxidising agent which will, like H2Se04, oxidise hydrogen chloride to chlorine. It dissolves in hot water to give telluric(VI) acid. This is a weak acid and quite different from sulphuric and selenic acids. Two series of salts are known. 

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 ... 

Aqueous hydrogen fluoride is a weak acid (see above) and dissolves silica and silicates to form hexafluorosilicic acid hence glass is etched by the acid, which must be kept in polythene bottles. 

The acids of chlorine(I), bromine(I) and iodine(I) are weak acids, the pKa values being 7.4, 8.7 and 12.3 respectively. They are good oxidising agents, especially in acid solutions. The acids decrease in stability from chloric(I) to iodic(I). 

Only chlorine forms a -t-3 acid, HCIO2. This is also a weak acid and is unstable. The - -5 acids, HXO3, are formed by chlorine, bromine and iodine they are strong acids. They are stable compounds and all are weaker oxidising agents than the corresponding +1 acids. 

Chloric(III) acid is a fairly weak acid, and is an oxidising agent, for example it oxidises aqueous iodide ion to iodine. Sodium chlorate(III) (prepared as above) is used commercially as a mild bleaching agent it bleaches many natural and synthetic fibres without degrading them, and will also bleach, for example, oils, varnishes and beeswax. 

These are acids which can be regarded, in respect of their formulae (but not their properties) as hydrates of the hypothetical diiodine heptoxide, liO-j. The acid commonly called periodic acid , I2O7. 5H2O, is written HglO (since the acid is pentabasic) and should strictly be called hexaoxoiodic(VII) acid. It is a weak acid and its salts are hydrolysed in solution. It can be prepared by electrolytic oxidation of iodic(V) acid at low temperatures ... 

In dilute aqueous solution hydrogen fluoride is a weak acid but the acid strength increases with the concentration of hydrogen fluoride. 

Since hydroxylamine is usually available only in the form of its salts, e.g., the hydrochloride or sulphate, the aqueous solution of these salts is treated with sodium acetate or hydroxide to liberate the base before treatment with the aldehyde or ketone. Most oximes are weakly amphoteric in character, and may dissolve in aqueous sodium hydroxide as the sodium salt, from which they can be liberated by the addition of a weak acid, e.g., acetic acid. 

The addition of the sulphuric acid first neutralises the sodium hydroxide, and then gives a weakly acidic and therefore colourless solution. The sodium derivative (A) then undergoes further partial hydrolysis in order to re-establish the original equilibrium, and the sodium hydroxide thus formed again produces the pink coloration, which increases in depth as the hydrolysis proceeds. 

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