Brpnsted theory

On the Brpnsted theory (p. 51), solutions with concentrations of H3O+ greater than that in pure water are acids (proton donors), and solutions rich in OH are bases (proton acceptors). The same classifications follow from the solvent-system theory of acids and bases... 

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 ... 

An acid is defined as a proton donor within the Lowry-Brpnsted theory (see Chapter 6). Molecules of acid ionize in aqueous solution to form an anion and a proton, both of which are solvated. An acid such as ethanoic acid (VI) is said to be weak if the extent to which it dissociates is incomplete we call it strong if ionization is complete (see Section 6.2). 

All aqueous solutions naturally contain hydroxide ions in consequence of the auto-protolytic reaction in Equation (6.2). As we have seen, there will be equal numbers of solvated protons and solvated hydroxide ions unless we add an acid or base to it. A solution containing more solvated protons than hydroxide ions is said to be an acid within the Lowry-Brpnsted theory, and a solution comprising more hydroxide ions than solvated protons is said to be a base. 

We must treat with caution one further aspect of the Brpnsted theory multiple proton-donation reactions. Consider the example of the bicarbonate ion HC03 in water. When titrating bicarbonate with a base such as hydroxide, the ion behaves as an acid to form the carbonate anion and water ... 

Seeing that the Brpnsted definitions of acids and bases are not related to a specific solvent, this theory can readily explain the reaction shown in Eq. (5.4). In that case, HCI donates a proton to NH3, resulting in the formation of the ionic salt NH4C1. Therefore, HCI is an acid. Because NH3 accepts a proton, it is acting as a base. Likewise, the Brpnsted theory is applicable to many reactions in which there is a solvent other than water, which makes the Brpnsted theory much more generally applicable than the Arrhenius theory. 

One of the general principles related to the Brpnsted theory is that a stronger acid or base displaces or produces a weaker one. For example, H2SO4 is a stronger acid than HC2H3O2 so the following reaction illustrates the principle ... 

Brpnsted theory, 23 Definition of Kb, 38 Lewis theory, 24 HSAB theory, 12 Base saturation (%), 163 Basic organic compounds, 356 Bicarbonate, 30-33 Biotite, 104, 108 Boltzmann equation, 143 Bonding, 6-12 Covalent, 7 Ionic, 7 Boron, 127 Buffer capacity, 86... 

Section 19.1 discusses the Brpnsted theory of acids and bases, which extends the concepts of add and base beyond aqueous solutions and also explains the acidic or basic nature of solutions of most salts. Dissociation constants, the equilibrium constants for the reactions of weak acids or bases with water, are introduced in Section 19.2. The concept of the ionization of covalent compounds is extended to water itself in Section 19.3, which also covers pH, a scale of acidity and basicity. Section 19.4 describes buffer solutions, which resist change in their acidity or basicity even when some strong acid or base is added. Both the preparation and the action of buffer solutions are explained. Section 19.5 discusses the equilibria of acids containing more than one ionizable hydrogen atom per molecule. 

In 1923, Johannes N. Brpnsted (1879-1947) and Thomas M. Lowry (1874-1936) independently defined acids and bases in a different way from the Arrhenins definitions. The resulting theory is sometimes called the Brpnsted-Lowry theory, bnt more often is referred to as just the Brpnsted theory. The Brpnsted theory extends the definitions of acid and base in a way that explains more than the Arrhenins definitions can explain. According to this theory, a Brpnsted acid is a proton donor, and a Brpnsted base is a proton acceptor. 

The Brpnsted theory extends the concepts of acid and base beyond reactions in aqueous solution. For example, it describes the reaction of ammonia gas with hydrogen chloride gas to form solid ammonium chloride ... 

The Brpnsted theory expands the definitions of acids and bases to explain the acidity or basicity of solutions previously regarded as salts chemskiii Builder 18.1 and to explain reactions in other than aqueous solutions. 

The Brpnsted theory of acids and bases extends the definition of acids and bases, which allows an explanation of why most salts dissolved in water do not form... 

A salt in Arrhenius terminology is composed of two types of ions. Describe them in terms of the Brpnsted theory. 

Brpnsted theory (19.1) A theory of acids and bases that is broader than the Arrhenius theory in that it does not limit acids and bases to water solutions and it defines ions as acids or bases depending on their tendency to react with water. 

The influence of solvents on the ionization equilibrium is related to their electrostatic and their solvation properties. The value of the ionization constant of an analyte is closely determined, in practice, by the pH scale in the particular solvent. It is clear that it is most desirable to have a universal scale which is able to describe acidity (and basicity) in a way that is generally valid for all solvents. It is, in principle, not the definition of an acidity scale in theory which complicates the problem it is the difficulty of approximating the measured values in practice to the specifications of the definition. The pH scale, as is common in water, is applicable only to some organic solvents (i.e., mainly those for which the solvated proton activity is compatible with the Brpnsted theory of acidity). The applicability of an analog to the pH scale in water decreases with decreasing relative permittivity of the solvents and with their increasing aprotic character. 

The Bronsted theory expands these definitions of acids and bases to explain much more of solution chemistry. For example, the Brpnsted theory explains why a solution of ammonium chloride tests acidic and a solution of sodium acetate tests basic. In the Brpnsted theory, an acid is defined as a substance that donates a proton to another substance. In this sense. 

So far we have discussed acid-base properties in terms of the Brpnsted theory. To behave as a Brpnsted base, for example, a substance must be able to accept protons. By this definition both the hydroxide ion and ammonia are bases ... 

As another example, the Brpnsted theory of acids and bases replaced the older Arrhenius theory because of its more general applicability (e.g., it applied also to non-aqueous solutions and even to gases). Attempts have been made to familiarise secondary school students with principles of quantum mechanics and thermodynamics, albeit often in a simplified and sometimes distorted form. It was expected that this would save much time because formulas and physical and chemical properties of specific compounds could then simply be derived from theoretical principles instead of having to be memorised. It was decided that many aspects of descriptive chemistry could be left out when students were provided with a small handbook covering most of the relevant facts and formulas. 

In summary, the Brpnsted theory of acids and bases has a much broader scope than previous acid-base concepts. According to the Bronsted theory, acids and bases can be ions as well as molecular substrates acid-base reactions are not restricted to aqueous solutions various chemical entities can act either as acids... 

Assuming that differences in zero-point motion of hydronium and acid can be ignored, pKa is the difference in deprotonation free energies adjusted by a constant. The constant, -3.2 units, is effectively the pKa of the constrained hydronium ion, because, in this case, the deprotonation integrals and zero-point motion terms rigorously cancel. The model H3O+ is therefore 1.5 pX units more acid than the pKa = -1.74 of the hydronium in the Brpnsted theory. 

In the Brpnsted theory, the amphoteric character requires the participation of two water molecules to be explained ... 

In this example, ethanoic acid is the solvent, but it also plays the part of a base since it accepts a proton from the hydrogen chloride to give the solvated proton CH3 COOH2+, which has sometimes been called the acetyhum ion. Quite obviously, ethanoic acid is less basic than water since the proton transfer to it is equilibrated, whereas it is total to water. Hydrogen chloride is a weak acid in ethanoic acid and is a strong one in water. Conversely, these are the reasons why it is called hydrochloric acid and why ethanoic acid is called acetic acid in water. It is also interesting to notice that in this example the chloride ion exhibits a true basic character in acetic acid, in agreement with the Brpnsted theory. 

It is interesting to note that all the relations given in this section are found in either the Arrhenius or Brpnsted theory. 

The rapid growth of the list of basic substances evident since the advent of the Brpnsted theory has already made acceptable to most chemists the idea that almost any atom or group of atoms may act as a base if a strong enough acid is used. The electronic theory supplies the explanation. Most atomic groupings are surrounded by some lone pairs of electrons which can be utilized by... 

The effect of the solvent upon neutralization will be treated in Chapter 8 since a previous discussion of displacement is necessary. The type equation of the Brpnsted theory eliminates neutralization from all acid-base reactions covered by the theory. This is because the equation represents displacement reactions of secondary acids and bases. Since the Lewis theory includes the Br0nsted... 

Examples of the second kind of displacement (of a weak base by a stronger one) are more familiar, since the type equation of the Brpnsted theory represents the displacement of one base by a stronger base in competition for the proton, for example ... 

Some writers have indicated that hydrogen acids require special consideration in the Lewis terminology. No doubt such a misunderstanding arises from the emphasis of the Br0nsted theory upon displacement of one base by another as the only criterion of acid-base phenomena. Actually displacement is only one of the four phenomenological criteria of acids and bases, and the Brpnsted type equation is only one of the two types of acid-base displacement. The Brpnsted theory is thus included in the Lewis theory and requires no special consideration. 

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