Robert Boyle added to that list the abilities to precipitate sulfur from sulfide solutions and to turn blue plant dyes red. In , Rouelle introduced the term bases for a class of substances that reacts with acids to form salts Finston and Rychtman Acid-base reactions are of great importance in almost all areas of chemistry, and the historical development of acid-base theories is reviewed in most general chemistry textbooks.
The theories have become more generalized, and the more recent ones can be thought of as including the older as special cases. The Lewis definition views acids as substances that can employ a lone electron pair of another molecule to complete their own electron structure and bases as being able to share an electron pair with another molecule. One consequence of the small size is a high charge density i. The terminology is that an acid, after giving up a proton, turns into its conjugate base , B.
Under practical conditions, the hydrogen ion cannot exist by itself and will always attach to a free electron pair of another atom. The molecule containing that atom then reacts by definition as a base. An exchange of a hydrogen ion is therefore synonymous with a reaction between an acid and a base.
Many substances can react as acids or bases, depending on other substances present. The definition of acidity is therefore relative.
Acid Base Practice Problems Quiz
An illustration of typical acid-base reactions, as well as of the ability of water to act as acid or base, is given in Figure 1. Figure 1 Examples of acid-base reactions with water: towards hydrochloric acid HCl , water reacts as a base, towards ammonia NH 3 , it reacts as an acid. According to the laws of thermodynamics, any chemical reaction establishes an equilibrium between the products and reactants, characterized by a number, K, called the chemical equilibrium constant.
For the reaction given in Equ. Castellan The activity of a substance is related to its concentration by a factor, the activity coefficient which is a function of temperature and the activities of all other substances present. The majority of the important reactions occur in aqueous solution, and water can react readily either as acid or as base see Figure 1. Therefore, one can use water as a common denominator and compare the strengths of other acids and bases by the extent of their reactions with water. In dilute solutions, the activity coefficient of a substance approaches unity, and the concentration  of water can be considered constant.
Weast Some examples are given in Table 1.
Water can take the place of both acid 1 and base 2 in Equ. The extent of this reaction is very small so that the water remains essentially pure, with a concentration of The pH values of some common dilute aqueous solutions found in the literature Weiss , Streitwieser are shown in Table II. It can be seen that measuring the pH is a feasible way of comparing apples and oranges. Though the concept of pH has meaning only for dilute aqueous solutions, Table II lists values for apparently solid materials like wood, paper or cheese.
In all cases, however, only dilute aqueous solutions are measured. Food stuffs consist to a large extent of water, which may be extracted and used for pH determinations. Solids like paper and wood contain adsorbed water and, more importantly, water soluble substances, again allowing determination of a pH value by measurement of dilute aqueous solutions.
Table II pH values: examples Soft drinks 2. Detailed discussions of this property can be found in Bell This is used in very old but impractical methods of measuring acid strength. This approach exploits the fact that some molecules, like blue plant dyes, change their color when they become protonated or deprotonated, i. Through relations like Equ. Substances behaving in this manner, like the well-known litmus, are called indicators.
Through most of the range of possible pH values, an indicator will be present almost exclusively in either its acid or its base form.
11.1: Introduction to Acid/Base Equilibria
Only in a very narrow pH region will both forms be present at the same time. Any one indicator therefore is used only to determine whether a solution is below or above the pH value at which this particular indicator changes color. Indicator papers that allow determination of a range of pH values contain a mixture of several indicators, each of which changes color at a different pH. Indicators can be put to use to determine acidities of systems other than dilute aqueous solutions by basing an acidity scale on the concentrations of the indicator's acid and base forms.
An example is the Hammet acidity function Rochester , which defines a scale of actual acidity using the spectroscopically measured exact concentrations of indicator molecules in their protonated HIn and deprotonated In states. The useful range of this definition can be extended by using a sequence of chemically similar indicators with different pK a,w HIn. This involves the exchange of electrons written usually as e -. Their combination is known as an oxidation-reduction process or, more commonly, redox reaction. In Equ.
If the two half-reactions of a redox reaction take place in spatially separated solutions, there will be a difference in the availability of electrons in the two solutions. If these two solutions are electrically connected, one can measure a voltage between them.
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This setup is the principle of galvanic cells e. The theory of such electrochemical pH determinations is discussed in detail by Bates An outline of the operational definitions of pH can be found in Weast Practical considerations like establishment of the required galvanic cell, and the equipment available are addressed by Westcott Total Acid and Buffering The three methods of measuring acidity discussed above, namely kinetic, colorimetric and electrochemical, compare the actual acidity of dilute aqueous solutions containing any number of components to that of pure water.
An alternative approach of quantifying the acidity of a solution is to determine total acid or total alkali , respectively. To determine the total acid of a solution, small quantities of a strong base are added until the pH reaches a predetermined value, the so-called equivalence point where the amount of added base equals the amount of total acid in the solution.
The base is added in the form of a dilute solution of known concentration. The process is known as acid-base titration. Sometimes the term neutralization is used but the equivalence point is not necessarily pH 7. The equivalence point is defined as the pH at which the amount of base added is equal to the amount of proton-donating molecules. For a particular acid, this point can be found by rearranging Equ.
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If one plots the resulting pH against the fraction of dissociated acid molecules, i. As an example, the buffering curves for the two conjugate bases of carbonic acid are shown in Figure 2. Acid-base titrations are very useful methods for quantifying acidity since they can be applied to systems other than dilute aqueous solutions, provided that a method for recognizing the equivalence point exists. Titrations may be used with suspensions of insoluble materials, allowing the determination of acidic groups in the molecules of the solids. Other applications of acid-base titrations, as well as potential problems and errors, are discussed in detail by Kolthoff and Laitinen and by Mattock Figure 2.
Buffering curves of the dibasic acid HO 2 CO The concept of buffering , mentioned above in the context of acid-base titrations, designates the property of a solution to show only negligible changes in pH on the addition of small quantities of strong acids or bases. Looking at Figure 2 again, one notices that over a sizeable range of values for the fraction of conjugate base, the pH changes very little.
This shape is the reason for the name buffering curves. The term buffered solution usually refers to a solution containing a weak acid and its salt, e.
One property of many heteroatom containing molecules is a certain acidity or basicity at physiological conditions that can contribute to the reactivity and physiochemical properties of the molecule. Although drug molecules are often designed to have little or no chemical reactivity as administered, once absorbed they are metabolized i. This differential solubility impacts the absorption of the drug and hence the amount that needs to be administered to achieve the desired effect. In this model, the proton never exists in solution as an isolated ion because it is energetically too unstable.
Instead it is always covalently bonded to another atom, but can be transferred between lone pairs of electrons on two different atoms. In this proton transfer reaction, the Forgot Password?
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