File Name: ionisation constants of inorganic acids and bases in aqueous solution .zip
The relative strength of an acid or base is the extent to which it ionizes when dissolved in water.
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- Ionisation Constants of Inorganic Acids and Bases in Aqueous Solution
Aqueous Arrhenius acids have characteristic properties which provide a practical description of an acid. Chemicals or substances having the property of an acid are said to be acidic.
Common aqueous acids include hydrochloric acid a solution of hydrogen chloride which is found in gastric acid in the stomach and activates digestive enzymes , acetic acid vinegar is a dilute aqueous solution of this liquid , sulfuric acid used in car batteries , and citric acid found in citrus fruits. As these examples show, acids in the colloquial sense can be solutions or pure substances, and can be derived from acids in the strict  sense that are solids, liquids, or gases.
Strong acids and some concentrated weak acids are corrosive , but there are exceptions such as carboranes and boric acid. The second category of acids are Lewis acids , which form a covalent bond with an electron pair.
An example is boron trifluoride BF 3 , whose boron atom has a vacant orbital which can form a covalent bond by sharing a lone pair of electrons on an atom in a base, for example the nitrogen atom in ammonia NH 3.
Hydronium ions are acids according to all three definitions. Thus, an Arrhenius acid can also be described as a substance that increases the concentration of hydronium ions when added to water.
Examples include molecular substances such as hydrogen chloride and acetic acid. This decreases the concentration of hydronium because the ions react to form H 2 O molecules:.
Due to this equilibrium, any increase in the concentration of hydronium is accompanied by a decrease in the concentration of hydroxide. Thus, an Arrhenius acid could also be said to be one that decreases hydroxide concentration, while an Arrhenius base increases it. Since pH is defined as the negative logarithm of the concentration of hydronium ions, acidic solutions thus have a pH of less than 7.
While the Arrhenius concept is useful for describing many reactions, it is also quite limited in its scope. Consider the following reactions of acetic acid CH 3 COOH , the organic acid that gives vinegar its characteristic taste:. In the second example CH 3 COOH undergoes the same transformation, in this case donating a proton to ammonia NH 3 , but does not relate to the Arrhenius definition of an acid because the reaction does not produce hydronium.
Hydrogen chloride HCl and ammonia combine under several different conditions to form ammonium chloride , NH 4 Cl. In aqueous solution HCl behaves as hydrochloric acid and exists as hydronium and chloride ions. The following reactions illustrate the limitations of Arrhenius's definition:.
As with the acetic acid reactions, both definitions work for the first example, where water is the solvent and hydronium ion is formed by the HCl solute. The next two reactions do not involve the formation of ions but are still proton-transfer reactions. In the second reaction hydrogen chloride and ammonia dissolved in benzene react to form solid ammonium chloride in a benzene solvent and in the third gaseous HCl and NH 3 combine to form the solid.
A third, only marginally related concept was proposed in by Gilbert N. Lewis , which includes reactions with acid-base characteristics that do not involve a proton transfer. A Lewis acid is a species that accepts a pair of electrons from another species; in other words, it is an electron pair acceptor. Contrast how the following reactions are described in terms of acid-base chemistry:.
Fluoride "loses" a pair of valence electrons because the electrons shared in the B—F bond are located in the region of space between the two atomic nuclei and are therefore more distant from the fluoride nucleus than they are in the lone fluoride ion. BF 3 is a Lewis acid because it accepts the electron pair from fluoride. The second reaction can be described using either theory. Depending on the context, a Lewis acid may also be described as an oxidizer or an electrophile.
This reaction is referred to as protolysis. The protonated form HA of an acid is also sometimes referred to as the free acid. Acid-base conjugate pairs differ by one proton, and can be interconverted by the addition or removal of a proton protonation and deprotonation , respectively. In solution there exists an equilibrium between the acid and its conjugate base. The equilibrium constant K is an expression of the equilibrium concentrations of the molecules or the ions in solution.
Brackets indicate concentration, such that [H 2 O] means the concentration of H 2 O. The acid dissociation constant K a is generally used in the context of acid-base reactions.
The stronger of two acids will have a higher K a than the weaker acid; the ratio of hydrogen ions to acid will be higher for the stronger acid as the stronger acid has a greater tendency to lose its proton. Stronger acids have a smaller p K a than weaker acids. Arrhenius acids are named according to their anions. In the classical naming system, the ionic suffix is dropped and replaced with a new suffix, according to the table following. The prefix "hydro-" is used when the acid is made up of just hydrogen and one other element.
For example, HCl has chloride as its anion, so the hydro- prefix is used, and the -ide suffix makes the name take the form hydrochloric acid. The strength of an acid refers to its ability or tendency to lose a proton. In contrast, a weak acid only partially dissociates and at equilibrium both the acid and the conjugate base are in solution. Two key factors that contribute to the ease of deprotonation are the polarity of the H—A bond and the size of atom A, which determines the strength of the H—A bond.
Acid strengths are also often discussed in terms of the stability of the conjugate base. Stronger acids have a larger acid dissociation constant , K a and a more negative p K a than weaker acids. Sulfonic acids, which are organic oxyacids, are a class of strong acids. A common example is toluenesulfonic acid tosylic acid. Unlike sulfuric acid itself, sulfonic acids can be solids. In fact, polystyrene functionalized into polystyrene sulfonate is a solid strongly acidic plastic that is filterable.
Examples of superacids are fluoroantimonic acid , magic acid and perchloric acid. Superacids can permanently protonate water to give ionic, crystalline hydronium "salts". They can also quantitatively stabilize carbocations. While K a measures the strength of an acid compound, the strength of an aqueous acid solution is measured by pH, which is an indication of the concentration of hydronium in the solution.
The pH of a simple solution of an acid compound in water is determined by the dilution of the compound and the compound's K a. Lewis acids have been classified in the ECW model and it has been shown that there is no one order of acid strengths. Monoprotic acids, also known as monobasic acids, are those acids that are able to donate one proton per molecule during the process of dissociation sometimes called ionization as shown below symbolized by HA :.
Common examples of monoprotic acids in mineral acids include hydrochloric acid HCl and nitric acid HNO 3. On the other hand, for organic acids the term mainly indicates the presence of one carboxylic acid group and sometimes these acids are known as monocarboxylic acid. Polyprotic acids, also known as polybasic acids, are able to donate more than one proton per acid molecule, in contrast to monoprotic acids that only donate one proton per molecule.
Specific types of polyprotic acids have more specific names, such as diprotic or dibasic acid two potential protons to donate , and triprotic or tribasic acid three potential protons to donate.
A diprotic acid here symbolized by H 2 A can undergo one or two dissociations depending on the pH. Each dissociation has its own dissociation constant, K a1 and K a2. The first dissociation constant is typically greater than the second; i. The large K a1 for the first dissociation makes sulfuric a strong acid. An inorganic example of a triprotic acid is orthophosphoric acid H 3 PO 4 , usually just called phosphoric acid.
Even though the positions of the three protons on the original phosphoric acid molecule are equivalent, the successive K a values differ since it is energetically less favorable to lose a proton if the conjugate base is more negatively charged. An organic example of a triprotic acid is citric acid , which can successively lose three protons to finally form the citrate ion. Although the subsequent loss of each hydrogen ion is less favorable, all of the conjugate bases are present in solution.
A plot of these fractional concentrations against pH, for given K 1 and K 2 , is known as a Bjerrum plot. A pattern is observed in the above equations and can be expanded to the general n -protic acid that has been deprotonated i -times:. Neutralization is the reaction between an acid and a base, producing a salt and neutralized base; for example, hydrochloric acid and sodium hydroxide form sodium chloride and water:. Neutralization is the basis of titration , where a pH indicator shows equivalence point when the equivalent number of moles of a base have been added to an acid.
It is often wrongly assumed that neutralization should result in a solution with pH 7. Neutralization with a base weaker than the acid results in a weakly acidic salt.
An example is the weakly acidic ammonium chloride , which is produced from the strong acid hydrogen chloride and the weak base ammonia. Conversely, neutralizing a weak acid with a strong base gives a weakly basic salt, e.
In order for a protonated acid to lose a proton, the pH of the system must rise above the p K a of the acid. Solutions of weak acids and salts of their conjugate bases form buffer solutions. To determine the concentration of an acid in an aqueous solution, an acid-base titration is commonly performed.
A strong base solution with a known concentration, usually NaOH or KOH, is added to neutralize the acid solution according to the color change of the indicator with the amount of base added. The pH of the solution always goes up as the base is added to the solution. For each diprotic acid titration curve, from left to right, there are two midpoints, two equivalence points, and two buffer regions. Due to the successive dissociation processes, there are two equivalence points in the titration curve of a diprotic acid.
The second equivalence point occurs when all hydrogen ions are titrated. For a weak diprotic acid titrated by a strong base, the second equivalence point must occur at pH above 7 due to the hydrolysis of the resulted salts in the solution. Because the buffer regions consist of the acid and its conjugate base, it can resist pH changes when base is added until the next equivalent points. Acids exist universally in our life.
There are both numerous kinds of natural acid compounds with biological functions and massive synthesized acids which are used in many ways. Acids are fundamental reagents in treating almost all processes in today's industry. Sulfuric acid, a diprotic acid, is the most widely used acid in industry, which is also the most-produced industrial chemical in the world. It is mainly used in producing fertilizer, detergent, batteries and dyes, as well as used in processing many products such like removing impurities.
In the chemical industry, acids react in neutralization reactions to produce salts. For example, nitric acid reacts with ammonia to produce ammonium nitrate , a fertilizer. Additionally, carboxylic acids can be esterified with alcohols, to produce esters. Acids are often used to remove rust and other corrosion from metals in a process known as pickling.
Articles citing this article
Although acid—base properties have been investigated most thoroughly in aqueous solutions, partly because of their practical importance, water is in many respects an abnormal solvent. In particular, it has a higher dielectric constant a measure of the ability of the medium to reduce the force between two electric charges than most other liquids, and it is able itself to act either as an acid or as a base. The behaviour of acids and bases in several other solvents will be described briefly here. The effect of the solvent on the dissociation of acids or bases depends largely upon the basic or acidic properties of the solvent, respectively. Since many acid—base reactions involve an increase or decrease in the number of ions, they are also influenced by the dielectric constant of the solvent, for a higher dielectric constant favours the formation of ions. It is usually not easy to separate these three effects and, in particular, the effects of dielectric constant and solvation merge into one another. These points are illustrated with examples of several of the more important solvents.
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Ionisation Constants of Inorganic Acids and Bases in Aqueous Solution
In this research work, a potentiometric technic was used to measure the acidic dissociate constants pK a ,s for glycyl aspartic acid GLY-ASP at temperatures In addition, the value of the acid dissociation constants pK a1 , pK a2, and pK a3 , the optimized structure, and the thermodynamic properties of GLY-ASP were calculated in aqueous solution at various temperatures by ab initio and DFT methods. Thomasi's method was used to analyze the formation of intermolecular hydrogen bonding between the water molecule and various species of GLY-ASP.
All definitions agree that bases are substances which react with acids as originally proposed by G. Rouelle in the midth century. Such aqueous hydroxide solutions were also described by certain characteristic properties. They are slippery to the touch, can taste bitter  and change the color of pH indicators e.
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The gas phase and aqueous thermochemistry and reactivity of nitroxyl nitrosyl hydride, HNO were elucidated with multiconfigurational self-consistent field and hybrid density functional theory calculations and continuum solvation methods. The pK a of HNO is predicted to be 7. HNO is predicted to exist as a discrete species in solution and is a viable participant in the chemical biology of nitric oxide and derivatives. The discoveries of nitric oxide NO biosynthesis in mammalian cells and the diverse biological activity associated with NO and NO-derived species 1 have brought intense interest in the physiological chemistry of nitrogen oxides. Additionally, HNO might be expected to be electrophilic, and hydration under physiological conditions would serve to attenuate its aqueous reactivity.