Acid-Base Neutralization Reactions

Acid-base reactions (neutralization). An acid, which contributes H+ (H30+) ions, and a base, which contributes OH- ions, undergo metathesis to produce water (HOH or H2O) and a salt. Isn t this a special case of a double displacement reaction ... 

Acids and bases are essential substances in home, industry, and the environment. In aqueous solution, water combines with the proton released from an acid to form the hydrated species represented by HgO laq). In the Arrhenius definition, acids contain H and yield HaO in water, bases contain OH and yield OH in water, and an acid-base reaction (neutralization) is the reaction of and OH to form HgO. Acid strength depends on [HaO" ] relative to [HA] in aqueous solution. Strong acids dissociate completely and weak acids slightly. The extent of dissociation is expressed by the acid-dissociation constant, K. Weak acids have values ranging from about 10 to 10 . Many acids and bases can be classified qualitatively as strong or weak based on their formulas. 

Acid-base reactions neutralize one another, and the product of a balanced reaction will always render a salt product and water. 

By the Arrhenius definition, acids contain H and yield H3O+ in water, bases contain OH and yield OH in water, and an acid-base reaction (neutralization) is the reaction of H+ and OH to form HjO. 

Hydroxylation of the metal cation may be obtained through an acid-base reaction (neutralization, thermolysis, etc.) or through an oxidation-reduction reaction. The charge-pH diagram (Figure 1.6) shows that reduction and oxidation of cations as 0x0 and aquo species, respectively, allows them to reach the stability domain of hydroxo forms. Hydroxylation is the initiation stage of the process and the hydro-xylated complex is the precursor of the condensation products. 

The electric field-jump method is applicable to reactions of ions and dipoles. Application of a powerful electric field to a solution will favor the production of ions from a neutral species, and it will orient dipoles with the direction of the applied field. The method has been used to study metal ion complex formation, the binding of ions to macromolecules, and acid-base reactions. 

In acid-base reactions, the heat of neutralization of aqueous acids and bases can be sufficient to cause spitting from containers when the concentrated reagents interact. This is also encountered when concentrated sulphuric acid is diluted (refer to Table 6.1) the acid should always be added cautiously to water and not vice versa. Eye protection is obligatory when using such reagents. 

Indicators are chemical dyes that change color with a change of pH. Litmus paper and phenolphthalein are two common indicators used in acid-base reactions. They are chosen because they change color at or very near solution neutrality. Litmus paper is red in acidic solutions and blue in basic solutions. Phenolphthalein is colorless in acidic solutions and turns red in basic solutions. 

Water is always one product of a neutralization reaction. The other product is a salt. In the reaction of hydrochloric acid and sodium hydroxide, the salt is sodium chloride, which is, literally, table salt. Not all acid-base reactions make sodium chloride, but they do make a salt. Salts are ionic compounds. An ionic compound is a compound that is made up of cations (positively... 

The hydrogenation of simple alkenes using cationic rhodium precatalysts has been studied by Osborn and Schrock [46-48]. Although kinetic analyses were not performed, their collective studies suggest that both monohydride- and dihydride-based catalytic cycles operate, and may be partitioned by virtue of an acid-base reaction involving deprotonation of a cationic rhodium(III) dihydride to furnish a neutral rhodium(I) monohydride (Eq. 1). This aspect of the mechanism finds precedent in the stoichiometric deprotonation of cationic rhodium(III) dihydrides to furnish neutral rhodium(I) monohydrides (Eq. 2). The net transformation (H2 + M - X - M - H + HX) is equivalent to a formal heterolytic activation of elemental... 

A fundamental point in both molecular and surface chemistry concerns the involvement of donor atoms (from the ancillary ligand or the surface) in acid/base reactions.77 The reaction of H+ (from PyHCl) and Me+ (from MeOTf) with the anionic 1-metallacyclopropene 161 (Scheme 29) has been investigated. Although protonation gave back the starting material as the only product observed in solution ( h NMR), the reaction with MeOTf led to the neutral 1-metallacyclopropene, 162,... 

Acid-base reactions are called neutralization reactions because the reaction of an acid with a base generally produces a salt with little or no acid-base character and, in many cases, water. 

A neutral salt solution is produced by an acid-base reaction in two cases ... 

Scheme 2.19 depicts a typical example of the coupling of acid-base reactions, here protonations, with electron transfer. In a dry aprotic solvent [e.g., /V./V-dimethylformamide (DMF)], an aromatic hydrocarbon such as anthracene exhibits two successive reversible cyclic voltammetric waves (suspensions of neutral alumina may be used efficiently to dry the solvent... 

The Arrhenius theory explains acid-base reactions as a combination of H (aq) and OH (aq). It provides insight into the heat of neutralization for the reaction between a strong acid and a strong base. (Strong acids and bases dissociate completely into ions in solution.) For example, consider the following reaction. 

The acidic or basic property of an aqueous solution of a salt results from reactions between water and the dissociated ions of the salt. Some ions do not react with water. They are neutral in solution. Ions that do react with water produce a solution with an excess of HsO iaq) or OH (aq). The extent of the reaction determines the pH of the solution. As you will see, the reaction between an ion and water is really just another acid-base reaction. 

Changes in the pH of subsurface aqueous solutions may lead to an apparent increase or decrease in the solubility of organic contaminants. The pH effect depends on the structure of the contaminant. If the contaminant is sensitive to acid-base reactions, then pH is the governing factor in defining the aqueous solubility. The ionized form of a contaminant has a much higher solubility than the neutral form. However, the apparent solubility comprises both the ionized and the neutral forms, even though the intrinsic solubility of the neutral form is not affected. 

It is interesting to note that in the reaction of (188) with [TcOCU] , the 0x0 group is replaced by a doubly deprotonated amino group. The mixed imido-amido Tc complex [Tc(app)Cl2(Happ)j (263) was synthesized and structurally characterized with Re. Neutral, mixed amino-phosphino ligands can stabilize the soft [Tc=N] + core in the expected way, whereas the harder [Tc=0] + core imposes subsequent acid/base reactions. Stabilization is then mainly achieved by deprotonation, in order to compensate for the relatively high charge. ... 

Nitric acid undergoes both wet and dry deposition rapidly and can be neutralized by ammonia, the major gaseous base found in the atmosphere. As discussed in Section E.2, the neutralization reaction is an equilibrium reaction so that by itself, this does not result in permanent removal from the atmosphere. However, as seen in this chapter and in Chapter 9, this acid-base reaction has some important implications for visibility in the atmosphere and for the nitrate concentrations found in respirable particles. 

In a related application, polyelectrolyte microgels based on crosslinked cationic poly(allyl amine) and anionic polyfmethacrylic acid-co-epoxypropyl methacrylate) were studied by potentiometry, conductometry and turbidimetry [349]. In their neutralized (salt) form, the microgels fully complexed with linear polyelectrolytes (poly(acrylic acid), poly(acrylic acid-co-acrylamide), and polystyrene sulfonate)) as if the gels were themselves linear. However, if an acid/base reaction occurs between the linear polymers and the gels, it appears that only the surfaces of the gels form complexes. Previous work has addressed the fundamental characteristics of these complexes [350, 351] and has shown preferential complexation of cationic polyelectrolytes with crosslinked car-boxymethyl cellulose versus linear CMC [350], The departure from the 1 1 stoichiometry with the non-neutralized microgels may be due to the collapsed nature of these networks which prevents penetration of water soluble polyelectrolyte. 

Simply, an electron-deficient species that accepts an electron pair is called an acid, e.g. hydrochloric acid (HCl), and a species with electrons to donate is a base, e.g. sodium hydroxide (NaOH). A neutral species does not do either of these. Most organic reactions are either acid-base reactions or involve catalysis by an acid or base at some point. 

According to the Br0nsted-Lowry definitions, any species that contains hydrogen can potentially act as an acid, and any compound that contains a lone pair of electrons can act as a base. Therefore, neutral molecules can also act as bases if they contain an oxygen, nitrogen or sulphur atom. Both an acid and a base must be present in a proton transfer reaction, because an acid cannot donate a proton unless a base is present to accept it. Thus, proton-transfer reactions are often called acid-base reactions. 

The heat of neutralization is the amount of heat produced by neutralization (acid-base) reactions. 

All these electrolytes are neutral in Bronsted acid-base properties. Although rather exceptional, an acid, a base, or a pH buffer may be added to the supporting electrolyte of neutral salts. The acid-base system to be selected depends on the purpose of the measurement. We often use trifluoromethanesulfonic acid (CF3S03F1) as a strong acid acetic acid, benzoic acid, or phenol as a weak acid an amine or pyridine as a weak base and tetraalkylammonium hydroxide (ILtNOH) as a strong base. Examples of buffer systems are the mixtures of picric acid and its R4N-salt and amines and their PlCl04-salts. Here, we should note that the acid-base reactions in aprotic solvents considerably differ from those in water, as discussed in Chapter 3. 

Note that A is called the conjugate base of HA and BH+ the conjugate acid of B. Proton transfer reactions as described by Eq. 8-1 are usually very fast and reversible. It makes sense then that we treat such reactions as equilibrium processes, and that we are interested in the equilibrium distribution of the species involved in the reaction. In this chapter we confine our discussion to proton transfer reactions in aqueous solution, although in some cases, such reactions may also be important in nonaqueous media. Our major concern will be the speciation of an organic acid or base (neutral versus ionic species) in water under given conditions. Before we get to that, however, we have to recall some basic thermodynamic aspects that we need to describe acid-base reactions in aqueous solution. 

Any ionic solid, such as ammonium chloride, is called a salt. In a formal sense, a salt can be thought of as the product of an acid-base reaction. When an acid and base react, they are said to neutralize each other. Most salts containing cations and anions with a single positive and negative charge are strong electrolytes—they dissociate nearly completely into ions in dilute aqueous solution. Thus, ammonium chloride gives NH and Cl- in water ... 

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