salt
(noun)
An ionic compound composed of cations and anions that are held together by electrostatic attraction.
(noun)
in acid-base chemistry, one of the products in a neutralization reaction
Examples of salt in the following topics:
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Salts that Produce Basic Solutions
- When dissolved in water, a basic salt yields a solution with pH greater than 7.0.
- There are several varieties of salts, and in this section we will consider basic salts.
- What makes a basic salt basic?
- It is due to the fact that the anion in the salt is the conjugate base of a weak acid.
- An example of a basic salt is sodium bicarbonate, NaHCO3.
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Salts that Produce Acidic Solutions
- When dissolved in water, acidic salts form solutions with pH less than 7.0.
- Salts with acidic protons in the cation are most commonly ammonium salts, or organic compounds that contain a protonated amine group.
- Acid salts can also contain an acidic proton in the anion.
- From the previous concept, we know that salts containing the bicarbonate ion (HCO3-) are basic, whereas salts containing bisulfate ion (HSO4-) are acidic.
- Anilinium chloride is an example of an acid salt.
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Overview of the Acid-Base Properties of Salt
- Acid salts are the converse of basic salts; they are formed in the neutralization reaction between a strong acid and a weak base.
- The conjugate acid of the weak base makes the salt acidic.
- The ammonium ion contains a hydrolyzable proton, which makes it an acid salt.
- A good example of such a salt is ammonium bicarbonate, NH4HCO3; like all ammonium salts, it is highly soluble, and its dissociation reaction in water is as follows:
- This video examines the hydrolysis of an acid salt, a basic salt, and a salt in which both ions hydrolyze.
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Salt Formation
- Because of their enhanced acidity, carboxylic acids react with bases to form ionic salts, as shown in the following equations.
- In the case of alkali metal hydroxides and simple amines (or ammonia) the resulting salts have pronounced ionic character and are usually soluble in water.
- Heavy metals such as silver, mercury and lead form salts having more covalent character (3rd example), and the water solubility is reduced, especially for acids composed of four or more carbon atoms.
- Carboxylic acids and salts having alkyl chains longer than six carbons exhibit unusual behavior in water due to the presence of both hydrophilic (CO2) and hydrophobic (alkyl) regions in the same molecule.
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Precipitation Reactions
- Precipitation reactions transform ions into an insoluble salt in aqueous solution.
- Precipitation refers to a chemical reaction that occurs in aqueous solution when two ions bond together to form an insoluble salt, which is known as the precipitate.
- A precipitation reaction can occur when two solutions containing different salts are mixed, and a cation/anion pair in the resulting combined solution forms an insoluble salt; this salt then precipitates out of solution.
- For instance, if silver nitrate is added to a solution of an unknown salt and a precipitate is observed, the unknown solution might contain chloride (Cl-).
- To determine the solubility of an given salt, find the cationic component along the left-hand side, match it to the anionic component along the top, then check to see if it is S - soluble, I - insoluble, or sS - slightly soluble.
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Nucleophilicity of Sulfur Compounds
- Sulfides, for example, react with alkyl halides to give ternary sulfonium salts (equation # 1) in the same manner that 3ยบ-amines are alkylated to quaternary ammonium salts.
- Although equivalent oxonium salts of ethers are known, they are only prepared under extreme conditions, and are exceptionally reactive.
- Remarkably, sulfoxides (equation # 2), sulfinate salts (# 3) and sulfite anion (# 4) also alkylate on sulfur, despite the partial negative formal charge on oxygen and partial positive charge on sulfur.
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Nucleophilicity of Phosphorus Compounds
- The nucleophilicity of trivalent phosphorus results in rapid formation of phosphonium salts when such compounds are treated with reactive alkyl halides.
- For example, although resonance delocalization of the nitrogen electron pair in triphenylamine, (C6H5)3N, renders it relatively unreactive in SN2 reactions, the corresponding phosphorus compound, triphenylphosphine, undergoes a rapid and exothermic reaction to give a phosphonium salt, as shown below in the first equation.
- Phosphite esters react in the same manner, but the resulting phosphonium salts (shaded box) are often unstable, and on heating yield dialkyl phosphonate esters by way of a second SN2 reaction (equation 2 below).
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Solubility
- The solubility chart shows the solubility of many salts.
- Salts of alkali metals (and ammonium), as well as those of nitrate and acetate, are always soluble.
- Carbonates, hydroxides, sulfates, phosphates, and heavy metal salts are often insoluble.
- The solubilities of salts formed from cations on the left and anions on the top are designated as: soluble (S), insoluble (I), or slightly soluble (sS).
- Recognize the various ions that cause a salt to generally be soluble/insoluble in water.
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Important Reagent Bases
- Most base reagents are alkoxide salts, amines or amide salts.
- Since alcohols are much stronger acids than amines, their conjugate bases are weaker than amide bases, and fill the gap in base strength between amines and amide salts.
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Reactions of Aryl Diazonium Salts
- Aryl diazonium salts are important intermediates.
- Those substitution reactions that are catalyzed by cuprous salts are known as Sandmeyer reactions.
- Stable diazonium tetrafluoroborate salts may be isolated, and on heating these lose nitrogen to give an arylfluoride product.
- (iii) The activating character of an amine substituent may be attenuated by formation of an amide derivative (reversible), or even changed to deactivating and meta-directing by formation of a quaternary-ammonium salt (irreversible).
- Aryl diazonium salts may be reduced to the corresponding hydrazines by mild reducing agents such as sodium bisulfite, stannous chloride or zinc dust.