Acid: Difference between revisions
imported>Johan Förberg (added paragraph on weak/strong acids) |
imported>Henry A. Padleckas (acids are opposite of bases, qualified statement on pH of acid, mentioned acidic substances, qualified comparison of strong and weak acids) |
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'''Acids''' are generally defined as those chemical substances which release [[hydrogen]] [[ions]] on dissolving in water. Although acids are most often thought of as proton (H<sup>+</sup>) or [[hydronium ion]] ([H<sub>3</sub>O]<sup>+</sup>) donors, as defined by the [[Brønstad-Lowry]] theory, the [[Lewis acid-base theory]] is more comprehensive and defines acids as electron pair acceptors. | '''Acids''' are generally defined as those chemical substances which release [[hydrogen]] [[ions]] on dissolving in water. Although acids are most often thought of as proton (H<sup>+</sup>) or [[hydronium ion]] ([H<sub>3</sub>O]<sup>+</sup>) donors, as defined by the [[Brønstad-Lowry]] theory, the [[Lewis acid-base theory]] is more comprehensive and defines acids as electron pair acceptors. As such, acids are the opposite of [[base (chemistry)|bases]] in chemistry. Acids dissolved in otherwise pure water have a [[pH]] less than 7 at 25°C. Compounds, substances, or materials which overall have the nature of an acid can be called '''acidic'''; for example, aqueous solutions (solutions of water) having a pH less than 7 can be considered acidic. | ||
Acids play an integral role in the metabolic functions of animals, in chemistry, material science and the food industry. Acids, such as the twenty common [[amino acid]]s and [[carboxylic acid|carboxylic acids]] (including [[valeric acid]], and [[propionic acid]]), are necessary for life. | Acids play an integral role in the metabolic functions of animals, in chemistry, material science, and the food industry. Acids, such as the twenty common [[amino acid]]s and [[carboxylic acid|carboxylic acids]] (including [[valeric acid]], and [[propionic acid]]), are necessary for life. | ||
A distinction is made between strong and weak acids. Strong acids, e.g. hydrochloric acid (HCl), | A distinction is made between strong and weak acids. Strong acids, e.g. hydrochloric acid (HCl), nitric acid (HNO<sub>3</sub>), are completely dissociated in a solution. This means for solutions of equivalent concentration, there will be a higher concentration of H<sup>+</sup> ions for a strong acid than a weak acid. Weak acids, e.g. ethanoic acid or [[acetic acid]] (CH<sub>3</sub>COOH), do not dissociate completely but release only a small fraction of their hydrogens as H<sup>+</sup> ions. There are also intermediate strength acids. | ||
==[[Bronstad-Lowry acid-base theory|Brønstad-Lowry acids]]== | ==[[Bronstad-Lowry acid-base theory|Brønstad-Lowry acids]]== | ||
According to the theory proposed by Brønstad and Lowry, an acid is defined as a species which donates a proton (H<sup>+</sup> ion) or a [[hydronium]] ([H<sub>3</sub>O]<sup>+</sup>) ion. For example, [[hydrochloric acid]] (HCl) donates a H<sup>+</sup> ion. Therefore it is an acid. [[Sodium hydroxide]] (NaOH), by contrast, donates a OH<sup>-</sup> ion, and so, it is not considered an acid.<br />There is a drawback in this theory: acids which do not release protons or hydronium ions are not considered acids. For example, in the following reaction, [[boron trifluoride]] does not donate a proton or hydronium ion, so it is not a Bronstad-Lowry acid.<br /><br /><math>BF_3 \; + \; :NH_3 \; \longrightarrow \; F_3B:NH_3</math><br /><br />Yet by nature BF<sub>3</sub> is an acid.<br />Consequently, this definition of acids has been discarded in favor of Lewis' definition. | According to the theory proposed by Brønstad and Lowry, an acid is defined as a species which donates a proton (H<sup>+</sup> ion) or a [[hydronium]] ([H<sub>3</sub>O]<sup>+</sup>) ion. For example, [[hydrochloric acid]] (HCl) donates a H<sup>+</sup> ion. Therefore it is an acid. [[Sodium hydroxide]] (NaOH), by contrast, donates a OH<sup>-</sup> ion, and so, it is not considered an acid.<br />There is a drawback in this theory: acids which do not release protons or hydronium ions are not considered acids. For example, in the following reaction, [[boron trifluoride]] does not donate a proton or hydronium ion, so it is not a Bronstad-Lowry acid.<br /><br /><math>BF_3 \; + \; :NH_3 \; \longrightarrow \; F_3B:NH_3</math><br /><br />Yet by nature BF<sub>3</sub> is an acid.<br />Consequently, this definition of acids has been discarded in favor of Lewis' definition. | ||
Revision as of 02:56, 28 February 2011
Acids are generally defined as those chemical substances which release hydrogen ions on dissolving in water. Although acids are most often thought of as proton (H+) or hydronium ion ([H3O]+) donors, as defined by the Brønstad-Lowry theory, the Lewis acid-base theory is more comprehensive and defines acids as electron pair acceptors. As such, acids are the opposite of bases in chemistry. Acids dissolved in otherwise pure water have a pH less than 7 at 25°C. Compounds, substances, or materials which overall have the nature of an acid can be called acidic; for example, aqueous solutions (solutions of water) having a pH less than 7 can be considered acidic.
Acids play an integral role in the metabolic functions of animals, in chemistry, material science, and the food industry. Acids, such as the twenty common amino acids and carboxylic acids (including valeric acid, and propionic acid), are necessary for life.
A distinction is made between strong and weak acids. Strong acids, e.g. hydrochloric acid (HCl), nitric acid (HNO3), are completely dissociated in a solution. This means for solutions of equivalent concentration, there will be a higher concentration of H+ ions for a strong acid than a weak acid. Weak acids, e.g. ethanoic acid or acetic acid (CH3COOH), do not dissociate completely but release only a small fraction of their hydrogens as H+ ions. There are also intermediate strength acids.
Brønstad-Lowry acids
According to the theory proposed by Brønstad and Lowry, an acid is defined as a species which donates a proton (H+ ion) or a hydronium ([H3O]+) ion. For example, hydrochloric acid (HCl) donates a H+ ion. Therefore it is an acid. Sodium hydroxide (NaOH), by contrast, donates a OH- ion, and so, it is not considered an acid.
There is a drawback in this theory: acids which do not release protons or hydronium ions are not considered acids. For example, in the following reaction, boron trifluoride does not donate a proton or hydronium ion, so it is not a Bronstad-Lowry acid.
Yet by nature BF3 is an acid.
Consequently, this definition of acids has been discarded in favor of Lewis' definition.
Lewis acids
For a discussion of Lewis acids and Lewis bases, see Lewis acid-base theory.