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Transcript
Acids are substances that ionize in aqueous solutions to form hydrogen ions, thereby increasing the concentration of H+ ions. Because hydrogen atom consists of a proton and an electron, H+ is simply a proton. Thus, acids are often called proton donors. Molecules of different acids can ionize to form different numbers of H+ ions. Both hydrochloric acid and nitric acid are monoprotic acids, which yield one per molecule of acid. Sulfuric acid is a diprotic acid, one that yields two H+ per molecule of acid. Bases are substances that accept H+ ions. Bases produce hydroxide ions when they dissolve in water. When dissolved in water, they dissociate into their component ions, introducing OH- ions into the solution. ACID BASE Sour taste Bitter taste Neutralizes bases Neutralizes acids Turns litmus paper blue to red Turns litmus paper red to blue Soapy and slippery feeling Indicator Color in strongly acidic solution pH at which color changes Color in strongly alkaline solution Methyl orange Red 4 Yellow Litmus Red 7 Blue Phenolphthalein Colorless 9 Red Screened Methyl Orange Red 4 Green Acids and bases that are strong electrolytes (completely ionized in solution) are called strong acids and strong bases. Those that are weak electrolytes (partly ionized) are called weak acids and weak bases. STRONG ACIDS STRONG BASES Hydrochloric acid Lithium hydroxide Hydrobromic acid Sodium hydroxide Hydroiodic acid Potassium hydroxide Chloric acid Rubidium hydroxide Perchloric acid Cesium hydroxide Nitric acid Calcium hydroxide Sulfuric acid Strontium hydroxide Barium hydroxide To classify a soluble substance as a strong electrolyte, weak electrolyte, or nonelectrolyte, we simply use the following table: Strong Electrolyte Weak Electrolyte Nonelectrolyte Ionic All None None Molecular Strong acids Weak acids Weak bases All other compounds If an acid is not listed, it is probably a weak electrolyte. NH3 is only a weak base that we consider. When a solution of an acid and that of a base are mixed, a neutralization reaction occurs. The products of the reaction have none of the characteristics properties of either the acidic and the basic solutions. By analogy to this reaction, the term salt has come to mean any ionic compound whose cation comes from a base and whose anion comes from an acid. A neutralization reaction between an acid and a metal hydroxide produces water and salt. 1. Arrhenius Acids and Bases 2. Bronsted-Lowry Acids and Bases 3. Lewis Acids and Bases Swedish chemist Svante Arrhenius (1859-1927) proposed a revolutionary way of defining and thinking about acids and bases. He said that acids are hydrogen-containing compounds that ionize to yield hydrogen ions (H+) in aqueous solutions. He also said that bases are compounds that ionize to yield hydroxide ions (OH-) in aqueous solutions. Acids that contain one ionizable hydrogen, such as nitric acid, are called monoprotic acids. Acids that contain two ionizable hydrogens, such as sulfuric acid, are called diprotic acids. Acids that contain three ionizable hydrogens, such as phosphoric acid, are called triprotic acids. In 1923, the Danish chemist Johannes Bronsted and the English chemist Thomas Lowry independently proposed a new definition. Defines an acid as a hydrogen-ion donor. Defines a base as a hydrogen-ion acceptor. A conjugate base is the particle that remains when an acid has donated a hydrogen ion. A conjugate base is the particle that remains when an acid has donated a hydrogen ion. Conjugate acids and bases are always paired with a base or an acid, respectively. A conjugate acid-base pair consists of two substances related by the loss or gain of a single hydrogen ion. The third theory of acids and bases was proposed by Gilbert Lewis. Lewis focused on the donation or acceptance of a pair of electrons during a reaction. This concept is more general than either the Arrhenius theory or the Bronsted-Lowry theory. A Lewis acid is a substance that can accept a pair of electrons to form a covalent bond. A Lewis base is a substance that can donate a pair of electrons to form a covalent bond. A hydrogen ion (Bronsted-Lowry acid) can accept apair of electrons in forming a bond. A hydrogen ion, therefore, is also a Lewis acid. A Bronsted-Lowry base, or a substance that accepts a hydrogen ion, must have a pair of electrons available and is also a Lewis base. A widely used system for expressing [H+] is the pH scale, proposed in 1909 by the Danish scientist Soren Sorenson. It ranges from 0-14, neutral solutions have a pH of 7. A pH of 10 is strongly basic. The pH of a solution is the negative logarithm of the hydrogen-ion concentration. The pH may be represented mathematically using the following equation: pH = - log [H+] Similarly, the pOH of a solution equals the negative logarithm of the hydroxide-ion concentration. pOH = - log [OH-] A neutral solution has a pOH of 7. A solution with a pOH less than 7 is basic. A solution with a pOH greater than 7 is acidic. A simple relationship between pH and pOH makes it easy to find either one when the other is known. pH + pOH = 14 pH = 14 – pOH pOH = 14 - pH 2 3 4 5 6 Increasing Acidity 7 NEUTRAL 1 8 9 10 11 12 Increasing Basicity 10-1 10-2 10-3 10-4 10-5 10-6 10-7 10-8 10-9 10-10 10-1110-12 10-1310-14 13 14 What is the pH of a solution with a hydrogen-ion concentration of 1.0 x 10-10 M? Find the pH of each solution: ◦ [H+] = 1.0 x 10 -4 M ◦ [H+] = 0.0010 M ◦ [H+] = 1.0 x 10 -9 M ◦ [H+] = 1.0 x 10 -12 M ◦ [H+] = 0.010 M