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Acids & Bases

Acids & Bases. Chapter 19 Chemistry. Section 1 - Acid-Base Theories. Introduction. Bracken Cave, near San Antonio, Texas, is home to twenty to forty million bats.

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Acids & Bases

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  1. Acids & Bases Chapter 19 Chemistry

  2. Section 1 - Acid-Base Theories Introduction • Bracken Cave, near San Antonio, Texas, is home to twenty to forty million bats. • Visitors to the cave must protect themselves from the dangerous levels of ammonia in the cave. Ammonia is a byproduct of the bats’ urine. • You will learn why ammonia is considered a base.

  3. 1. Properties of Acids and Bases • Acids • Give foods a tart or sour taste • Cause certain chemicals, called indicators, to change color (ex. Turns litmus paper red) • Can be strong or weak electrolytes in aqueous solution. • A strong electrolyte is a solution in which a large portion of the solute exists as ions • A weak electrolyte isa solution that conducts electricity poorly because only a fraction of the solute exists as ions. • Reacts with many metals to in aqueous solution to produce hydrogen gas. • Reacts with compound containing hydroxide ions to form water and a salt. Citrus fruits contain citric acid. Tea contains tannic acid.

  4. Properties of Acids and Bases (cont.) • Bases • Taste bitter, but tasting most bases is hazardous. • With a few exceptions, none of the foods you eat are bases. • Feels slippery • Ex. Soap • Will change the color of an acid-base indicator • Turns litmus paper blue • Can be strong or weak electrolytes in aqueous solution. Antacids use bases to neutralize excess stomach acid. The base calcium hydroxide is a component of mortar.

  5. 2. Arrhenius Acids and Bases • Swedish chemist Svante Arrhenius (1859-1927) proposed that acids are hydrogen-containing compounds that ionize to yield hydrogen ions (H+) in aqueous solution. • He also said that bases are compounds that ionize to yield hydroxide ions (OH–) in aqueous solution.

  6. Arrhenius Acids and Bases (cont.) • Arrhenius Acids • Acids that contain one ionizable hydrogen, such as nitric acid (HNO3), are called monoprotic acids. • Acids that contain two ionizable hydrogens, such as sulfuric acid (H2SO4), are called diprotic acids. • Acids that contain three ionizable hydrogens, such as phosphoric acid (H3PO4) are called triprotic acids.

  7. Arrhenius Acids and Bases (cont.) • Not all compounds that contain hydrogen are acids • Not all hydrogens may be released as hydrogen ions. • Only hydrogen ions in very polar bonds are ionizable • In such bonds, hydrogen is joined to a very electronegative element. • When a compound that contains such bonds dissolves in water, it releases hydrogen ions because the hydrogen ions are stabilized by solvation. • Solvation is the process that occurs when an ionic solute dissolves. • Solvent molecules surround the positive and negative ions. Hydrochloric Acid is a polar covalent molecule. It ionizes to form an aqueous solution of hydronium ions and chloride ions.

  8. Arrhenius Acids and Bases • Arrhenius Bases • Bases that contain alkali metals are very soluble in water and dissociate into ions. • Hydroxide ions are one of the products of the dissolution of an alkali metal in water. • Bases that contain alkaline earth metals are not very soluble in water. • Their solutions are always very dilute, even when saturated.

  9. 3. Brønsted-Lowry Acids and Bases • The Arrhenius definition of acids and bases is not a very comprehensive one. • It defines acids and bases rather narrowly and does not include certain substances that have acidic or basic properties. • For example, aqueous solutions of sodium carbonate (Na2CO3 (aq)) and ammonia (NH3 (aq)) are basic. • Neither is a hydroxide, and neither would be classified as a base under the Arrhenius definition • In 1923, the Danish chemist Johannes Brønsted (1879-1947) and the English chemist Thomas Lowry (1874-1936) proposed that an acid is a hydrogen-ion donor, and a base is a hydrogen-ion acceptor

  10. Brønsted-Lowry Acids and Bases (cont.) • Conjugate Acids and Bases • Because all gases become less soluble in water as temperature increases, increasing the temperature of an aqueous solution releases gas. • For example, a solution of ammonia gas dissolved in water releases ammonia gas as the temperature increases, which shifts the equilibrium to the left. • A conjugate acid is the particle formed when a base gains a hydrogen ion. • A conjugate baseis the particle that remains when an acid has donated a hydrogen ion.

  11. Brønsted-Lowry Acids and Bases (cont.) • A conjugate acid-base pairconsists of two substances related by the loss or gain of a single hydrogen ion. • A water molecule that gains a hydrogen ion becomes a positively charged hydronium ion (H3O+). • A substance, such as water, that can act as both an acid and a base is said to be amphoteric.

  12. 4. Lewis Acids and Bases • Gilbert Lewis (1875-1946) proposed that an acid accepts a pair of electrons during a reaction, while a base donates a pair of electrons. • This concept is more general than either the Arrhenius or Brønsted-Lowry theory. • A Lewis acidis a substance that can accept a pair of electrons to form a covalent bond. • A Lewis baseis a substance that can donate a pair of electrons to form a covalent bond.

  13. END OF SECTION 1

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