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Acids and Bases

Explore the origins of acids and bases in ancient times, their properties, and how they interact. Learn about key theories including Arrhenius and Brønsted-Lowry, and the operational definitions of acids and bases. Understand how acids and bases behave, their pH levels, and the roles of conjugate acid-base pairs. Discover the versatile nature of amphiprotic substances and the Lewis Theory of acids and bases, proposed by Gilbert Lewis.

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Acids and Bases

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  1. Acids and Bases Properties and Theories

  2. Acids in Ancient Times • Latin “acidus” (sour) was the original name for vinegar • Hydrochloric acid (aka muriatic acid) was discovered around the year 800 by Islamic alchemistJabir Ibn Hayyan (Gaber). • Nitric acid was known to the alchemists as aqua fortis • Aqua regia, a mixture of nitric and hydrochloric acids, contributed to the effort of alchemists tried to find the philosopher's stone

  3. Bases in Ancient Times • Milk of magnesia—magnesium hydroxide • Used as antacid, laxative • Lye—sodium hydroxide • used in tanning, cloth production, and soapmaking • Potash—crude mixture of potassium carbonate and potassium hydroxide • Used as fertilizer • Limewater—solution of calcium hydroxide • or calcium carbonate • Used as a test for CO2, in calamine lotion, and as an antacid

  4. How do acids behave? • Acids turn blue litmus paper red (but have no effect on red litmus paper) • Sour taste

  5. Properties of Acids • Acids react with certain metals to release hydrogen gas • Mg + HCl  MgCl2 + H2 • Form electrolytes in aqueous solutions • Ionize in solution • support electric currents in water

  6. How do bases behave? • Bases turn red litmus paper blue (but have no effect on blue litmus paper) • Turns phenolphthalein pink • Bitter taste and • slippery feel • Electrolytes in solution

  7. When acids and bases mix • Neutralization reaction occurs • Acid + base  water + salt • First studied quantitatively by German chemist Jeremias Richter

  8. pH scale • Proposed by Danish scientist Søren Sørenson in 1909 • Acids have pH < 7 • Bases have pH > 7 • Neutral substances have pH = 7

  9. Acids pH < 7 React with metals to release H2(g) Sour taste Turn litmus paper red Bases pH > 7 Turn phenolphthalein pink Turns litmus paper blue Bitter taste Slippery feel Operational Definitions

  10. 1st Acid-Base Theory • Proposed by Swedish chemist Svante Arrhenius in 1887 • 1903 Nobel Prize in • Chemistry • Studied dissociation of • substances in solution

  11. Arrhenius Theory • Acids release H+ ions in aqueous solution • Examples of Arrhenius acids • HNO3, HCl, : monoprotic acids • H2SO4 : diprotic acid • H3PO4: triprotic acid • Bases produce OH- ions in aqueous solution • Examples of Arrhenius bases • NaOH, KOH, Mg(OH)2

  12. What about ammonia? • NH3 • Turns litmus paper blue • Neutralizes acid • Behaves as a base • Arrhenius theory doesn’t explain it well

  13. Brønsted-Lowry Theory • Both independently proposed this theory in 1923 • Refined definition of bases • Acid definition very similar to Arrhenius’ • All Arrhenius acids are also B-L acids Johannes Brønsted—Danish chemist Thomas Lowry—English chemist

  14. Brønsted-Lowry Theory • Acids are H+ ion donors or “proton donors” • Bases are H+ acceptors or “proton acceptors”

  15. Conjugate Acid-Base Pairs • Acids and bases occur in conjunction • Conjugate acid-base pairs • Differ only by a proton

  16. Amphiprotic Substances • Some substances can act as an acid and as a base • Also called “amphoteric”

  17. Gilbert Lewis • Massachusetts native and MIT chemistry professor • Lewis structures • Lewis acid-base theory an extension of his theory of electron pairs

  18. Lewis Theory • Proposed same year as B-L theory • Lewis bases: a substance that can donate a pair of electrons to form a covalent bond • Lewis acids: a substance that can accept a pair of electrons to form a covalent bond

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