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The Chemistry of Acids and Bases

The Chemistry of Acids and Bases. Acids. Acids. Bases. Some Properties of Acids. Produce H + Taste sour Corrode metals Electrolytes React with bases to form a salt and water pH is less than 7 Turns blue litmus paper to red “Blue to Red A-CID”

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The Chemistry of Acids and Bases

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  1. The Chemistry of Acids and Bases

  2. Acids

  3. Acids

  4. Bases

  5. Some Properties of Acids • Produce H+ • Taste sour • Corrode metals • Electrolytes • React with bases to form a salt and water • pH is less than 7 • Turns blue litmus paper to red “Blue to Red A-CID” • React with certain metals to produce hydrogen gas. • React with carbonates and bicarbonates to produce carbon dioxide gas

  6. Some Common Acids Monoprotic acids: one acidic (ionizable)hydrogen HNO3 - nitric acid HCl - hydrochloric acid Diprotic acids: two acidic (ionizable)hydrogens H2SO4 - sulfuric acid Triprotic acids: three acidic (ionizable)hydrogens H3(PO4)phosphoric acid H3(C6H5O7) citric acid

  7. Some Properties of Bases • Produce OH- ions in water • Taste bitter, chalky • Are electrolytes • Feel soapy, slippery • React with acids to form salts and water • pH greater than 7 • Turns red litmus paper to blue “Basic Blue”

  8. Some Common Bases NaOH sodium hydroxide lye KOH potassium hydroxide liquid soap Ba(OH)2 barium hydroxide stabilizer for plastics Mg(OH)2 magnesium hydroxide Milk of magnesia Al(OH)3 aluminum hydroxide Maalox (antacid)

  9. Acid/Base definitions Definition #1: Arrhenius (traditional) Acids – produce H+ ions (H3O+)HX  H+(aq) + X-(aq) Bases– produce OH- ions XOH -----> X+(aq) + OH-(aq) (problem: some bases don’t have hydroxide ions!)

  10. Arrhenius acid is a substance that produces H+ (H3O+) in water Arrhenius base is a substance that produces OH- in water But: some bases don’t have hydroxide ions! Now what?????

  11. Acid/Base Definitions Definition #2: Brønsted – Lowry Acids – proton donor Bases – proton acceptor

  12. A Brønsted-Lowry acid is a proton donor A Brønsted-Lowry base is a proton acceptor conjugateacid conjugatebase acid base H+ + Base = Conjugate acid of Base+Acid = H+ + Conjugate base of Acid-

  13. Conjugate Pairs

  14. Water can be an acid or a baseAmphoteric

  15. Learning Check! Label the acid, base, conjugate acid, and conjugate base in each reaction: HCl + OH-Cl- + H2O H2O + H2SO4   HSO4- + H3O+

  16. Acids & Base Definitions Definition #3 – Lewis Acids and Bases Lewis acid – a substance that accepts an electron pair Lewis base – a substance that donates an electron pair

  17. Lewis Acids & Bases Formation of hydronium ion is also an excellent example. Electron pair of the new O-H bond originates on the Lewis base

  18. Lewis Acid Base Reaction

  19. Water Water molecules collide with one another: self-ionization reaction 2H2O H3O+ + OH- The reaction does not form very much H3O+ or OH-. 1 Liter water = ~55 moles of water molecules, but only forms 1.0 x 10-7 moles of H3O+ and OH- [H3O+] and [OH-] in pure water = 1.0 x 10-7 M. It is the 7 in the exponent or power of this number that gives neutral water a pH of 7.

  20. Ionization constant of water Why isn’t water in the equilibrium constant Kw ? Neutral solution: [H3O+] = [OH-] = [10-7] pH = - log [H+] pOH = - log [OH-] Neutral solution: pH = pOH= 7 Acidic Solutions: [H+] > [OH-] Basic Solutions (Alkaline): [OH-] > [H+] pH + pOH must always add up to 14

  21. pH Scale: indicates strength of acid or base Example: If [H+] = 1 X 10-10pH = - log 1 X 10-10 pH = - (- 10) pH = 10 basic pOH = 4 Example: If [H+] = 1.8 X 10-5pH = - log 1.8 X 10-5 pH = - (- 4.74) pH = 4.74 acidic pOH = 9.26

  22. Try These! Find the pH of these: 1) A 0.15 M solution of Hydrochloric acid 2) A 3.00 X 10-7 M solution of Nitric acid 3) A 0.15 M solution of sodium hydroxide

  23. pH testing • There are several ways to test pH • Blue litmus paper (red = acid) • Red litmus paper (blue = basic) • pH paper (multi-colored) • pH meter (7 is neutral, <7 acid, >7 base) • Universal indicator (multi-colored) • Indicators like phenolphthalein • Natural indicators like red cabbage, radishes

  24. pH indicators • Indicators are dyes that can be added that will change color in the presence of an acid or base. • Some indicators only work in a specific range of pH

  25. Titration Buret with standardized solution End point – Visual Equivalence Point: Stoichiometric The point at which neutralization is achieved (acid = base) Unknown solution Past End point End point

  26. Titration Curves

  27. Strong Acids vs Weak Acids • An acid that nearly completely dissociates • All molecules of the acid break up to form the ions soluble in water • An acid that only slightly dissociates in a water solution • Only a small percent of acid molecules donate their hydrogen, and most remain the same.

  28. A strong acid essentially ionizes 100%. HCl(g) + H2O(l) H3O+(aq) + Cl-(aq) 0.10 mol 0.10 mol 0.10 mol After ionization: a few molecules;but many ions Weak acids have much lower percent ionization. CH3COOH(l) + H2O(l) H3O+(aq) + CH3COO-(aq)0.10 mol << 0.10 mol << 0.10 molmany molecules few ions Only about 5% ionization at 25C

  29. Strong Acids and Bases Strong Acids Strong Bases HClO4perchloric acid LiOHlithium hydroxide HCl hydrochloric acid NaOHsodium hydroxide HNO3 nitric acid KOH potassium hydroxide H2SO4 sulfuric acid RbOHrubidium hydroxide HBrhydrobromic acid CsOHcesium hydroxide HI hydriodic acid Ca(OH)2calcium hydroxide HClO3Chloric acid*Sr(OH)2strontium hydroxideBa(OH)2barium hydroxide Not included in every list

  30. Acids: Concentration vs. Strength CONCENTRATED DILUTE H+ A- H+ A- H+ A- H+ A-HA A- H+ A- H+ A- H+ A- H+ A - H+ A- HA H+ A- H+ A- H+ A- A- H+ A- H+ A- H+ A- H+ A- H+ H+ A - H + A - H + A -HA H + A - A- H+ A- H+ A- H+ A- H+ A– H+ A- H+ A- H+ A- H+ A- H+ A- H+ A- H+ A- H+ A- H+ A- HA A- H+ A- H+ A- H+ A- H+ H+ A - H+ A - HA A - H+ A - H+ A – H+ A - H+ A - H+ A - HA H+ A - H+ A - H+ A - H+ STRONG HA HAH+ A- HA HAHA HA HAHAHAHAH+ A- H+ A- HA HAHAHAHA HA HAH+ A- HA HAHA HA HAHAH+A- HA HA H+ A- HA HAHAHAHA HA HAHAH+A- HA HA H+ A- HA HAHAHAHA HA HAH+ A- HA HAHA HA HAHAHA HA HAHA H+ A - HA HA HA HAH + A – HA H + A – HA HA WEAK

  31. Acid Dissociation Constant

  32. Bases • All information and equations reference bases also.

  33. Strengths Of Conjugate Acid-Base Pairs • The stronger an acid, the weaker is its conjugate base. • The stronger a base, the weaker is its conjugate acid. • An acid-base reaction is favored in the direction from the stronger member to the weaker member of each conjugate acid-base pair.

  34. For biological systems: • Ionization of a strong acid is TOO BIG! • Ionization of water itself is way TOO LITTLE! • Ionization of a weak acid is JUST RIGHT!

  35. Weak acids, their conjugate bases, and buffers… • Weak acids have only a modest tendency to shed their protons (definition of an acid). • When they do, the corresponding negatively charged anion becomes a willing proton acceptor, and is called the conjugate base. • The properties of a buffer rely on a balance between a weak acid and its conjugate base. • And a titration curve looks like this…

  36. Buffers • A bufferis a solution of a weak acid and its conjugate base that resists changes in pH in both directions—either up or down. • A buffer works best in the middle of its range, where the amount of undissociated acid is about equal to the amount of the conjugate base. • One can soak up excess protons (acid), the other can soak up excess hydroxide (base).

  37. pH 7 _ _ _ _ _ _ _ 0 equiv. of NaOH 1.0 added pKa = 4.76 Titration of acetic acid with sodium hydroxide Buffering range: only small pH changes result from addition of base or acid 50% dissociation

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