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Acids and Bases

Acids and Bases. Properties of Acids and Bases. Acids taste sour Bases feel slippery and taste bitter Acids react with metals to form H 2 gas and a salt ( Mg + 2HCl  MgCl 2 + H 2 ) Metal carbonates react with acids to produce CO 2 (limestone (CaCO 3 ) +HCl)

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Acids and Bases

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  1. Acids and Bases

  2. Properties of Acids and Bases • Acids taste sour • Bases feel slippery and taste bitter • Acids react with metals to form H2 gas and a salt ( Mg + 2HCl  MgCl2 + H2) • Metal carbonates react with acids to produce CO2 (limestone (CaCO3) +HCl) • Bases turn litmus paper blue, acids turn litmus paper red

  3. Ions in Solution • All water solutions contain hydronium(H3O+ or H+) and hydroxide (OH— ) ions • The relative amounts of the two ions determines whether the solution is basic, acidic, or neutral • Acidic solutions H3O+ > OH— • Basic solutions H3O+ < OH— • Neutral solutions H3O+ = OH—

  4. Arrhenius Model of Acids and Bases • Acid is a substance that contains hydrogen and ionizes to produce or hydrogen ions in aqueous solutions • A base is a substance that contains a hydroxide group and ionizes to produce a hydroxide ion in aqueous solution • HCl(g)  H+(aq) + Cl— (aq) • NaOH(s)  Na+ (aq) + OH— (aq) • Some drawbacks exist with this model. NH3 does not have hydroxide but produces OH— in solution • A better model is needed • This leads us to Bronsted-Lowry Model

  5. Bronsted-Lowry Model of Acids and Bases • An acid is a hydrogen ion (proton) donor and a base is a hydrogen ion (proton) acceptor • In this example X and Y represent nonmetals or negative polyatomic ions • HX(aq) + H2O H3O+ (aq) + X— (aq)

  6. Conjugates • In Bronsted-Lowry model acids and bases can be labeled as conjugate pairs • HX(aq) + H2O H3O+ (aq) + X— (aq) • Conjugate base – results when an acid donates its proton • Conjugate acid- results when base accepts the proton • Conjugate pairs = HX, X— and H2O, H3O+ acid base Conjugate acid Conjugate base

  7. Conjugate pairs • HF + H2O H3O+ + F— • Conjugate pairs= HF, F— and H2O, H3O+ • What about ammonia? Does Bronsted Lowry define why ammonia (NH3) is a base? base Conjugate base Conjugate acid acid

  8. Ammonia • NH3(aq) + H2O NH4+ (aq) + OH— (aq) • We can see from the examples that substances classified as acids and bases by Arrhenius model are classified as acids and base by Bronsted Lowry model • We also can see some substances NOT classified as bases by Arrhenius model ARE classified as bases by Bronsted Lowry model (NH3 for example) • Water is amphoteric. It can sometimes act as a base and sometimes as an acid base acid Conjugate acid Conjugate base

  9. Monoprotic and Polyprotic Acids • Some acids (HF, HCl, HNO3 for example) have only one hydrogen to donate • These acids are called monoprotic acids • Diprotic acids have 2 hydrogens to donate (H2SO4 for example) • Triprotic acids have 3 hydrogens to donate (H3PO4 for example) • Any acid with 2 or hydrogens to donate can be called polyprotic

  10. Anhydrides • Oxides that can become acids or base when added to water • CO2 (g) + H2O(l)  H2CO3 (aq) • CaO(s) + H2O (l)  Ca+2 (aq) + 2OH— (aq) • Oxides of metallic elements usually form basic solutions • Oxides of nonmetallic elements (C, S, N) usually produce an acid in aqueous solutions

  11. Strengths of Acids • Strong acids are defined as acids that completely ionize when mixed with water • Strong acids = HCl, HBr, HI, HClO4 , HNO3, and H2SO4 • HCl + H2O H3O+ + Cl— • Weak acids ionize only partially in water • H2C2H3O2 + H2O H3O+ + C2H3O2—

  12. Strengths of Acids Continued • Strong acids are good conductors of electricity because they ionize in water • Weak acids are poor conductors because they don’t ionize in water

  13. Strengths of Bases • Same rules apply to base as acids • Bases that ionize completely in water are strong bases (NaOH, KOH, RbOH, CsOH, Ca(OH)2 , Ba(OH)2 ) • NaOH Na+ + OH— • Strong bases are good conductors and weak bases are not

  14. pH • In pure water,the concentration of H3O+(H+) ions is equal to OH— ions. • [H3O+] = [OH—] ( [ ] is short for concentration) • An important mathematical property of water is that the product of the H3O+ and OH— concentrations is always a constant • We call this the ion product constant for water(Kw) • [H+] x [OH—] = (1.0 x10—7 )x (1.0 x10—7 ) = Kw = 1.0 x 10—14

  15. pH continued • Remember that ……. • Acidic solutions [H+] > [OH—] • Basic solutions [H+] < [OH—] • Neutral solutions [H+] = [OH—] • Regardless of the concentrations of each, the product always equals (1.0 x 10—14 )

  16. pH scale • Since H+ and OH— concentrations deal with such small numbers, scientists came up with scale that is easier to read (pH scale) • pH is the negative log of the H+ concentration (pH = -log[H+]) • pH range goes from 0 to 14

  17. pH scale continued • 0-6.99 = acidic • 7.00 = neutral • 7.01 –14.00 = basic • Remember that a change of 1 on the pH scale is a change in 10 times the concentration of H+ ions (pH 4  pH 3)

  18. pOH Scale • Sometimes its easier for scientists to analyze using the OH— concentration instead of the H+ concentration • This scale is the pOH scale: • 0-6.99 = basic • 7.00 = neutral • 7.01 –14.00 = acidic • The pOH is negative log of the OH—concentration (pOH = -log[OH—] ) • Another helpful conversion is pH + pOH = 14.00

  19. Calculating pH and pOH • Calculate the pH for the following • pH = -log[H+] • [H+] = 3.6 x 10—9 M pH = • [H+] = .025 M pH = • Calculate the pOH for the following • pOH = -log[OH—] • [OH—] = 6.5 x 10—4 MpOH = 8.44 basic 1.60 acidic 3.19 basic

  20. Calculating H+ and OH— Concentrations • To go from pH  [H+] Take the antilog of the negative pH (antilog(-pH) = [H+] ) • To go from pOH[OH—] Take the antilog of the negative pOH (antilog(-pOH)=[OH—]) • pH = 4.56 [H+] = • pH = 11.05 [H+] = • pOH = 1.23 [OH—] = • pOH = 9.87 [OH—] = 2.8 x 10—5 M 8.9 x 10—12 M .059 M 1.3 x 10—10 M

  21. Neutralization Reactions • Rxn in which base and an acid in solution react to form a salt and water • A salt is defined as an ionic compound formed from the cation off the base and the anion off the acid • Always a double replacement rxn • NaOH + HCl(aq) NaCl(aq) + H2O (l) base acid a salt water

  22. Acid/Base Indicators • Many methods are used to measure pH • Limus paper, pH paper, pH meter • Acid/ base indicators are chemical dyes whose colors are affected by acidic and basic solutions • Many are listed on pg 619 along with their range

  23. Buffers • Solutions that resist changes in pH • Added to aquariums to keep water at safe pH • The body has natural buffers to keep your blood at or around 7.4 • Because the scale for pH is exponential, a change of .3 either direction can be fatal to your body

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