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Acids and Bases

Acids and Bases. Acids Bases. Sour Bitter Also referred to as alkali H 2 O solutions feel slippery. Acid – Base Theories. Arrhenius Definition acid -- donates a hydrogen ion (H + ) in water base --donates a hydroxide ion in water (OH - )

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Acids and Bases

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  1. Acids and Bases

  2. AcidsBases Sour Bitter Also referred to as alkali H2O solutions feel slippery

  3. Acid – Base Theories Arrhenius Definition acid--donates a hydrogen ion (H+) in water base--donates a hydroxide ion in water (OH-) This theory was limited to substances with those "parts"; ammonia is a MAJOR exception!

  4. Bronsted-Lowry Definition acid--donates a proton in water base--accepts a proton in water This theory is better; it explains ammonia as a base! This is the main theory that we will use for our acid/base discussion.

  5. Lewis Definition acid--accepts an electron pair base--donates an electron pair This theory explains alltraditional acids and bases + a host of coordination compounds and is used widely in organic chemistry. Uses coordinate covalent bonds

  6. Conjugate acid-base concept • Acid becomes the conjugate base • Base becomes the conjugate acid

  7. Acidsdonate a proton (H+) HNO3 + H2O  H3O+ + NO3- acid base CA CB NH4+ + H2O  H3O+ + NH3 acid base CA CB H2PO4- + H20  H3O+ + HPO42- acid base CA CB

  8. In each of the acid examples---notice the formation of H3O+ This species is named the hydronium ion. It lets you know that the solution is acidic!

  9. Basesaccept a H+ NH3 + H2O  NH4+ + OH- base acid CA CB CO32- + H2O  HCO3- + OH- base acid CA CB PO43- + H2O  HPO42- + OH- base acid CA CB

  10. In each of the basic examples-- notice the formation of OH- -- this species is named the hydroxide ion. It lets you know that the solution is basic!

  11. Identify the acid, base, conjugate acid and conjugate base. HBr + NH3 NH4+ + Br- What is the conjugate base of H2S? What is the conjugate acid of NO3-?

  12. Acids only donate one H+ at a time • monoprotic--acids donating one H+ (ex. HC2H3O2) • diprotic--acids donating two H+'s (ex. H2C2O4) • polyprotic--acids donating many H+'s (ex. H3PO4)

  13. Polyprotic Bases accept more than one H+ anions with -2 and -3 charges (example: PO43- ; HPO42-)

  14. Amphiprotic or Amphoteric molecules or ions that can behave as EITHER acids or bases: water, anions of weak acids (look at the examples above—sometimes water was an acid, sometimes it acted as a base)

  15. Relative Strengths of Acids and Bases Strength is determined by the position of the "dissociation" equilibrium. Do Not confuse concentration with strength!

  16. Strong acids/Strong bases dissociate completely in water have very large K values

  17. Weak acids/Weak bases dissociate only to a slight extent in water dissociation constant, Ka, is very small

  18. Strong Weak

  19. Strong Acids Hydrohalic acids: HCl, HBr, HI (except HF) Nitric: HNO3 Sulfuric: H2SO4 Perchloric: HClO4

  20. Strong Bases Hydroxides OR oxides of IA and IIA metals Solubility plays a role (those that are very soluble are strong!)

  21. The stronger the acid, the weaker its CB. The converse is also true.

  22. The equilibrium expression for acids is known as the Ka (the acid dissociation constant). It is set up the same way as in general equilibrium.

  23. For Weak Acid Reactions: HA + H2O  H3O+ + A- Ka = [H3O+][A-] Ka < 1 [HA]

  24. For Weak Base Reactions: B + H2O  HB+ + OH- Kb = [H3O+][OH-] Kb < 1 [B]

  25. Notice that Ka and Kb expressions look very similar. The difference is that a base produces the hydroxide ion in solution, while the acid produces the hydronium ion in solution.

  26. Another note on this point: H+ and H3O+ are both equivalent terms here. Often water is left completely out of the equation since it does not appear in the equilibrium. This has become an accepted practice. (*However, water is very important in causing the acid to dissociate.)

  27. H2O(l) + H2O(l) <=> H3O+(aq) + OH-(aq) Keq[H2O]2 = Kw = [H3O+][OH-] Kw = 1.0 x 10-14 Kw = Ka x Kb (Another useful equation) Knowing this value allows us to calculate the OH- and H+ concentration for various situations.

  28. [OH-] = [H+] : solution is neutral (in pure water, each of these is 1.0 x 10-7) [OH-] > [H+] : solution is basic [OH-] < [H+] : solution is acidic

  29. Kw = [H+] [OH-] = 1.0 X 10-14 M Calculate [H+] or [OH-] as required for each of the following solutions at 25°C, and state whether the solution is neutral, acidic, or basic. a. 1.0 X 10-5M OH- b. 1.0 X 10-7M OH- c. 10.0 M H+

  30. Salts and pH

  31. Soluble salts dissociate in water to produce ions. Salts are basically ionic compounds that can be formed from the reaction from an acid and a base. • Not all aqueous salt solutions are neutral; their pH depends on how the cation and anion interact with water. This interaction is called hydrolysis.

  32. Neutral salts Salts from strong acids and strong bases are NEUTRAL Example: NaCl from NaOH and HCl LiCl from LiOH and HCl

  33. Basic salts Cations from strong base and anions from weak acid have a strong conjugate base. This interacts with water by gaining a proton and generating hydroxide ions. A- + H2O ↔ HA + OH- Salts from STONG bases and WEAK acids are BASIC Example: NaC2H3O2 from NaOH and HC2H3O2 C2H3O2- + H3O+↔ OH- + HC2H3O2

  34. Acidic Salts Cations from WEAK base and anions from STRONG acid have a strong conjugate acid. This interacts with water by gaining a proton and generating hydronium ions. HB+ + H2O ↔ B + H3O+ Salts from WEAK base and STRONG acid are ACIDIC Example: NH4Cl from NH4OH and HCl NH4+ + H2O ↔ NH3 + OH-

  35. Cations from weak base and anions from weak acid have both strong conjugates. Since both can interact with water, the resulting pH depends upon the respective strength of the conjugate acid or base. If Ka > Kb then salt is acid If Ka < Kb then salt is base If Ka =Kb then salt is neutral

  36. Would the following salts be acidic, basic or neutral? NaNO3 NaNO2 C5H5NHClO4 NH4NO2 KOCl KCl CH3NH3Cl KF NH4F CH3NH3CN

  37. What is the pH of a 0.10 M solution of sodium acetate?

  38. What is the pH of a 0.10 M solution of ammonium nitrate?

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