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Unit 2 Acids and Bases

Unit 2 Acids and Bases. http://www.cdli.ca/courses/ http://www.cbhs.k12.nf.ca/adrianyoung/. Topics Properties / Operational Definitions Acid-Base Theories pH & pOH calculations Equilibria ( K w , K a , K b ) Indicators Titrations STSE: Acids Around Us. Operational Definitions.

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Unit 2 Acids and Bases

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  1. Unit 2 Acids and Bases http://www.cdli.ca/courses/ http://www.cbhs.k12.nf.ca/adrianyoung/

  2. Topics • Properties / Operational Definitions • Acid-Base Theories • pH & pOH calculations • Equilibria (Kw, Ka,Kb) • Indicators • Titrations • STSE: Acids Around Us

  3. Operational Definitions • An Operational Definition is a list of properties, or operations that can be performed, to identify a substance. • See p. 550 for operational definitions of acids and bases

  4. Acids pH < 7 taste sour react with active metals (Mg, Zn) to produce hydrogen gas Bases pH > 7 taste bitter no reaction with active metals feel slippery Operational Definitions (Properties – see p. 550)

  5. Acids blue litmus turns red react with carbonates to produce CO2 gas Bases red litmus turns blue no reaction with carbonates Operational Definitions

  6. Acids conduct electric current neutralize bases to produce water and a “salt” Bases conduct electric current neutralize acids to produce water and a “salt” Operational Definitions any ionic compound any ionic compound

  7. Acid-Base Theories 1. Arrhenius Theory (p. 549 ) acid – any substance that dissociates in water to produce H+ ions ie. an acid must contain H+ ions

  8. Arrhenius Theory eg. HCl(aq)→ H2SO4(aq) → HSO4-(aq) →

  9. Arrhenius Theory base – any substance that dissociates in water to produce OH- ions ie. a base must contain OH- ions

  10. Arrhenius Theory eg. NaOH(aq) Ca(OH)2(aq)

  11. Arrhenius Theory Which is a Arrhenius base? Which is an Arrhenius acid? a) KOH c) CH4 b) HCN d) CH3OH

  12. Limitations of Arrhenius theory (p.551) • H+ cannot exist as an ion in water. The positive H+ ions are attracted to the polar water molecules forming HYDRONIUM ions or H3O+(aq) H+(aq) + H2O(l)→ H3O+(aq)

  13. Limitations of Arrhenius theory • CO2 dissolves in water to produce an acid. NH3 dissolves in water to produce a base. Neither of these observations can be explained by Arrhenius theory

  14. Limitations of Arrhenius theory • Some acid-base reactions can occur in solvents other than water. Arrhenius theory can explain only aqueous acids or bases.

  15. Limitations of Arrhenius theory • Arrhenius theory is not able to predict whether certain species are acids or bases. eg. NaHSO4 H2PO4- HCO3- Arrhenius theory needs some work

  16. Acid-Base Theories To be used when Arrhenius is inadequate 2. Modified Arrhenius Theory (p. 552) acid – any substance that reacts with water to produce H3O+ ions eg. HCl(g) + H2O(l) → H3O+(aq) + Cl-(aq)

  17. Modified Arrhenius Theory base – any substance that reacts with water to produce OH- ions eg. NH3(aq) + H2O(l))→ NH4+(aq) + OH-(aq) Acids & Bases #1: #’s 1 - 6 pp. 558, 559 #’s 1, 3, 8, & 9

  18. Acid-Base Theories 3. Brønsted-Lowry Theory (p. 553) acid – any substance from which a proton (H+) may be removed ie. an acid is a substance that loses a proton (H+)

  19. Brønsted-Lowry Theory base – any substance that can remove a proton (H+) from an acid. ie. a base is a substance that gains a proton (H+) In BLT , an acid-base reaction requires the transfer of a proton (H+) from an acid to a base.

  20. Brønsted-Lowry Theory conjugate acid base eg. HCN(aq) + NH3(aq) → ← CN-(aq) + NH4+(aq) conjugate base acid

  21. Brønsted-Lowry Theory What is a conjugate acid-base pair?? (p. 554) • Two particles (molecules or ions) that differ by one proton are called a conjugate acid-base pair. • The conjugate base forms when an acid loses a proton. • The conjugate acid forms when a base gains a proton (H+).

  22. Brønsted-Lowry Theory conjugate acid base conjugate base acid

  23. Brønsted-Lowry Theory conjugate acid base eg. NH3(aq) + H2O(l) → NH4+(aq) + OH-(aq) H2O(l) +H2O(l) → ← conjugate base acid ←

  24. Brønsted-Lowry Theory - an amphotericsubstance can be either an acid or a base • usually these are negative ions that contain at least one hydrogen atom eg. H2O, HCO3-(aq), H2PO4-(aq)

  25. Brønsted-Lowry Theory p.557 #’s 1 – 7 p. 558 #’s 8, 9 p. 559 #’s 2, 4-7, 10,11

  26. Strength of Acids and Bases • A strong acid is an acid that ionizes or dissociates 100% in water eg. HCl(aq)→ • Strong acids react 100% with water (BLT) eg. HCl(aq) + H2O(l) →

  27. Strength of Acids and Bases Strong acids produce more H+ ions OR more H3O+ ions than weak acids with the same molar concentration NOTE: The equilibrium symbol,  , is NOT used for strong acids because there is NO REVERSE REACTION.

  28. Strength of Acids and Bases • A weak acidis an acid that ionizes or dissociates LESS THAN 100% eg. HF(aq) • Weak acids react less than 100% with water eg. HF(aq) + H2O(l)

  29. Strength of Acids and Bases Weak acids produce fewer H+ ions OR fewer H3O+ ions than strong acids with the same molar concentration • For weak acids, an equilibrium is established between the original acid molecule and the ions formed. DO NOT confuse the terms strong and weak with concentrated and dilute.

  30. Strength of Acids and Bases eg. Classify the following acids: • 0.00100 mol/L HCl(aq) strong and dilute • 12.4 mol/L HCl(aq) strong and concentrated • 10.5 mol/L CH3COOH(aq) weak and concentrated

  31. Strength of Acids and Bases monoprotic – acids that contain or lose one proton diprotic – acids that contain or lose two protons polyprotic –

  32. Strength of Acids and Bases • A strong base is a base that dissociates 100% in water, or reacts 100% with water, to produce OH- ion. • The only strong bases are hydroxide compounds of most Group 1 and Group 2 elements eg. NaOH(s)→ Na+(aq) + OH-(aq) Ca(OH)2(s) → Ca2+(aq) + 2 OH-(aq)

  33. Strength of Acids and Bases • A weak base is a base that reacts less than 100% in water to produce OH- ion. eg. S2-(aq) + H2O(l)º HS-(aq) + OH-(aq)

  34. Writing Acid-Base Equations (BLT) Step 1: List all the molecules/ions present in the solution • ionic compounds form cations and anions • strong acids exist as hydronium ion and the anion (conjugate base) • for weak acids use full formula of the compound (i.e. un-ionized molecule) • always include water in the list.

  35. Writing Acid-Base Equations (BLT) Step 2: Identify the strongEST acid and the strongEST base from Step 1. Step 3: Write the equation for the reaction by transferring a proton from the strongest acid to the strongest base.

  36. Writing Acid-Base Equations (BLT) • Step 4: Determine the type of reaction arrow to use in the equation. Stoichiometric(100%) reactions occur between: • Hydronium (H3O+) and bases stronger than nitrite (NO2-) • hydroxide (OH-)and acids stronger than hypochlorous acid (HOCl)

  37. Writing Acid-Base Equations (BLT) Step 5: Determine the position of the equilibrium by comparing the strengths of both acids in the equation. The favoured side is the side with the weaker acid! 

  38. Writing Acid-Base Equations (BLT) Sample problems: • Write the net ionic equation for the acid-base reaction between: - aqueous sodium hydroxide (NaOH(aq)) and hydrochloric acid (HCl(aq)).

  39. Na+(aq) OH-(aq) H3O+(aq) Cl-(aq) H2O(l) species present strongest acid strongest base H3O+(aq) + OH-(aq) H2O(l) + H2O(l) OR H3O+(aq) + OH-(aq) →2 H2O(l)

  40. Writing Acid-Base Equations (BLT) Sample problems: • Write an equation for the acid-base reaction between nitrous acid (HNO2(aq)) and aqueous sodium sulfite (Na2SO3(aq)).

  41. HNO2(aq) Na+(aq) SO32-(aq) H2O(l) species present SB SA º HNO2(aq) + SO32-(aq) NO2-(aq) + HSO3 -(aq) Weaker Acid Stronger Acid Products favored

  42. Write the Net Ionic Equation for each aqueous reaction below: • Na2CO3(aq) and CH3COOH(aq) • NH3(aq) and HNO2(aq) • HNO3(aq) and RbOH • H2SO4(aq) and K3PO4(aq) • HF(aq) and NH4CH3COO(aq) • CaCl2(aq) and PbSO4(aq) p. 564 #’s 10 &11 Acids & Bases #3

  43. H2O(l) Na+(aq) CH3COOH(aq) CO32-(aq) 1.a) species present SB SA º CH3COOH(aq) + CO32-(aq) CH3COO-(aq) + HCO3-(aq) Stronger Acid Weaker Acid Products Favoured

  44. NH3(aq) HNO2(aq) H2O(l) species present SA SB º HNO2(aq) + NH3(aq) NO2-(aq) + NH4 +(aq) Weaker Acid Products favored Stronger Acid

  45. NO3-(aq) Rb+(aq) H3O+(aq) OH-(aq) H2O(l) species present strongest acid strongest base H3O+(aq) + OH-(aq) H2O(l) + H2O(l) OR H3O+(aq) + OH-(aq) →2 H2O(l)

  46. HSO4-(aq) K+(aq) PO43-(aq) H3O+(aq) H2O(l) species present SA SB H3O+(aq) + PO43-(aq) H2O(l) + HPO42-(aq)

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