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UNIT IV

UNIT IV. Strong and Weak Acids and Bases. What is a Strong Acid?. Strong Acid - An acid which is 100% ionized in a water solution .  Ex.) HCl (g) + H 2 O (l)  H 3 O + (aq) + Cl - (aq) S ingle a rrow (goes to completion) = S trong a cid Ex.) What is the [HCl (g) ] in 1 M HCl?

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UNIT IV

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  1. UNIT IV Strong and Weak Acids and Bases

  2. What is a Strong Acid? • Strong Acid- An acid which is 100% ionized in a water solution.  Ex.) HCl(g) + H2O(l) H3O+(aq)+ Cl-(aq) Single arrow (goes to completion) = Strong acid Ex.) What is the [HCl(g)] in 1 M HCl? Ex.) What is [H3O+] in 0.20 M HCl

  3. What is a Strong Acid? • In a Strong Acid: [H3O+] = [Acid] Ex.) What is [H3O+] in 0.60 M HNO3

  4. What is a Weak Acid? • Weak Acid: An acid which is less than 100% ionized in solution. •  (In Chem 12 WA’s are usually significantly less than 100% ionized…usually < 5% ionized)  • In a solution of a weak acid, most of the molecules don’t ionize. Ex.) HF (g) + H2O(l) H3O+ (aq) + F-(aq) (Molecules) (Double arrow!) (Ions) http://www.mhhe.com/physsci/chemistry/essentialchemistry/flash/acid13.swf

  5. Strength Versus Concentration • Any acid (weak or strong) could have high or low concentration. • Weak & Strong refers to % ionization • Concentration the moles of acid dissolved per litre Examples: • 10.0 M HCl conc. and strong [H3O+] = 10.0 M • 0.001 M HCl dilute and strong [H3O+] = 0.001 M • 10.0 M HF  conc. and weak [H3O+] = low • 0.001 M HF  dilute and weak [H3O+] = very low *Concentration of IONS determines CONDUCTIVITY!

  6. The Strong Acids • The strong acids are the top six acids listed on the “Relative Strengths of Bronsted-Lowry Acids and Bases”, namely: Name Formula Perchloric acid HClO4 Hydriodic acid HI Hydrobromic acid HBr Hydrochloric acid HCl Nitric acid HNO3 Sulphuric acid H2SO4 (Note: H2SO4 is only strong for the first dissociation: H2SO4 → H+ + HSO4-)

  7. The Weak Acids • All the acids listed on the LEFT side of the table in the white section are “weak” acids. H3O+  H+ + H2O net result of adding any strong acid to water HIO3  H+ + IO3- THE WEAK ACIDS H2O H+ + OH- Note: The bottom two species on the left NEVER act as acids....SINGLE ARROW GOING BACKWARDS!

  8. What is a Strong Base? Strong Base- A base which is 100% ionized in a water solution.  Ex. Ba(OH)2(s) Ba2+(aq) + 2OH-(aq) Ex.) What is the [OH­­-] in 0.10 M NaOH? Ex.) What is the [OH­-] in 0.10 M Ba(OH)2 ?

  9. What is a Strong Base? • For a strong base: [OH-] = [Base] x # of OH’s in Formula

  10. What is a Weak Base? • Weak Base: A base which is less than 100% ionized in solution. Ex.) NH3(aq) + H2O(l) NH4+ (aq) + OH- (aq)

  11. The Strong Bases • Any substance which dissociates completely to produce OH-, O2- or NH2- is a Strong Base: • Alkali Metal Hydroxides (Group 1) Ex. LiOH, NaOH, KOH, RbOH, CsOH • Alkaline Earth Metal Hydroxides (Group 2) Ex. Ca(OH)2, Ba(OH)2 , Sr(OH)2 • The hydroxide ion, OH-, found on the lower right side of the table, is the net result of adding any strong base to water.

  12. The Weak Bases • All the bases listed on the RIGHT side of the table in the white section are “weak” bases. H2O IO3- . THE WEAK BASES PO43- H2O  H+ + OH- net result of adding any strong base to water

  13. Spectators • The CONJUGATE of a STRONG ACID or a STRONG BASE is a SPECTATOR. • These are the top 5 (not 6) “bases” on the right. CIO4- I- Br- They are so weak that they cannot react with H2O to form OH- Cl- NO3-

  14. Spectators H----Cl + H2O  H3O+ (aq) + Cl-(aq) weak So weak , it cannot take an H+ from H2O or even H3O+ • Also, the alkali metal ions and alkaline earth metal ions are considered spectators since they are the conjugates of strong bases.

  15. Amphiprotic Species • are found on both sides of the table e.g.) HSO4- • can act as acids (donate H+’s) or as bases (accept H+’s) • to look at an amphiprotic species as an acid, you must find it on the left side: Example • HCO3- is a ________er acid than C6H5OH • HCO3- is a ________er acid than H2O2

  16. Amphiprotic Species • to look at an amphiprotic species as a base, you must find it on the rightside: Example • HCO3- is a _________er base than C6H5O73- • HCO3- is a _________er base than Al(H2O)5(OH)2+ • HSO4- in shaded region on top right will not act as a base in water….but it is not a spectator (like NO3- is). Why not?  • (HSO4‑ is also found on the left side quite a way up, it is a relatively “strong” weak acid.)

  17. The Levelling Effect • What is [H3O+] in 1.0 M H3O+ ? ______ • What is [H3O+] in 1.0 M HNO3? ______  • What is [H3O+] in 1.0 M HCl ? ______ • Acids from HClO4 to H2SO4 are 100% ionized in water • So, even though HClO4 is above HCl on the chart, it is no more acidic in a water solution.

  18. The Levelling Effect H3O+ is the strongest acid that can exist in undissociated form in water solution. All stronger acids ionize to form H3O+. • The only way you can tell which strong acid is “stronger” is to react them in a non-aqueous (not H2O) solvent. • Eg) HClO4 + H2SO4 H3SO4+ + ClO4- •  It is found that HClO4 donates a proton to H2SO4, not the other way around, so HClO4 is a stronger acid than H2SO4. This is not important in Chemistry 12. •  This would not happen in a water solution…in H2O, they would both form H3O+.

  19. The Levelling Effect The strongest base which can exist in high concentrations in water solution is OH- The two stronger bases below it will react with water completely to form OH-. Eg) O2- + H2O  OH- + OH- OR…O2- + H2O  2OH- Ex. What is the final [O2-] in 1.0 M Na2O ?

  20. The Levelling Effect Write the equation for NH2- reacting with H2O. • Hebden Textbook Pages 125-126 Questions #21-27

  21. Acid-Base Equilibria & Relative Strengths of Acids & Bases Example Mix some H2PO4- and some CO32- H2PO4- + CO32- HCO3- + HPO42- (A) (B) (A) (B) At equilibrium, which will be favoured, reactants or products?

  22. Acid-Base Equilibria & Relative Strengths of Acids & Bases Equilibrium favours the side with the weaker acid. Example Will HSO3- + HCO3- H2CO3 + SO32- favour reactants or products?

  23. Acid-Base Equilibria & Relative Strengths of Acids & Bases Mixing Two Amphiprotic Ions(products not given) Example Complete the reaction and determine which is favoured, reactants or products? (Also, compare the relative strengths of the bases.) HSO4- + H2PO4-

  24. Acid-Base Equilibria & Relative Strengths of Acids & Bases NOTICE: • The STRONGER ACID is on the same side as the STRONGER BASE. (The stronger acid has the weaker conjugate base.) • The WEAKER ACID is on the same side as the WEAKER BASE. Equilibrium favours the side with the weaker conjugate acid and the weaker conjugate base.

  25. Acid-Base Equilibria & Relative Strengths of Acids & Bases Starting with “Salts” • The amphiprotic ions are often products of the dissociation of salts. • Spectator ions must be discarded. Example Complete the net ionic reaction and state whether equilibrium favours reactants or products NaHSO3 + K2HPO4 →

  26. Keq and Acid Strength • If products are favored Keq is large (>1) • If reactants are favored Keq is small (<1) Ex) Given: HA + B- HB + A- Keq = 0.003 Which acid is stronger, HA or HB? • Hebden Textbook Page 133 Questions # 38-46 

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