Ionic Equilibria (I) Acids and bases
Ionic Equilibria (I) 1. How do we identify acids and bases?
Ionic Equilibria (I) <ul><li>Bronsted-Lowry Definition </li></ul><ul><li>An  acid  is a  proton (H + ) donor . </li></ul><...
Ionic Equilibria (I) 2. How do we identify conjugate acids and bases?
Ionic Equilibria (I) <ul><li>Example : </li></ul><ul><li>HCl  Cl – </li></ul><ul><li>NH 3  NH 4 +   </li></ul>2. How do ...
Ionic Equilibria (I) 3. How do we know whether an acid/ base is strong or weak?
Ionic Equilibria (I) 3. How do we know whether an acid/ base is strong or weak?   Strong acids/ bases ionise  completely  ...
Ionic Equilibria (I) 4. How do we calculate pH of an acid/ base?
Ionic Equilibria (I) 4. How do we calculate pH of an acid/ base? (I) Learn the important terms/ relations <ul><li>pH, pOH ...
Ionic Equilibria (I) 4. How do we calculate pH of an acid/ base? (II) Determine strong or weak Strong acids ionise  comple...
Ionic Equilibria (I) 4. How do we calculate pH of an acid/ base? (II) Determine strong or weak Weak bases ionise  partiall...
Ionic Equilibria (II) Salts
Ionic Equilibria (II) 5.  How do we know if a salt is neutral, acidic or basic?
Ionic Equilibria (II) <ul><li>Split the salt into its cation/ anion </li></ul><ul><li>Identify acid that produced anion (a...
Ionic Equilibria (II) <ul><li>Split the salt into its cation/ anion </li></ul><ul><li>Identify acid that produced anion (a...
Ionic Equilibria (II) <ul><li>Split the salt into its cation/ anion </li></ul><ul><li>Identify acid that produced anion (a...
Ionic Equilibria (II) 6.  How do we calculate the pH of a salt solution?
Ionic Equilibria (II) <ul><li>Identify  ion  that hydrolyses H 2 O </li></ul><ul><li>Write equation of the  ion  with H 2 ...
Ionic Equilibria (II) <ul><li>Find K a / K b  of the  ion  from K b / K a  of the parent acid/ base    use K w </li></ul>...
Ionic Equilibria (III) Solubility Equilibria - sparingly soluble salts
Ionic Equilibria (III) 7.  How do know if a salt is sparingly soluble?
Ionic Equilibria (III) <ul><li>From O level QA knowledge. </li></ul><ul><li>When K sp  is given. </li></ul><ul><li>When it...
Ionic Equilibria (III) <ul><li>There are mainly 2 types of question: </li></ul><ul><li>A sparingly soluble salt is dissolv...
Ionic Equilibria (III) Type 1 Questions K sp solubility conc. of ions Given the value of one of the above (e.g. K sp ), fi...
Ionic Equilibria (III) <ul><li>CaF 2  (s)  ⇌ Ca 2+   (aq)  + 2F –  (aq) </li></ul><ul><li>I ?   0  0 </li></ul><ul><li>C  ...
Ionic Equilibria (III) <ul><li>Ca 2+   (aq)  + 2F –  (aq)   ppt? K sp  = …… </li></ul>Type 2 Questions Given the conc. of...
Ionic Equilibria (III) <ul><li>Identify sparingly soluble salt (look for K sp ) </li></ul><ul><li>Write ionic product for ...
Ionic Equilibria (I) Tutorial Q1cii) A weak acid dissociates partially producing less moles of H +  ions than moles of aci...
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Ionic Equilibria

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Ionic Equilibria

  1. 1. Ionic Equilibria (I) Acids and bases
  2. 2. Ionic Equilibria (I) 1. How do we identify acids and bases?
  3. 3. Ionic Equilibria (I) <ul><li>Bronsted-Lowry Definition </li></ul><ul><li>An acid is a proton (H + ) donor . </li></ul><ul><li>A base is proton acceptor . </li></ul><ul><li>In an acid-base reaction, the transfer of protons occurs from an acid to a base. </li></ul>1. How do we identify acids and bases? Tip : It may be helpful for beginners to memorise some common acids and bases.
  4. 4. Ionic Equilibria (I) 2. How do we identify conjugate acids and bases?
  5. 5. Ionic Equilibria (I) <ul><li>Example : </li></ul><ul><li>HCl  Cl – </li></ul><ul><li>NH 3  NH 4 + </li></ul>2. How do we identify conjugate acids and bases? Acid Conjugate Base – H + + H + Base Conjugate Acid + H + – H +
  6. 6. Ionic Equilibria (I) 3. How do we know whether an acid/ base is strong or weak?
  7. 7. Ionic Equilibria (I) 3. How do we know whether an acid/ base is strong or weak? Strong acids/ bases ionise completely in aqueous solution. Weak acids/ bases ionise partially in aqueous solution. <ul><li>Tip : An acid/ base can be predicted to be weak when: </li></ul><ul><li>It is stated so in the question </li></ul><ul><li>K a or K b value is given </li></ul><ul><li>Degree/ percent of ionisation is given </li></ul><ul><li>[H 3 O + ] < [HA]; [OH – ] < [B] </li></ul>
  8. 8. Ionic Equilibria (I) 4. How do we calculate pH of an acid/ base?
  9. 9. Ionic Equilibria (I) 4. How do we calculate pH of an acid/ base? (I) Learn the important terms/ relations <ul><li>pH, pOH </li></ul><ul><li>K a , pK a </li></ul><ul><li>K b , pK b </li></ul><ul><li>K w </li></ul>pH + pOH = 14 pK a + pK b = 14 [H + ][OH – ] = 10 -14 K a . K b = K w = 10 -14
  10. 10. Ionic Equilibria (I) 4. How do we calculate pH of an acid/ base? (II) Determine strong or weak Strong acids ionise completely in aqueous solution. [H 3 O + ] = [HA]  pH = - log [HA] Strong bases ionise completely in aqueous solution. [OH - ] = [B]  pOH = - log [B] Strong Acid Strong Base
  11. 11. Ionic Equilibria (I) 4. How do we calculate pH of an acid/ base? (II) Determine strong or weak Weak bases ionise partially in aqueous solution. [OH - ] < [B]  pOH = - log [OH – ] Weak Acid Weak Base Weak acids ionise partially in aqueous solution. [H 3 O + ] < [HA]  pH = - log [H 3 O + ] [H 3 O + ] = K a × [HA] [OH – ] = K b × [B]
  12. 12. Ionic Equilibria (II) Salts
  13. 13. Ionic Equilibria (II) 5. How do we know if a salt is neutral, acidic or basic?
  14. 14. Ionic Equilibria (II) <ul><li>Split the salt into its cation/ anion </li></ul><ul><li>Identify acid that produced anion (add H + ) </li></ul><ul><li>Identify base that produced cation </li></ul><ul><li>If the acid and base are both strong, the salt is neutral </li></ul>5. How do we know if a salt is neutral, acidic or basic?
  15. 15. Ionic Equilibria (II) <ul><li>Split the salt into its cation/ anion </li></ul><ul><li>Identify acid that produced anion (add H + ) </li></ul><ul><li>Identify base that produced cation </li></ul><ul><li>If the acid is weak, the anion hydrolyses water and acts as a weak base  basic salt </li></ul>5. How do we know if a salt is neutral, acidic or basic?
  16. 16. Ionic Equilibria (II) <ul><li>Split the salt into its cation/ anion </li></ul><ul><li>Identify acid that produced anion (add H + ) </li></ul><ul><li>Identify base that produced cation </li></ul><ul><li>If the base is weak, the cation hydrolyses water and acts as a weak acid  acidic salt </li></ul>5. How do we know if a salt is neutral, acidic or basic?
  17. 17. Ionic Equilibria (II) 6. How do we calculate the pH of a salt solution?
  18. 18. Ionic Equilibria (II) <ul><li>Identify ion that hydrolyses H 2 O </li></ul><ul><li>Write equation of the ion with H 2 O </li></ul><ul><li>Find pH by treating ion as weak acid or weak base </li></ul><ul><li>Recall : </li></ul>6. How do we calculate the pH of a salt solution? [H 3 O + ] = K a × [HA] [OH – ] = K b × [B]
  19. 19. Ionic Equilibria (II) <ul><li>Find K a / K b of the ion from K b / K a of the parent acid/ base  use K w </li></ul><ul><li>2. Find [salt] = </li></ul>6. How do we calculate the pH of a salt solution? [H 3 O + ] = K a × [HA] [OH – ] = K b × [B] n (limiting reagent) V total V total = V acid + V base
  20. 20. Ionic Equilibria (III) Solubility Equilibria - sparingly soluble salts
  21. 21. Ionic Equilibria (III) 7. How do know if a salt is sparingly soluble?
  22. 22. Ionic Equilibria (III) <ul><li>From O level QA knowledge. </li></ul><ul><li>When K sp is given. </li></ul><ul><li>When it is stated so in the question. </li></ul>7. How do know if a salt is sparingly soluble?
  23. 23. Ionic Equilibria (III) <ul><li>There are mainly 2 types of question: </li></ul><ul><li>A sparingly soluble salt is dissolved in water to give a saturated solution </li></ul><ul><li>e.g. CaSO 4 (s) ⇌ Ca 2+ (aq) + SO 4 2 – (aq) </li></ul><ul><li>2. 2 ions are mixed to form a sparingly soluble salt. </li></ul><ul><li>e.g. Ca 2+ (aq) + SO 4 2 – (aq)  CaSO 4 (s) </li></ul>General Tips
  24. 24. Ionic Equilibria (III) Type 1 Questions K sp solubility conc. of ions Given the value of one of the above (e.g. K sp ), find the other 2 values (solubility and conc. of ions)
  25. 25. Ionic Equilibria (III) <ul><li>CaF 2 (s) ⇌ Ca 2+ (aq) + 2F – (aq) </li></ul><ul><li>I ? 0 0 </li></ul><ul><li>C -x +x +2x </li></ul><ul><li>E ? x 2x </li></ul>Type 1 Questions (Strategy) K sp solubility conc. of ions Expressing each term in x can help in the interconversion. solubility = x K sp = 4x 3 [Ca 2+ ] = x [F – ] = 2x
  26. 26. Ionic Equilibria (III) <ul><li>Ca 2+ (aq) + 2F – (aq)  ppt? K sp = …… </li></ul>Type 2 Questions Given the conc. of Ca 2+ and F – and K sp , predict whether ppt is formed or Given K sp and conc. of one ion, predict the min. conc of the other ion that will cause precipitation.
  27. 27. Ionic Equilibria (III) <ul><li>Identify sparingly soluble salt (look for K sp ) </li></ul><ul><li>Write ionic product for salt </li></ul><ul><li>(same expression as for K sp ) </li></ul><ul><li>Calculate ionic product and compare: </li></ul><ul><li>Q ≤ K  no ppt </li></ul><ul><li>Q > K  ppt </li></ul>Type 2 Questions (Strategy)
  28. 28. Ionic Equilibria (I) Tutorial Q1cii) A weak acid dissociates partially producing less moles of H + ions than moles of acid. e.g. 10 moles of a weak acid HA may produce only 0.1 mole of H + But when a weak acid reacts with another reagent (e.g. metal or alkali), the weak acid will react completely. e.g. All 10 moles of HA will be reacted when added to excess NaOH
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