Ionic Equilibria - Acid & Base


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Ionic Equilibria - Acid & Base

  1. 1. Ionic Equilibria
  2. 2. Ionic Equilibria Acid-Base - Acid/ Base/ Salt/ Buffer - Titration Curve/ Indicator Solubility - Dissolving - Precipitation
  3. 3. Acid/ Base Equilibria
  4. 4. Ionic Equilibria - only valid at 25⁰C p = -log10 At 25°C: pH + pOH = pKw = 14 pKa + pKb = 14 [H+][OH–] = 10-14 Ka × Kb = Kw = 10-14
  5. 5. Problem Solving Approach 1. Determine what is in solution: - Acid/ Base/ Salt/ Buffer 2. Apply the corresponding eqns
  6. 6. Acid/ Base strong acid weak acid [H+] = [HA] [H+] = Ka [HA] pH = -log [HA] pH = -log [H+] strong base weak base [OH–] = [B] [OH–] = Kb [B] pOH = -log [B] pOH = -log [OH–]
  7. 7. Salt How to determine if salt is acidic/ basic/ neutral? 1. 2. Split the salt into its ions. Determine the parent acid/ base of the ions: Acid Base Salt Strong Strong Neutral Strong Weak Acidic conj acid of weak base hydrolyses H2O to form H+ Weak Strong Basic conj base of weak acid hydrolyses H 2O to form OH– Weak Weak Depends
  8. 8. Salt Nature of salt Formed from Finding pH Neutral strong acid + strong base =7 Acidic strong acid + weak base treat as weak acid; Basic weak acid + strong base conj acid of weak base hydrolyses H2O to form H+ use weak acid formula treat as weak base; conj base of weak acid hydrolyses H2O to form OH– use weak base formula
  9. 9. Salt How to calculate pH of a salt solution: [H3O+] = Ka × [salt] [OH–] = Kb × [salt] 1. Find Ka/ Kb of the ion from Kb/ Ka of the parent acid/ base  use Kw 2. Find [salt] = n (limiting reagent) Vtotal Vtotal = Vacid + Vbase
  10. 10. Buffer Type Formed from pH Acidic weak acid + conjugate base <7 weak base + conjugate acid >7 Basic pH = pKa + log [salt] [acid] [salt] pOH = pKb + log [base]
  11. 11. Buffer How to calculate pH of a buffer on adding small amounts of H+ and OH–: 1. Determine which species is reacted and which is formed. 2. Calculate new amounts (in moles) 3. Substitute into buffer equation.
  12. 12. Buffer 1. When using buffer equation, just substitute no. of moles of salt and acid/base since total volume is the same and cancels out. nsalt/V pH = pKa + log nacid/V 2. Look out for [salt] = [acid] which simply means pH = pKa (same for basic buffer) 3. When unsure if buffer is acidic or basic, look at pKa e.g. HCO3–/CO32– pKa = 10 since pKa is large, buffer is more basic than acidic  use basic buffer formula
  13. 13. Titration Curve There are three important points in a titration curve. e.g. titration of weak acid against strong base
  14. 14. Titration Curve Titration curve What to observe What can be found 1 Initial pt - pH 2 Equivalence pt - pH 3 Half equivalence pt [H+] due to dissociation of acid [H+] due to hydrolysis of salt; - volume of base Given [base], [acid] can be found and vice versa - pH pKa of acid
  15. 15. Titration Curve Calculate pH at various points in titration curve: 1. 2. 3. 4. At the beginning In the middle (sometimes at half eq. pt) At equivalence pt Beyond eq. pt. Strategy 1. 2. Determine what is in solution - Acid/ Base/ Salt/ Buffer Use the relevant eqns
  16. 16. Titration Curve e.g. titration of weak acid against strong base Titration curve What is in solution Equations Initial pt weak acid Between initial acidic buffer pt and equivalence pt At equivalence acidic salt pt Beyond equivalence pt strong base pH = -log Ka × [acid] [salt] pH = pKa + log [acid] pH = -log Ka × [salt] pOH = -log [B]