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EQUILBRIUM OF Acids and Bases

EQUILBRIUM OF Acids and Bases. Chapter 17. Water. AUTOIONIZATION. H 2 O can function as both an ACID and a BASE. Equilibrium constant for water = K w K w = [H 3 O + ] [OH - ] = 1.00 x 10 -14 at 25 o C. Water. K w = [H 3 O + ] [OH - ] = 1.00 x 10 -14 at 25 o C

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EQUILBRIUM OF Acids and Bases

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  1. EQUILBRIUM OF Acids and Bases Chapter 17

  2. Water AUTOIONIZATION H2O can function as both an ACID and a BASE. Equilibrium constant for water = Kw Kw = [H3O+] [OH-] = 1.00 x 10-14 at 25 oC

  3. Water Kw = [H3O+] [OH-] = 1.00 x 10-14 at 25 oC In a neutralsolution [H3O+] = [OH-] so [H3O+] = [OH-] = 1.00 x 10-7 M Autoionization

  4. [H3O+], [OH-] and pH A common way to express acidity and basicity is with pH pH = - log [H3O+] In a neutral solution, [H3O+] = [OH-] = 1.00 x 10-7 at 25 oC pH = -log (1.00 x 10-7) = - (-7) = 7

  5. [H3O+], [OH-] and pH What is the pH of the 0.0010 M NaOH solution? [H3O+] = 1.0 x 10-11 M pH = - log (1.0 x 10-11) = 11.00 General conclusion — Basic solution pH > 7 Neutral pH = 7 Acidic solution pH < 7

  6. [H3O+], [OH-] and pH If the pH of Coke is 3.12, it is ________. log [H3O+] = - pH Take antilog and get [H3O+] = 10-pH [H3O+] = 10-3.12 = 7.6 x 10-4 M

  7. pX Scales In general pX = -log X pOH = - log [OH-] pH = - log [H+] pKw = 14 = pH + pOH

  8. Weak Base Step 3. Calculate pH [OH-] = 4.2 x 10-4 M so pOH = - log [OH-] = 3.37 Because pH + pOH = 14, pH = 10.63

  9. Equilibria Involving Weak Acids and Bases Aspirin is a good example of a weak acid, Ka = 3.2 x 10-4

  10. Weak Acids and Bases AcidConjugate Base acetic, CH3CO2H CH3CO2-, acetate ammonium, NH4+ NH3, ammonia bicarbonate, HCO3- CO32-, carbonate A weak acid (or base) is one that ionizes to a VERY small extent (< 5%).

  11. Weak Acids and Bases acetic acid, CH3CO2H (HOAc) HOAc + H2O D H3O+ + OAc- Acid Conj. base (K is designated Ka for ACID) [H3O+] and [OAc-] are SMALL, Ka << 1.

  12. Equilibrium Constants for Weak Acids Weak acid has Ka < 1 Leads to small [H3O+] and a pH of 2 - 7

  13. 0.0001 M Calculations with Equilibrium Constants 0.003 M pH of an acetic acid solution. What are your observations? 0.06 M 2.0 M a pH meter, Screen 17.9

  14. Equilibria Involving A Weak Acid You have 1.00 M HOAc. Calc. the pH. And the equilibrium concs. of EACH Step 1. ICE table. [HOAc] [H3O+] [OAc-] I C E

  15. Equilibria Involving A Weak Acid [HOAc] [H3O+] [OAc-] I 1.00 0 0 C -x +x +x E 1.00-x x x Note that we neglect [H3O+] from H2O.

  16. Equilibria Involving A Weak Acid Step 2. Write Ka expression This is a quadratic. Use quadratic formula or method of approximations (see Appendix A). HOWEVER

  17. Equilibria Involving A Weak Acid Assume x is very small because Ka is so small. Now we can more easily solve this approximate expression.

  18. Equilibria Involving A Weak Acid Step 3. Solve Kaapproximate expression x = [H3O+] = [OAc-] = [Ka • 1.00]1/2 x = [H3O+] = [OAc-] = 4.2 x 10-3 M pH = -log (4.2 x 10-3) = 2.37

  19. Equilibria Involving A Weak Acid For many weak acids [H3O+] = [conj. base] = [Ka • Co]1/2 where C0 = initial conc. of acid Useful Rule of Thumb: If 100•Ka < Co, then [H3O+] = [Ka•Co]1/2

  20. Equilibria Involving A Weak Acid Calculate the pH of a 0.0010 M solution of formic acid, HCO2H. HCO2H + H2O D HCO2- + H3O+ Ka = 1.8 x 10-4 Approximate solution [H3O+] = [Ka • Co]1/2 = 4.2 x 10-4 M, pH = 3.4 Exact Solution [H3O+] = [HCO2-] = 3.4 x 10-4 M [HCO2H] = 0.0010 - 3.4 x 10-4 = 0.0007 M pH = 3.5

  21. Weak Bases

  22. Equilibrium Constants for Weak Bases Weak base has Kb < 1 Leads to small [OH-] and a pH of 12 - 7

  23. Equilibria Involving A Weak Base You have 0.010 M NH3. Calc. the pH. NH3 + H2O D NH4+ + OH- Kb = 1.8 x 10-5 Step 1; ICE table [NH3] [NH4+] [OH-] I C E

  24. Weak Base Step 1. ICE table [NH3] [NH4+] [OH-] I 0.010 0 0 C -x +x +x E 0.010 - x x x

  25. Weak Base Step 2. Solve the equilibrium expression Assume x is small (100•Kb < Co), so x = [OH-] = [NH4+] = 4.2 x 10-4 M [NH3] = 0.010 - 4.2 x 10-4 ≈ 0.010 M The approximation is valid !

  26. Acids Conjugate Bases

  27. Relation of Ka, Kb, [H3O+] and pH

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