1. 1 Acids and Bases 23-3-2011

2. 2 Definition of Acids Arrhenius acid: A substance that releases H + in water ( e.g. HCl) H + + H 2 O  H 3 O + Hydronium Ion Acid: A substance which increases [H + ] when dissolved in water. Base: A substance which increases [OH - ] when dissolved in water.

3. 3 Arrhenius Acids -Strong acids completely ionize in water to release H + (H 3 O + ) and an anion; Strong Acids: HClO 4 H 2 SO 4 HI HBr HCl HNO 3 HI + H 2 O  H 3 O + + I - or HI  H + + I - (100%)

4. 4 Arrhenius Acids - Weak acids only partially ionize in water; Weak Acids: HC 2 H 3 O 2 HF H 3 PO 4 HCN H 2 S HNO 2 NH 4 + HC 2 H 3 O 2 H + + C 2 H 3 O 2 -

5. 5 Brønsted-Lowry Acids and Bases The Arrhenius definition does not apply when water is not the solvent; so, a broader definition (Brønsted-Lowry) was generated. Acid:A proton donor Base:A proton acceptor Example: HCl (aq)  H + (aq) + Cl - H + (aq) + :NH 3(aq)  NH 4 + (aq)

6. 6 Conjugate Acids and Bases: From the Latin word conjugare, meaning “to join together.” Reactions between acids and bases always yield their conjugate bases and acids.

7. 7 Examples: HF + HCO 3 - H 2 CO 3 + F - HF = Acid F - = Conjugate Base HCO 3 - = BaseH 2 CO 3 = Conjugate Acid CH 3 NH 2 + HI  CH 3 NH 3 + + I - Which is conjugate acid & which is conjugate base? I - = conjugate base CH 3 NH 3 + = conjugate acid

8. 8 Strong Acids You will recall that the six strong acids are HCl, HBr, HI, HNO 3, H 2 SO 4, and HClO 4. These are, by definition, strong electrolytes and exist totally as ions in aqueous solution. For the monoprotic strong acids, [H 3 O + ] = [acid].

9. 9 Strong Bases Strong bases are the soluble hydroxides, which are the alkali metal and heavier alkaline earth metal hydroxides (Ca 2+, Sr 2+, and Ba 2+ ). Again, these substances dissociate completely in aqueous solution.

10. 10 Autoionization of Water H 2 O (l) + H 2 O (l) H 3 O + (aq) + OH − (aq) Autoionization is the self-ionization of a neutral molecule into positive and negative ions. What is the equilibrium-constant expression for this equilibrium?

11. 11 At 25°C, K w = 1.0  10 −14 and [H + ] = [OH - ] = 10 -7 K w is also called ion-product of water as well as autoprotolysis of water The expression is…. 2 H 2 O H 3 O + + OH - K = [H 3 O + ][OH - ]/[H 2 O] 2

12. 12 pH Scale In most cases, the hydrogen ion concentration is very small which makes it difficult to practically express a meaningful concept for such a small value. Currently, the pH scale is used to better have an appreciation of the value of the hydrogen ion concentration where: pH = - log [H + ] pH + pOH = 14

13. 13 In the case of pure water at 25 o C, the pH is 7 and the solution is said to be neutral.

14. 14 What is the concentration of OH - ions in a HCl solution whose hydrogen ion concentration is 0.013 M? K w = [H + ][OH - ] = 1.0 x 10 - 14 [H + ] = 0.013 M [OH - ] = KwKw [H + ] 1 x 10 -14 0.013 = = 7.7 x 10 -13 M

15. 15 The OH - ion concentration of a blood sample is 2.5 x 10 -7 M. What is the pH of the blood? pH + pOH = 14.00 pOH = -log [OH - ] = -log (2.5 x 10 -7 ) = 6.60 pH = 14.00 – pOH = 14.00 – 6.60 = 7.40

16. 16 What is the pH of a 2 x 10 -3 M HNO 3 solution? HNO 3 is a strong acid – 100% dissociation. HNO 3 (aq) + H 2 O (l) H 3 O + (aq) + NO 3 - (aq) pH = -log [H + ] = -log [H 3 O + ] = -log(0.002) = 2.7 Start End 0.002 M 0.0 M

17. 17 More detailed manipulation: Find the pH of a 0.1 M HCl solution. Solution HCl is a strong acid that completely dissociates in water, therefore we have HCl  H + + Cl - H 2 O  H + + OH - [H + ] Solution = [H + ] from HCl +[H + ] from water

18. 18 However, [H + ] from water = 10 -7 in absence of a common ion, therefore it will be much less in presence of HCl and can thus be neglected as compared to 0.1 (0.1>>[H + ] from water ) Always neglect [H + ] from water when [H + ] from ACID is larger than 10 -6 M [H + ] solution = [H + ] HCl = 0.1 pH = -log 0.1 = 1

19. 19 Calculate the pH of the solution that is 0.04 M Ca(OH) 2 in water. [OH - ] = [OH - ] Ca(OH)2 + [OH - ] water Always neglect [OH - ] water when [OH - ] base is larger than 10 -6 M [OH - ] Ca(OH)2 = 0.08 M [H + ] = 10 -14 /0.08 = 1.2x10 -12 M, Therefore: [OH - ] = [OH - ] Ca(OH)2 = 0.08 M pOH = 1.097 pH = 14 – 1.097 = 12.9

20. 20 What is the pH of a 1.8 x 10 -2 M Ba(OH) 2 solution? Ba(OH) 2 is a strong base – 100% dissociation. Ba(OH) 2 (s) Ba 2+ (aq) + 2OH - (aq) Start End 0.018 M 0.036 M0.0 M pH = 14.00 – pOH = 14.00 + log(0.036) = 12.6

21. 21 The pH of rain water in northeastern United States on a particular day was 4.82. Find [H + ]. pH = -log [H + ] [H + ] = 10 -pH [H + ] = 10 -4.8 [H + ] = 1.5*10 -5 M

22. 22 Acid - Base Strengths - Two factors influence acid strength: 1) Bond Polarity- The more polar the bond, the stronger the acid. - HCl (∆EN = 0.9) > > H 2 S (∆EN = 0.4) 2) Bond Strength- The weaker the bond, the stronger the acid. - HI >> HF. HI bond is weaker than that of HF bond. - Bond strength is the larger of the two factors.

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24. 24 Strength of Oxyacids