3. Equilibrium in Aqueous Solutions Acids and Bases The ideas we have discussed about chemical equilibrium are very useful to understand the behavior of acids and bases in water.
4. Water’s Role as Acid or Base Water acting as a Base HA + H 2 O H 3 O + + A - base acid Water acting as an Acid B + H 2 O BH + + OH - acid base Water behaves like a base when an acid is present. Water behaves like an acid when a base is present.
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9. Calculating [H 3 O + ] and [OH - ] Now, do it yourself: Calculate the concentration of OH - and H 3 O + in a 6.0 M solution of nitric acid ( HNO 3 ). Analyze the results ( How acidic is this solution? How do you know )
10. Acidity and Basicity A solution is considered acidic when [H 3 O + ] > [OH - ] A solution is considered basic when [H 3 O + ] < [OH - ] A solution is neutral when [H 3 O + ] = [OH - ]
11. A common way to express acidity and basicity is with pH pH = - log [H 3 O + ] or [H 3 O + ]=10 -pH Acidity and Basicity The # of sig figs in the concentration = the number of decimal places in pH In an acidic solution, [H 3 O + ]= 1.25 x 10 -4 M at 25 o C pH =-log (1.25 x 10 -4 )= -(-3.903) = 3.903
12. What is the pH of Black Coffee, [H 3 O + ]= 1.0 x 10 -5 M?
13. What is the H 3 O + concentration in sea water if pH=8.30?
14. What is the OH ¯ concentration in acid rain if pH=5.25?
21. HNO 3 , HCl, HBr, HI, H 2 SO 4 and HClO 4 are strong acids Strong Acids The H 3 O + ion forms strong hydrogen bonds with surrounding water molecules
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25. Weak Bases Weak bases are molecules that have an affinity to ACCEPT an H + Weak base: only a small percentage ionized in water One of the best known weak bases is ammonia NH 3 (aq) + H 2 O(liq) NH 4 + (aq) + OH - (aq) H + How would you express K b for this reaction?
26. Conjugate Acid-Base Pairs NH 4 + / NH 3 is a conjugate acid-base pair — related by the gain or loss of H + Every acid has a conjugate base and vice-versa. Consider the acid-base reaction: H + H + Acid
27. The Conjugate Pairs in Some Acid-Base Reactions Conjugate Pair Conjugate Pair Conjugate Acid-Base Pairs HPO 4 2- SO 3 2- + PO 4 3- HSO 3 - + Identify the acid, the base, and the conjugate acid/base pairs: Base Acid + Acid Base +
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32. Relative Strengths of Acid and Bases As acid strength decreases, base strength increases; the weaker the acid, the stronger its conjugate base. As base strength decreases, acid strength increases; the weaker the base, the stronger its conjugate acid.
33. Relative Strengths of Acid and Bases The stronger acid and the stronger base will always react to form a weaker conjugate base and a weaker conjugate acid. Would you expect these acids and bases to react? What would be the outcome? CN - (aq) + HCl(aq) HF(aq) + Cl - (aq) H 2 O(l) + S 2- (aq)
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38. 2. What is the [H 3 O + ] of a 0.0050 M Ca(OH) 2 solution?
39. 3. What is the pOH of a 0.0028 M HNO 3 solution?
