Chapter 15 notes

Chapter 15 notes - 15.1 Solutions of Acids or Bases...

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15.1 Solutions of Acids or Bases Containing a Common Ion common ion effect- a shift in equilibrium position due to the addition of an ion already involved in the equilibrium reaction Example HF and NaF H F (s) ! Na + (aq) + F - (aq) HF (aq) H + (aq) + F - (aq) " Equilibrium Shift " The additional F - decrease the pH by increasing the formation of molecular HF 15.2 Buffered Solutions Buffered Solutions- resist pH changes when additional OH - or H + are added . Buffered solutions contain weak acids or bases with their salts (ex. HF and NaF, or NH 3 and NH 4 Cl) SAMPLE EXERCISE 15.3 Calculate the pH change that occurs when 0.010 mol solid NaOH is added to 1.0 L of a buffered solution containing 0.50 M acetic acid and 0.50 M sodium acetate. OH - + HC 2 H 3 O 2 C 2 H 3 O 2 - + H 2 O Before 0.010 mol 0.50 mol 0.50 mol After 0.010-0.010 0.50-0.010 0.50+0.010 0 mol 0.49 mol 0.51 mol mol/L HC 2 H 3 O H + + C 2 H 3 O 2 - initial 0.49 0 0.51 x -x +x +x equilibrium 0.49 x 0.51 1.8 x 10 -5 = x(0.51) 0.49 1.7 x 10 -5 = x (5% rule satisfied) pH= -log[H+] = -log[1.7 x 10 -5 ] = 4.76 *pH without the addition of excess OH - is 4.74 (only an increase of +0.002 pH units) Hints in Solving Buffer Problems 1. Determine what the new concentrations of the species involved in equilibrium after the addition of H + or OH - ions. Assume the reaction between H and OH goes to completion. 2. Proceed with equilibrium calculations to determine pH Buffering: How does it work? - pH is governed by the ratio of [HA]/[A - ] [H+]= K a [HA] [A - ] - If the amounts of HA and A are large in comparison to the amount of H or OH added, then the change in the ratio is small.
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- If we can assume that the changes of concentration of H + and A - are insignificant (initial concentrations are equal to equilibrium concentrations), then we can use the Henderson-Hasselbalch equation to solve for pH. pH= pK a + log([base]/ [acid]) see SAMPLE EXERCISES 15.4, 15.5, 15.6 15.3 Buffer Capacity Buffer Capacity represents the amount of protons and hydroxide ions that can be absorbed without a significant change in pH - Buffer solutions containing large quantities of buffering components will have a high buffering capacity - Optimal buffering occurs when the ratio of [A - ] and [HA] is 1. ( the concentrations are equal). - The best buffer will be a weak acid with a pKa similar to the desired pH see SAMPLE EXERCISE 15.8 15.4 Titrations and pH Curves pH curve (titration curve) - a curve based on the plotting of the pH of solution being analyzed as a function of the amount of titrant (standard) added. Strong Acid-Strong Base Titrations
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Chapter 15 notes - 15.1 Solutions of Acids or Bases...

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