CHM 116 Ch 17-Aqueous Equilibria

CHM 116 Ch 17-Aqueous Equilibria - Chapter 17 Additional...

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1 1 1 Chapter 17 Additional Aspects of Aqueous Equilibria z Water is a common solvent. Dissolved materials can be involved in different types of chemical equilibria. z Acid-Base z Solubility 2 17.1 The Common Ion Effect z Metal ions or salts containing a conjugate weak acid or base ion can shift the pH of a solution. z We can predict pH changes using Le Chatelier’s principle z HA + H 2 O U A - + H 3 O + z B + H 2 O U HB + + OH - 3 Common Ion Effect z CH 3 CO 2 H + H 2 O U CH 3 CO 2 - + H 3 O + z Adding more CH 3 CO 2 - to the solution shifts the equilibrium to the left, making the solution less acidic (higher pH). z 0.100 M CH 3 CO 2 H pH = 2.879 z 0.100 M CH 3 CO 2 H, 0.050 M NaCH 3 CO 2 pH = 4.456 4 pH of Weak Acid Calculation z What is the pH of a 1.00 M HF solution? z K a = 7.0 x 10 - 4 z HF H 3 O + F - Initial 1.00 0 0 Change -x +x +x Equil. 1.00 - x x x z x 2 /(1.00 - x) = 7.0 x 10 - 4 z Assume x << 1.00: x 2 /1.00 = 7.0 x 10 - 4 z x = 2.65 x 10 - 2 , so the assumption was okay z pH = -log(2.65 x 10 - 2 ) = 1.576 5 Group Work z What is the pH of 1.00 M HF solution to which is added 0.500 M NaF? K a = 7.0 x 10 -4 z HF H 3 O + F - Initial 1.00 0 0.500 Change -x +x +x Equil. 1.00 - x x 0.500 + x z x(0.500 + x)/(1.00 - x) = 7.0 x 10 - 4 z Assume x << 0.500 and 1.00: x(0.500)/1.00 = 7.0 x 10 - 4 z x = 1.40 x 10 - 3 (.14%), so assumption was okay z pH = -log(1.40 x 10 - 3 ) = 2.854 6 17.2 Buffered Solutions z Buffer : Solution of a weak acid and its conjugate base in approximately equal concentrations z Used to control pH z Addition of a strong acid or base to a buffer shifts the pH only slightly
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2 2 7 Buffer Solutions z Make buffer: Mix HF and NaF in water. z HF(aq) + H 2 O(l) U H 3 O + (aq) + F - (aq) z Add HCl: Equilibrium shifts to left to consume some of added H 3 O + z Add NaOH: Some H 3 O + is consumed to neutralize the added OH - , and the equilibrium shifts to the right to replace some of the consumed H 3 O + . z The pH change is much smaller than if HCl or NaOH was added to water. 8 Buffer Solutions z pH of all body fluids is controlled in this way. In our blood, phosphate, carbonate, and hemoglobin all act as buffers. z H 2 CO 3 (aq) + H 2 O(l) U HCO 3 - (aq) + H 3 O + (aq) z Secondary Equilibrium: z H 2 CO 3 (aq) U H 2 O(l) + CO 2 (g) 9 Addition of acid or base to a buffer 10 Buffer Solutions z Recall that a solution of 1.00 M HF and 0.500 M NaF has a pH of 2.854 z Now add 0.100 M HCl (assume there is no volume change) z To find the new pH, first assume that all the added strong acid reacts to change the F - and HF concentrations. z [HF] = 1.00 + 0.100 = 1.100 M z [F - ] = 0.500 - 0.100 = 0.400 M 11 Buffer Solutions z Now solve the weak acid/conjugate base system: z HF H 3 O + F - Initial 1.100 0 0.400 Change -x +x +x Equil. 1.100 - x x 0.400 + x z x(0.400 + x)/(1.100 - x) = 7.0 x 10 - 4 z Assume x << 0.400 and 1.100: x(0.400)/1.100 = 7.0 x 10 - 4 z x = 1.93 x 10 - 3 , so the assumption was okay z pH = -log(1.93 x 10 -3 ) = 2.714 12 Buffer Solutions z Water: pure 0.100 M H 3 O + , pH 7 pH 1 z Buffer: pH 2.854 pH 2.714 z What will happen when a strong base (NaOH) is added to: z Water? z Buffer?
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3 3 13 Buffer Solutions z If base is added, assuming no volume change, reduce the concentration of HF and increase the concentration of F - by the corresponding amount. Then solve the weak acid/conjugate base system.
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CHM 116 Ch 17-Aqueous Equilibria - Chapter 17 Additional...

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