CHM152Chapter 15

Chemistry (MasteringChemistry Series)

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1 1 Chapter 15 - Applications of Aqueous Equilibria Neutralization Reactions Strong acid-strong base ± HCl (aq) + NaOH (aq) ´ NaCl (aq) + H 2 O (l) ± What’s in solution? i.e. look at the net ionic equation Weak acid-strong base ± HA (aq) + NaOH (aq) ´ NaA (aq) + H 2 O (l) ± What’s in solution? i.e. net ionic equation ± What will the pH be, roughly? ± Example: Acetic acid and sodium hydroxide Neutralization Reactions Strong acid-weak base ± HCl (aq) + NH 3 (aq) ´ NH 4 + (aq) + Cl - (aq) ± What’s in solution? ± What will the pH be roughly? What will the pH be, roughly? Weak acid-weak base ± CH 3 COOH (aq) + NH 3 (aq) Q NH 4 + (aq) + CH 3 COO - (aq) ± Larger K values than other combinations ± Reaction does not proceed as far toward completion as previous 3 reactions.
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2 2 Neutralization Reactions Predict whether the pH after neutralization will be greater than, less than, or equal to 7 for the following combinations: HNO 2 and KOH HCl and LiOH HBr and NH 3 The Common Ion Effect Metal ions or salts containing a conjugate weak acid or base can shift the pH of a solution. This is the mechanism that controls the pH This is the mechanism that controls the pH of your blood and other biological systems. The Common Ion Effect If we add a conjugate acid (base) to a solution of a weak base (acid), the pH will shift. This is called the Common Ion Effect . NaCH 3 CO 2 added to a solution of CH 3 CO 2 H will make the solution more basic. Why? A common ion will shift a chemical equilibrium in such a direction as to get rid of some of the added ion. (think Le Chatelier’s principle)
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3 3 Common Ion Effect CH 3 CO 2 H + H 2 O CH 3 CO 2 - + H 3 O + Adding more CH 3 CO 2 - to the solution shifts the equilibrium to the left, making the solution less acidic (higher pH). 0.100 M CH 3 CO 2 H pH = 2.879 0.100 M CH 3 CO 2 H, 0.050 M NaCH 3 CO 2 pH = 4.456 CH 3 CO 2 - is an ion common to CH 3 CO 2 H and NaCH 3 CO 2 solutions, consistent with Le Chatelier’s Principle Group Quiz #9 Which of the following salts will change the pH of a solution? NaCl KNO 2 CrCl 3 LiCN KClO 4 Na 2 CO 3 NH 4 Cl CaBr 2 Example Problem What is the pH of 1.00 M HF solution? K a = 7.0 x 10 - 4 HF H 3 O + F - Initia 100 0 0 Initial 1.00 Change -x +x +x Equil. 1.00 - x x x x 2 /(1.00 - x) = 7.0 x 10 - 4 Assume x << 1.00: x 2 /1.00 = 7.0 x 10 - 4 x = 2.65 x 10 -2 , so the assumption was okay pH = -log(2.65 x 10 -2 ) = 1.576
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4 4 Another Example 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 HF H 3 O + F - Initial 1.00 0 0.500 Change -x +x +x Equil. 1.00 - x x 0.500 + x x(0.500 + x)/(1.00 - x) = 7.0 x 10 - 4 Assume x << 0.500 and 1.00: x(0.500)/1.00 = 7.0 x 10 - 4 x = 1.40 x 10 -3 , so the assumption was okay pH = -log(1.40 x 10 -3 ) = 2.854 16.3 Buffer Solutions
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CHM152Chapter 15 - Chapter 15 - Applications of Aqueous...

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