Chapter%2017%20-%20Additional%20Aspects

Chapter%2017%20-%20Additional%20Aspects - Chapter 17...

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1 Chapter 17 Additional Aspect of Aqueous Equilibria
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2 In Chapter 16, we learned … 2 the ionization of strong acids & bases the ionization of weak acids the ionization of weak bases the ionization of salts (strong electrolytes) (weak electrolytes) (weak electrolytes) (strong electrolytes) What if a strong electrolyte is added to a weak electrolyte?
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3 For Example: In which direction the equilibrium will shift when you add NaC 2 H 3 O 2 to the acetic acid solution? Remember! NaC 2 H 3 O 2 is a strong electrolyte. It completely dissociates in water to produce C 2 H 3 O 2 - ion and Na + ion. Consider the ionization of acetic acid:
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4 17.1 The Common-Ion Effect According to Le Châtelier’s principle, if acetate ion is added, the equilibrium will shift to the left . Addition of C 2 H 3 O 2 - shifts equilibrium, reducing [H + ] “The extent of ionization of a weak electrolyte is decreased by adding to the solution a strong electrolyte that has an ion in common with the weak electrolyte” Common-ion Effect Thus, addition of a common ion could change the pH of the starting solution. pH will increase
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5 Practice Exercise Identify the common-ion and indicate whether the pH increase, decrease or remain constant in the following reactions? (a) NaNO 2 is added to a solution of HNO 2 . (b) (CH 3 NH 3 )Cl is added to a solution of CH 3 NH 2 . (c) Sodium formate is added to a solution of Formic acid. (d) KBr is added to a solution of HBr. 2 3 2 Try this. ..
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6 The Common-Ion Effect - Contd. .. Now, we need to find [H + ] eq after having the common ion F - ion from NaF and then calculate pH. First of all, identify the principal equilibrium. Calculate the pH of a solution that contains 0.50 M HF and 0.30 M NaF. K a for HF is 6.8 × 10 −4 . How do we calculate the pH of a solution made of a weak and a strong electrolyte?
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7 The Common-Ion Effect - Contd. .. Because NaF, a strong electrolyte is also present, the initial [F ] is not 0, but rather 0.30 M. [HF], M [H 3 O + ], M [F ], M I nitial C hange E quilibrium Now we can set up an ICE table… 0.50 0.30 x + x + x x 0.50 - x 0.30 + x
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8 The Common-Ion Effect - Contd. .. ( x ) (0.30) (0.50) Substitute the equilibrium concentrations from the ICE table and assume x is very small: keep x in multiplication, omit x in addition or subtraction. What about the % ionization of HF in presence of NaF? = x
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9 17.2 Buffered Solutions Composition: - Solutions of a weak acid and its conjugate base. or - Solution of weak base and its conjugate acid. Buffers are very important in biochemical and biological systems. - Blood is a complex aqueous mixture with a pH regulated at 7.4 for its function. Find out the buffer system that operates in our biological system. HF/NaF NH 3 /NH 4 Cl
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10 Buffered Solutions or Buffers - Contd. .. Behavior: Because a buffer contains both an acidic species to neutralize OH - ions and a basic species to neutralize H + ions! (see slide 16) How do they resist pH change?
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This note was uploaded on 05/05/2008 for the course CHEM 178 taught by Professor Gupta during the Spring '08 term at N. Arizona.

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Chapter%2017%20-%20Additional%20Aspects - Chapter 17...

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