Feb7-AcidBase (1)

Feb7-AcidBase (1) - Acid-Base Chemistry Ka/Kb pH Buffers...

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February 7, 2012 Acid-Base Chemistry, Ka/Kb, pH, Buffers
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Why do chemistry professors always teach a little bit about ammonia? Chemistry Joke of the Day It’s very basic material.
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Opening Prayer Reminder: I’m in the Tutorial lab 2-3 T, 12-1 and 2-3 Th, and 1-3 F. It’s a great place to come in and get help. I’m also available in the mornings (9-10 except for Th) if you e-mail me to set up an appointment (do it!). Practice problem suggestion: Mastering Chemistry has a “study area” which will link you to a site related to the textbook—it’ll have practice quizzes and other problems you can work for practice. Getting Started
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Two definitions (so far): Arrhenius acids/bases Brønstead-Lowry acids/bases Arrhenius definitions Acid: a compound that produces H+ in water Base: a compound that produces OH- in water Brønstead-Lowry definitions What are acids and bases?
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Acids Bases Strong (memorize these! You’ll need to recognize them if mentioned) HCl, H2SO4, HNO3, HI, HBr, HClO4, HClO3 LiOH, NaOH, KOH, RbOH, CsOH, Ca(OH)2, Sr(OH)2, Ba(OH)2 Weak (lots more than this) NH4+, CH3COOH, HF NH3, CH3COO-, F- Examples of Acids and Bases This is not a comprehensive list – anything that meets the definition of an acid or base given in the previous slide would fall into these categories. For most of this class, we’ll be working problems with Arrhenius acids and bases— these would also be Brønstead-Lowry acids and bases, but we’ll be worrying about the concentrations of H+ and OH- in water.
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Consider the following 2 reactions: CH3COOH + H2O D CH3COO- + H3O+ NH3 + H2O D NH4+ + OH- In each reaction, which reactant is the acid and which is the base? In the first reaction, CH3COOH is the acid and water is the base. In the second reaction, NH3 is the base and water is the acid. Conjugate Acid/Base pairs Key point: water is amphiprotic
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You’ll notice that in each reaction, there are 2 pairs of compounds which differ from each other by exactly one H+ ion. CH3COOH + H2O D CH3COO- + H3O+ NH3 + H2O D NH4+ + OH- Acetic acid/acetate ion Water/hydronium ion Ammonia/ammonium ion Water/hydroxide ion Conjugate Acid/Base Pairs, cont. These are conjugate pairs!
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As acid strength increases, conjugate base strength decreases. Describe the strength of the conjugate bases of: H2SO4 HF H2O CH4 Relationship between conjugate acid/base pairs
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Acid Base Strong Negligible Weak Negligible Strong Clarifying – acid/base comparison Remember, a strong acid (or strong base) means that it COMPLETELY dissociates in water. Consider HCl… it will entirely break into hydrogen and chlorine ions. So the conjugate base, Cl-, would never act as a base at all – a base would be accepting a proton (reforming HCl… if that happened, HCl wouldn’t be a strong acid!) In a weak acid or base, there is partial dissociation and thus at equilibrium, you will have some of the compound protonated and some not (e.g., CH3COOH in
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Feb7-AcidBase (1) - Acid-Base Chemistry Ka/Kb pH Buffers...

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