Chapter 14 - ACIDS, BASES AND SALTS Water (H2 O) is made of...

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ACIDS, BASES AND SALTS Water (H 2 O) is made of two ions H + (aq) – hydrogen ion OH - (aq) – hydroxide ion Acids – substances that increase H + (aq) conc. Strong acids - strong electrolytes, i. e., completely dissociate HCl (aq) = H + (aq) + Cl - (aq) - memorize list HCl, HBr, HI, HNO 3 , H 2 SO 4 , HClO 3 , HClO 4 Weak acids - weak electrolytes HF (aq) D H + (aq) + F - (aq) HF (aq) HF (aq) + H + (aq) + F - (aq) a lot a little a little H 3 PO 4 (aq) D H + (aq) + H 2 PO 4 - (aq) Bases – substances that increase OH - concentration Strong bases - strong electrolytes - all alkali metal hydroxides LiOH, NaOH, KOH, RbOH, CsOH - some alkaline earth metal hydroxides Ca(OH) 2 , Sr(OH) 2 , Ba(OH) 2 Weak bases - weak electrolytes - usually increase OH - (aq) conc. “indirectly” by decreasing H + (aq) conc. NH 3 (aq) + H 2 O (l) D NH 4 + (aq) + OH - (aq) Neutralization reaction Acid + Base Water + Salt Salt - ionic compound - cations and anions remaining after water is made HNO 3 (aq) + KOH (aq) H 2 O (l) + KNO 3 (aq) acid base water salt
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THE AUTOIONIZATION OF WATER - Water is not a nonelectrolyte, actually it is a very weak electrolyte. - One out of every 10 million water molecules dissociates into an H + ion and a OH - ion. H 2 O (l) D H + (aq) + OH - (aq) Ion product of water Write equilibrium expression [ ][ ] [ ] K H OH H O c = + - 2 - Since H 2 O is solvent, we will neglect it in equilibrium expression. No matter what conditions exist in an aqueous solution, the following expression is true. (at 25 ° C) [ ][ ] H OH M + - - = × 10 10 14 2 . Ion product is also known as K w . [ ][ ] K H OH M w = = + - - 10 14 2 Aside: Other solvents autoionize as well. NH 3 + NH 3 D NH 4 + + NH 2 - CH 3 OH + CH 3 OH D CH 3 OH 2 + + CH 3 O - Since [ ][ ] H OH M + - - = 10 14 2 is always true - if we know [H + ], we can calculate [OH - ] - if we know [OH - ], we can calculate [H + ] Example: If a solution has [H + ] = 1.8 x 10 -5 M, what is [OH - ]? Example: If a solution has [OH - ] = 9.4 x 10 -12 M, what is [H + ]?
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Example: What is the [H + ] and [OH - ] for pure water? H 2 O(l) D H + (aq) + OH - (aq) [ ][ ] K H OH w = + - [H+] [OH-] Initial 0 0 Change x x Equil. x x x M 2 14 2 10 = - x x M 2 7 10 = = - [ ] H M + - = 10 7 [ ] OH M - - = 10 7 Note: When [H + ] = [OH - ], the amount of acid and base are in equal amounts. The solution is called neutral . THE HYDRONIUM ION H + is simply a proton. - A proton will hydrogen bond with water H 3 O + is a hydronium ion . Current theories of water treat proton as surrounded by 4 or 6 water molecules. Therefore, H + may be truly H 9 O 4 + or H 13 O 6 + . H 3 O + is a concept, not an actual structure. Reconsider autoionization of water as H 2 O (l) + H 2 O (l) D H 3 O + (aq) + OH - (aq) H + H H O H H O H + O H H O H H O H H O H H H +
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pH SCALE Definition: pH = - log [H + ] = -log[H 3 O + ] pH scale is used strictly for convenience - convenient to avoid scientific notation. [H+] for pure water equals 10
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This note was uploaded on 09/27/2010 for the course CHEM 220 taught by Professor Bates during the Spring '10 term at Skyline College.

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Chapter 14 - ACIDS, BASES AND SALTS Water (H2 O) is made of...

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