Acid Base Lecture Kotz

Acid Base Lecture Kotz - Acid-Base Chemistry Weak Acid...

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1 Acid-Base Chemistry Strong vs Weak Acids and Bases Based on the extent of ionization in solution Weak Acid Ionization Constant 5 3 2 3 3 2 [ ][ ] 1.8 10 [ ] CH CO H O K CH CO H + = = × Bronsted Acid Proton Donor Bronsted Base Proton Acceptor Polyprotic Acids Capable of donating more than one proton
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2 Amphiprotic means able to act as an acid or a base [i.e. can gain or lose a proton] and amphoteric means able to react with an acid or a base. Ex. Aluminium oxide is amphoteric in acid: Al 2 O 3 + 6HCl 2AlCl 3 + 3H 2 O in base: Al 2 O 3 + 2NaOH + 3H 2 O 2NaAl(OH) 4 NaAl(OH) 4 is sodium aluminate Water is both amphoteric and amphiprotic Exercise 17.1 a) Write a balanced equation for the reaction that occurs when H 3 PO 4 , phosphoric acid, donates a proton to water to form the dihydrogen phosphate ion. Is the dihydrogen phosphate ion an acid, a base, or amphiprotic? b) Write a balanced equation for the reaction that occurs when the cyanide ion, CN - , accepts a proton from water to form HCN. Is CN - a Bronsted acid or base? a) H 3 PO 4(aq) + H 2 O (l) H 2 PO 4 - (aq) + H 3 O + (aq) Since H 3 PO 4 is a weak acid, H 2 PO 4 - can act as both an acid and a base (amphiprotic). It’s preferred direction is going to be dependent upon the K a value of the dihydrogen phosphate ion. b) CN - (aq) + H 2 O (l) HCN (aq) + OH - (aq) The CN- is acting as a Bronsted base because it is a proton acceptor. Conjugate Acid-Base Pairs Differ by one H +
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3 Exercise 17.2 In the following reaction, identify the acid on the left and its conjugate base on the right. Similarly, identify the base on the left and its conjugate acid on the right. HNO 3(aq) + NH 3(aq) NH 4 + (aq) + NO 3 - (aq) Answer HNO 3 and NH 4 + are acids; NH 3 and NO 3 - are bases. The Autoionization of Water Pure water will autoionize to an extent that is temperature dependent Where Does the pH Scale Come From? In any sample of pure water a certain number of water molecules will interact with one another H 2 O (l) + H 2 O (l) H 3 O + (aq) + OH - (aq) This is known as the autoionization of water Hydronium and hydroxide are produced in a 1:1 ratio At 25 o C, the concentration (molarity) of H 3 O + or OH - at any given time (in pure water) is 1.0x10 -7 M The product of the concentrations is given by: [H 3 O + ][OH - ] = [1.0x10 -7 ][1.0x10 -7 ] = 1.0x10 -14 = K w K w = Ion product constant of water The product of the hydronium and hydroxide ion concentration in any solution must always be concentration in any solution must always be K w Implications: As the concentration of H 3 O + goes up, OH - goes down and vice versa such that the product is always K w Concentrations of H 3 O + greater than 1.0x10 -7 will have a pH less than 7 (given pH = -log[H 3 O + ]) and be acidic
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This note was uploaded on 12/07/2011 for the course CHEM 100 taught by Professor Feebeck during the Fall '10 term at Purdue.

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Acid Base Lecture Kotz - Acid-Base Chemistry Weak Acid...

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