16 Acid and Bases

16 Acid and Bases - 16. Acids and Bases Note : Practice...

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© David Hanson, Stony Brook University, 2008 Reproduction or distribution by any means is prohibited by law. 1 16. Acids and Bases Note : Practice problems in the MSJ textbook Chapter 16.are given in parentheses. 16.1. Brønsted-Lowry Acids and Bases A Brønsted-Lowry acid is a substance that donates protons (hydrogen ions, H + ) to another substance. (MSJ QRT 16.12) A Brønsted-Lowry base is a substance that accepts protons from another substance. (MSJ QRT 16.16) An acid-base reaction consists of a proton transfer between an acid and a base. (MSJ QRT 16.28) In a neutralization reaction an acid and base react completely with each other to form a salt. Water can act either as an acid or a base. Water can donate a proton and become a hydroxide ion, OH - , or it can accept a proton and form the hydronium ion, H 3 O + . A substance like water that can both donate and accept protons is called amphiprotic . A substance that can act both as an acid or a base is called amphoteric . All amphiprotic substances also are amphoteric. ZnO is an example of a substance that is amphoteric but not amphiprotic. As a base neutralizing an acid: ZnO + 2HCl ZnCl 2 + H 2 O, (16.1.1) and as an acid neutralizing a base and forming an hydroxide salt: ZnO + 2NaOH Na 2 [Zn(OH) 4 ]. (16.1.2) A conjugate acid-base pair is a pair of molecules or ions that are related to each other by the transfer of a single proton from one to the other, for example NH 4 + and NH 3 . (MSJ QRT 16.24) 16.2. Acid –Base Reaction Equilibria The reaction of acids (16.2.1) and bases (16.2.2) with water is called ionization . HA + H 2 O A - + H 3 O + (16.2.1) B + H 2 O HB + OH - (16.2.2) Water can react with itself. This reaction is called autoionization . H 2 O + H 2 O H 3 O + + OH - (16.2.3) All ionization reactions are characterized by equilibrium constants: K a for acid ionization, K b for base ionization, and K w for water autoionization. The equilibrium constant expressions for reactions 16.2.1 – 16.2.3 are in order (MSJ QRT 16.49)   3 a H O A K HA     (16.2.4)     b HB OH K B (16.2.5) 3 w K H O OH     (16.2.6)
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© David Hanson, Stony Brook University, 2008 Reproduction or distribution by any means is prohibited by law. 2 At 298 K, K w = 1.00 x 10 -14 . Pure water is called neutral because the hydronium ion concentration equals the hydroxide ion concentration, which follows from the stoichiometry in Equation 16.2.3. An acidic solution has the hydronium ion concentration greater than the hydroxide ion
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This note was uploaded on 06/29/2008 for the course CHE 132 taught by Professor Hanson during the Spring '08 term at SUNY Stony Brook.

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16 Acid and Bases - 16. Acids and Bases Note : Practice...

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