Chapter 8 Introducing acids and bases

Chapter 8 Introducing acids and bases Problems: 1, 3, 5, 10, 13, 17, 20, 21, 22, 27, 32, 33, 34, 35 8-1 What are Acids and Bases Let’s start with the ...
Author: Philip Allen
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Chapter 8 Introducing acids and bases Problems: 1, 3, 5, 10, 13, 17, 20, 21, 22, 27, 32, 33, 34, 35 8-1 What are Acids and Bases Let’s start with the simple Arrhenius concept of acid and base ACIDS are substance that increase the concentration of H3O+ when added to water. BASES decrease the concentration of H3O+ (Hence increase the concentration of OH-) The word PROTIC refers to any chemistry involving the transfer of H+ From one molecule to another. The Bronsted-Lowry concept of acids and bases is also useful. Here an ACID is a proton donor and a BASE is a proton acceptor. The difference between this definition and the one above is that this doesn’t include water, so it is more general (can extend into non-aqueous solvent and gas phase). The product of acid and base reaction are also acids and bases called conjugate acids and bases. A deprotonated acid is the conjugate base and a protonated base is a conjugate acid. Take the case of acetic acid and Methyl amine CH3COOH + CH3NH2 W CH3COO- + CH3NH3+ Acid Base CJ Base CJ Acid SALTS are any ionic solid.. In most cases the product of an acid-base reaction is a Salt. Many salts are strong electrolytes meaning that they completely dissociate into component ions in water. In the above reaction the CH3COO-CH3NH3 + is the salt product. 8-2 Relation Between [H+], [OH-] and pH Water can act as a base or an acid When it reacts with an acid it is a base and when it reacts with a base it is an acid. Autoproteolysis refers to water acting as both a base and an acid to react with itself. H2O + H2O WH2O+ + OHBecause this reaction goes on in water constantly , and most of our reaction are performed in water, this equilibrium is very important and has its own special designation and an easy to remember value Kw = 1x10-14 (This does vary with temp, see Table 5-1 but we won’t worry about that here) Autoproteolysis occurs in all protic solvents, so this is not a special property of water. This also means that in water if you know H+ you know OH- Because Kw=[H+][OH-

2 ] (what happened to the water in this equation?)

Let’s reinforce this concept with some calculations: The [H+] of a solution is 6.7x10-9 is this solution acidic or basic, and what is the [OH-] of the solution? Kw = 1x10-14 = [H+][OH-] = 6.7x10-9 × X X=1,x10-14/ 6.7x10-9 ; X = [OH-] = 1.5x10-6 [H+] < 10-7, [OH-] > 10-7 Basic

Now what is the definition of pH and pOH? pH=-log[H+], pOH = -log[OH-] What is the pH and pOH of the above solution? -log(6.7x10-9) = 8.17 -log(1.5x10-6) = 5.83 pH >7, pOH