Ch 17 Principles of Chemical Reactivity The Chemistry of Acids and Bases Acids and Bases are among the most common substances in nature 0 1 M HCl in stomach Drano solid NaOH with some Al chips Organic acids carboxylic acids occur in nature acetic acid vinegar Esters made from carboxylic acid and alcohol have pleasant odor 1 17 1 Acids and Bases A Review Arrhenius definition Acids are substances that contain hydrogen and produce H in aqueous solutions HCl aq H 2 O H 3O aq Cl aq HCO H H O H O HCO 2 aq 2 3 aq 2 aq Bases are substances that contain the hydroxyl OH group and produce hydroxide ions OH in aqueous solutions aq NaOH Na OH aq 2 Ca OH Ca 2 OH 2 aq aq 2 Br nsted Lowry definition An acid is a proton donor H A base is a proton acceptor NH3 H 2 O NH 4 OH base acid HBr H 2 O H 3O Br acid base 3 The Br nsted Lowry Theory An important concept in Br nsted Lowry theory involves the relative strengths of acidbase pairs Weak acids have strong conjugate bases Weak bases have strong conjugate acids The weaker the acid or base the stronger the conjugate partner The reason why a weak acid is weak is because the conjugate base is so strong it reforms the original acid Similarly for weak bases 4 NH 3 H 2O NH 4 OH Since NH3 is a weak base NH4 must be a strong acid NH4 gives up H to reform NH3 Compare that to NaOH Na aq OH aq Na must be a weak acid or it would recombine to form NaOH Remember NaOH ionizes 100 NaOH is a strong base 5 6 Table 3 1 p 129 17 2 The Br nsted Lowry Concept of Acids and Bases Extended Monoprotic acids acids that are capable of donating one proton examples Polyprotic acids acids that are capable of donating more than one proton examples Polyprotic bases can accept more than one proton examples 7 Amphiprotic molecule e g H2O that can behave like an acid or base 8 9 Table 17 1 p 759 Conjugate Acid Base Pairs An important part of Br nsted Lowry acid base theory is the idea of conjugate acid base pairs Two species that differ by a proton are called acid base conjugate pairs For example we can use this reaction HCl H Cl 10 Conjugate acid base pairs are species that differ by a proton 11 Conjugate acid base pairs are species that differ by a proton Some examples 12 HNO3 H2O H3O NO3 13 14 Table 17 2 p 761 17 3 Water and the pH Scale Water Autoionization and the Water Ionization Constant Kw Water can be either an acid or base in Bronsted Lowry theory Consequently water can react with itself This reaction is called autoionization One water molecule acts as a base and the other as an acid H 2O H 2O H 3O OH base1 acid 1 acid 2 base 2 15 Water does not do this extensively H3O OH 1 0 x 10 7 M Reason for pH scale 16 The autoionization of water H 2 O H 2 O H 3O aq OH aq Kc This equilibrium constant is known as the ion product for water Kw 17 Experimental measurements have determined that the concentration of each ion is 1 0 x 10 7 M at 25oC Note that this is at 25oC not every temperature And only for dilute aq solutions which means less than 1 M We can determine the value of Kw from this information 1 0 x 10 1 0 x 10 Kw H 3O OH 7 1 0 x10 7 14 18 Neutral solution H OH In basic solution more OH than H In acidic solution more H than OH 19 Calculate the concentrations of H3O and OH in 0 050 M HCl HCl H 2 O H 3O Cl 20 The pH and pOH scales A convenient way to express the acidity and basicity of a solution is the pH and pOH scales The pH of an aqueous solution is defined as pH log H 3O In general a lower case p before a symbol is read as the negative logarithm of the symbol Thus we can write the following notations pOH log OH 21 Calculating pH If either the H3O or OH is known the pH and pOH can be calculated Calculate the pH of a solution in which the H3O 0 030 M The pH of a solution is 4 597 What is the concentration of H3O 22 A convenient relationship between pH and pOH may be derived for all dilute aqueous solutions at 250C H 3O OH 1 0 10 14 Taking the logarithm of both sides of this equation gives log H 3O log OH 14 00 Multiplying both sides of this equation by 1 gives log H 3O log OH 14 00 Which can be rearranged to this form pH pOH 14 00 23 Very useful Memorize these H O OH 1 0 10 3 14 pH pOH 14 00 24 The usual range for the pH scale is 0 to 14 H O 1 0 M to H O 1 0 10 3 3 pH 0 14 M pH 14 00 to And for pOH the scale is also 0 to 14 but inverted from pH pH 0 has a pOH 14 and pH 14 has a pOH 0 OH 1 0 10 pOH 14 00 14 M up to OH 1 0M pOH 0 25 The pH and pOH scales In a pH value only the digits after the decimal point are significant figures because the ones place came from the log power of 10 26 Calculate the H3O pH OH and pOH for a 0 020 M Ba OH 2 solution Is Ba OH 2 a weak or strong base 27 17 4 Equilibrium Constants for Acids and Bases All strong acids and bases are completely ionized hence they are strong Weak acids only ionize slightly in dilute aq solutions Let s look at the dissolution of acetic acid a weak acid in water as an example The equation for the ionization of acetic acid is CH 3COOH H 2 O H 3O CH 3COO The equilibrium constant for this ionization is expressed as 28 We can define a new equilibrium constant for weak acid equilibria that uses the previous definition This equilibrium constant is called the acid ionization constant The symbol for the ionization constant is Ka Ka H O CH COO 1 8 10 3 3 CH3COOH 5 for acetic acid 29 In simplified form the dissociation equation and acid ionization expression are written as CH 3COOH H CH 3COO H CH COO 1 8 10 Ka 3 CH3COOH 5 30 NH3 H2O NH4 OH Kb For weak acids or weak bases the K is less than 1 because the product numerator is less than the equilibrium concentration of the weak acid or base 31 A large value of K indicates that ionization products are strongly favored …