2.1 16 Strong and weak acids By the end of this spread, you should be able to … 1Explain qualitatively the differences between strong and weak acids. K 1Explain that the acid dissociation constant, a , shows the extent of acid dissociation. K K 1Deduce expressions for a and p a for weak acids. Acid–base equilibria HCl hydrochloric acid In aqueous solution, acids dissociate and equilibrium is set up. The equilibrium below shows the dissociation of an acid, HA, in water: HNO nitric acid 3 + – HA(aq) + H2O(l) 5 H3O (aq) + A (aq) H SO sulfuric acid 2 4 or more simply: HBr hydrobromic acid HA(aq) 5 H+(aq) + A–(aq). HI hydriodic acid The strength of an acid HA is the extent of its dissociation into H+ and A– ions. HClO4 chloric(VII) (perchloric) acid Strong acids Table 1 Strong acids Some acids are strong – they are 100% dissociated in aqueous solution. There are comparatively few strong acids; the commonest are listed in Table 1. Almost every other Key definitions acid is weak. A strong acid is an acid that completely dissociates in aqueous solution. Weak acids A weak acid only partially dissociates in aqueous solution. Many naturally occurring A weak acid is an acid that partially acids are weak. dissociates in aqueous solution. The equilibrium that is set up when ethanoic acid, CH3COOH, dissociates in water is shown below: + – CH3COOH(aq) 5 H (aq) + CH3COO (aq) s 4HEEQUILIBRIUMPOSITIONLIESWELLOVERTOTHELEFT + – s 4HEREAREONLYSMALLCONCENTRATIONSOFDISSOCIATEDIONS ( (aq) and CH3COO (aq), compared with the concentration of undissociated CH3COOH(aq). K The acid dissociation constant, a The actual extent of acid dissociation is measured by an equilibrium constant called the acid dissociation constant, Ka. Figure 1 Formic acid (methanoic acid, A weak acid HA has the following equilibrium in aqueous solution: HCOOH) is a weak acid that occurs HA(aq) 5 H+(aq) + A–(aq) naturally in bee and ant stings. Formic acid makes up 55–60% of the body mass The expression for the acid dissociation constant is: of a typical ant; its name comes from [H+(aq)][A–(aq)] ‘formica’, the Latin word for ant. The acid K = _____________ a [HA(aq)] was once prepared by distilling ants! –3 The units of Ka are always mol dm . You can show this by cancelling the units in the Ka expression: Key definition (mol dm–3)(mol dm–3) K = __________________ = mol dm–3 a (mol dm–3) The acid dissociation constant, Ka, of an acid HA is defined as The terms strong and weak describe the extent of dissociation of an acid given by the Ka [H+(aq)][A–(aq)] value: ____________ Ka = [HA(aq)] s ALARGEKa value indicates a large extent of dissociation – the acid is strong s ASMALLKa value indicates a small extent of dissociation – the acid is weak. 142 179 A2 chemistry.U2 M1.indd 142 29/10/08 13:35:02 Module 1 Rates, equilibrium and pH K K Strong and weak acids a and p a + As with pH and [H (aq)] (see spread 2.1.15), values of Ka can be made more manageable if expressed in a logarithmic form called pKa. + Ka and pKa conversions are just like those between pH and H : s PKa = –log10 Ka –pKa s Ka = 10 s !LOWVALUEOFKa matches a high value of pKa K matches a low value of pK s !HIGHVALUEOF a a Key definition s 4HESMALLERTHEPKa value, the stronger the acid. pKa = –log10 Ka Table 2 below compares Ka and pKa values for some weak acids. –pKa Ka = 10 K –3 K Acid Relative strength a/mol dm p a –3 –3 Phosphoric acid H3PO4 Stronger acid 7.9 × 10 –log(7.9 × 10 ) = 2.10 –3 –2 Sulfurous acid H2SO3 1.5 × 10 –log(1.5 × 10 ) = 2.82 Methanoic acid HCOOH 1.6 × 10–4 –log(1.6 × 10–4) = 3.80 –5 –5 Ethanoic acid CH3COOH Weaker acid 1.7 × 10 –log(1.7 × 10 ) = 4.77 Table 2 Ka and pKa values of weak acids STRETCH and CHALLENGE Figure 2 Juices of citrus fruits, such as lemon and lime, contain citric acid Citric acid makes up as much as 8% of the dry mass of a citrus fruit. Citric acid is a weak tribasic acid. It releases its three acidic protons in three steps, each with its own pKa value: C H OH(COOH) H+(aq) + C H OH(COOH) (COO–)pK = 3.15 3 4 3 5 3 4 2 a C H OH(COOH) (COO–) H+(aq) + C H OH(COOH)(COO–) pK = 4.77 3 4 2 5 3 4 2 a C H OH(COOH)(COO–) H+(aq) + C H OH(COO–) pK = 6.40 3 4 2 5 3 4 3 a Notice how the acid becomes weaker at each dissociation. This is typical of a multibasic acid. Questions 1 For each of the following acid–base equilibria, write down the expression for Ka: (a) HCOOH(aq) 5 H+(aq) + HCOO–(aq) + – (b) CH3CH2COOH(aq) 5 H (aq) + CH3CH2COO (aq). 2 Calculate pKa from the following Ka values. Give you answers to two decimal places. –1 –3 (a) Ka = 2.3 × 10 mol dm –3 –3 (b) Ka = 2.5 × 10 mol dm –11 –3 (c) Ka = 4.8 × 10 mol dm . 3 Calculate Ka from the following pKa values. Give your answers to three significant figures. (a)pKa = 2.90; (b) pKa = 7.20; (c) pKa = 10.60. 143 179 A2 chemistry.U2 M1.indd 143 29/10/08 13:35:06.