Chemistry 102: Acids and Bases

Acids and Bases CHEM-102 1 Acids and Bases Acids ➢ Have a sour taste. Vinegar owes its taste to acetic acid. Citrus ➢ fruits contain citric acid. ➢ React with certain metals to produce hydrogen gas. ➢ React with carbonates and bicarbonates to produce carbon ➢ dioxide gas. Bases ➢ Have a bitter taste. ➢ Feel slippery. Many soaps contain bases. CHEM-102 2 Definition of Acid/Base Arrhenius acid is a substance that produces H+ (H3O+) in water Arrhenius base is a substance that produces OH- in water → limits bases to compounds that contain the hydroxide ion. A Brønsted- Lowry acid is a proton donor A Brønsted- Lowry base is a proton acceptor A Lewis acid is a substance that can accept a pair of electrons A Lewis base is a substance that can donate a pair of electrons CHEM-102 3 A Brønsted- Lowry acid is a proton donor A Brønsted- Lowry base is a proton acceptor base acid acid base base acid conjugate acid conjugate base CHEM-102 4 Example 1: Identify the conjugate acid-base pairs in the reaction between ammonia and hydrofluoric acid in aqueous solution NH3(aq) + HF(aq) NH+4 (aq) + F-(aq) Solution NH3 has one fewer H atom and one fewer positive charge than F− has one fewer H atom and one more negative charge than HF. The conjugate acid-base pairs are: (1) NH+4 and NH3 and (2) HF and F− CHEM-102 5 Example 2: In the following equations, label each species as an acid or base. Show the conjugate acid–base pairs. HCO3 − (aq ) +HF(aq ) Base Acid H2CO3 (aq ) +F − ( aq ) C. Acid C. Base [H2CO3 and HCO3–] and [HF and F–] are the conjugate acid– base pairs Acid Base C. Base C. Acid [HCO3– and CO32–] and [H2O and OH–] are the conjugate acid– base pairs CHEM-102 6 Lewis Acids and Bases A Lewis acid is a substance that can accept a pair of electrons A Lewis base is a substance that can donate a pair of electrons •• •• + OH •• acid base H+ H+ + acid CHEM-102 •• H N H H base •• H O H •• H + H N H H 7 Lewis Acids and Bases A Lewis acid is a substance that can accept a pair of electrons A Lewis base is a substance that can donate a pair of electrons + F B •• H F N H F H acid base F F B F H N H H No protons donated or accepted! CHEM-102 8 Example 3: Identify the Lewis acid and Lewis base in each of the following reactions: (a) C2H5OC2H5 + AlCl3 (C2H5)2OAlCl3 (b) Hg2+(aq) + 4CN-(aq) (a) Hg(CN)24 (aq) AlCl3 is a Lewis acid (b) Here the Hg2+ ion accepts four pairs of electrons from the CN- ions. → Hg2+ is the Lewis acid and CN- is the Lewis base. CHEM-102 9 Acid-Base Properties of Water H+ (aq) + OH- (aq) H2O (l) autoionization of water H O H + H [H O H H2O + H2O CHEM-102 ] H + H O - H base acid O + conjugate acid H3O+ + OHconjugate base 10 The Ion Product of Water H2O (l) H+ (aq) + OH- (aq) [H+][OH-] Kc = [H2O] [H2O] = constant Kc[H2O] = Kw = [H+][OH-] The ion-product constant (Kw) is the product of the molar concentrations of H+ and OH- ions at a particular temperature. At 250C Kw = [H+][OH-] = 1.0 x 10-14 CHEM-102 [H+] = [OH-] Solution Is neutral [H+] > [OH-] acidic [H+] < [OH-] basic 11 Example 4: The concentration of OH− ions in a certain household ammonia cleaning solution is 0.0025 M. Calculate the concentration of H+ ions. -14 K 1.0 × 10 + -12 w [H ] = = = 4.0 × 10 M 0.0025 [OH ] CHEM-102 12 pH – A Measure of Acidity pH = -log [H+] [H+] = [OH-] At 250C [H+] = 1.0 x 10-7 pH = 7 acidic [H+] > [OH-] [H+] > 1.0 x 10-7 pH < 7 basic [H+] < [OH-] [H+] < 1.0 x 10-7 pH > 7 Solution Is neutral pH CHEM-102 [H+] 13 Other important relationships pOH = -log [OH-] [H+][OH-] = Kw = 1.0 x 10-14 -log [H+] – log [OH-] = 14.00 pH + pOH = 14.00 CHEM-102 pH Meter 14 Example 5: The concentration of H+ ions in a solution is 3.2 x 10-4 M. The solution was left for a while, it was found that the hydrogen ion concentration equal to 1.0 x 10-3 M. Calculate the pH of the solution on these two occasions. pH = -log [H+] [H+] = 3.2 x 10-4 M pH = -log [H+] = -log (3.2 x 10-4) = 3.49 On the second occasion, [H+] = 1.0 x 10-3 M, so that pH = -log (1.0 x 10-3 ) = 3.00 CHEM-102 15 Example 6: The pH of rainwater collected in a certain region of the northeastern United States on a particular day was 4.82. Calculate the H+ ion concentration of the rainwater. pH = -log [H+] = 4.82 Therefore, log [H+] = −4.82 To calculate [H+], we need to take the antilog of −4.82 [H+] = 10−4.82 = 1.5 x 10−5 M CHEM-102 16 Example 7: In a NaOH solution [OH−] is 2.9 x 10−4 M. Calculate the pH of the solution. pOH = −log [OH−] = −log (2.9 x 10−4) = 3.54 pH + pOH = 14.00 pH = 14.00 − pOH = 14.00 − 3.54 = 10.46 Alternatively, we can use Kw = [H+][OH−] to calculate [H+], and then we can calculate the pH from the [H+]. Try it. pH = -log [H+] CHEM-102 17 Strength of acids and bases Strong Electrolyte – 100% dissociation NaCl (s) H 2O Na+ (aq) + Cl- (aq) Weak Electrolyte – not completely dissociated CH3COOH CH3COO- (aq) + H+ (aq) Strong Acids are strong electrolytes HCl (aq) + H2O (l) H3O+ (aq) + Cl- (aq) HNO3 (aq) + H2O (l) H3O+ (aq) + NO3- (aq) HClO4 (aq) + H2O (l) H3O+ (aq) + ClO4- (aq) H2SO4 (aq) + H2O (l) H3O+ (aq) + HSO4- (aq) CHEM-102 18 Weak Acids are weak electrolytes H3O+ (aq) + F- (aq) HF (aq) + H2O (l) HNO2 (aq) + H2O (l) H3O+ (aq) + NO2- (aq) HSO4- (aq) + H2O (l) H3O+ (aq) + SO42- (aq) H2O (l) + H2O (l) H3O+ (aq) + OH- (aq) Strong Bases are strong electrolytes NaOH (s) KOH (s) H2O H 2O Ba(OH)2 (s) CHEM-102 Na+ (aq) + OH- (aq) K+ (aq) + OH- (aq) H 2O Ba2+ (aq) + 2OH- (aq) 19 Weak Bases are weak electrolytes F- (aq) + H2O (l) NO2- (aq) + H2O (l) OH- (aq) + HF (aq) OH- (aq) + HNO2 (aq) Conjugate acid-base pairs: • The conjugate base of a strong acid has no measurable strength. • H3O+ is the strongest acid that can exist in aqueous solution. • The OH- ion is the strongest base that can exist in aqueous solution. CHEM-102 20 CHEM-102 ©2017 Cengage Learning. Ebbing and Gammon - General Chemistry. 21 CHEM-102 22 Strong Acid (HCl) CHEM-102 Weak Acid (HF) 23 CHEM-102 24 CHEM-102 25