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A CIDS AND B ASES II IB C HEMISTRY G R.12 Topic 18 1 Chem2_Dr. Dura
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T OPICS 18.1 Calculations Involving Acids and Bases 18.2 Buffer Solutions 18.3 Salt Hydrolysis 18.4 Acid-Base Titrations 18.5 Indicators 2 Chem2_Dr. Dura
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IB S TANDARDS 18.1.1 State the expression for the ionic product constant of water (K w ). 18.1.2 Deduce [H + (aq)] and [OH - (aq)], for water at different temperatures given K w values. 18.1.3 Solve problems involving [H + (aq)], [OH - (aq)], pH and pOH. 18.1.4 State the equation for the reaction for any weak acid or weak base with water, and hence deduce the expressions for K a and K b. Chem 2_Dr. Dura 3
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IB S TANDARDS 18.1.5 Solve problems involving solutions of weak acids and bases using the expressions: K a x K b = K w pK a + pK b = pK w pH + pOH = pK w 18.1.6 Identify the relative strengths of acids and bases using values of K a, K b, pK a and pK b. 4 Chem2_Dr. Dura
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5 H 2 O (l) H + (aq) + OH - (aq) The Ion Product of Water K c = [H + ][OH - ] [H 2 O] [H 2 O] = constant K c [H 2 O] = K w = [H + ][OH - ] The ion-product constant (K w ) is the product of the molar concentrations of H + and OH - ions at a particular temperature. At 25 0 C K w = [H + ][OH - ] = 1.0 x 10 - 14 [H + ] = [OH - ] [H + ] > [OH - ] [H + ] < [OH - ] Solution Is neutral acidic basic
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K W IS TEMPERATURE DEPENDENT Temp o CKwKw [H+] in pure water pH 01.5 x 10 -15 3.9 x 10 -6 7.47 206.8 x 10 -15 8.2 x10 -6 7.08 251.0 x 10 -14 1.0 x10 -7 7.00 6 Chem2_Dr. Dura
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7 pH = - log [H+], [H + ] = 10 -pH pOH = -log [OH-], [OH - ] = 10 -pOH [H + ] = [OH - ] [H + ] > [OH - ] [H + ] < [OH - ] Solution is neutral acidic basic [H + ] = 1 x 10 -7 [H + ] > 1 x 10 -7 [H + ] < 1 x 10 -7 pH = 7 pH < 7 pH > 7 At 25 0 C pH[H + ]
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Chem2_Dr. Dura 8 [H + ][OH - ] = K w = 1.0 x 10 -14 -log [H + ] – log [OH - ] = 14.00 pH + pOH = 14.00 At 25 0 C
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Chem2_Dr. Dura 9 Example 1: The pH of rainwater collected in a certain region of the northeastern United States on a particular day was 4.82. What is the H + ion concentration of the rainwater? pH = - log [H + ] [H + ] = 10 -pH = 10 -4.82 = 1.5 x 10 -5 M Example 2: The OH - ion concentration of a blood sample is 2.5 x 10 -7 M. What is the pH of the blood? pH + pOH = 14.00 pOH = -log [OH - ]= -log (2.5 x 10 -7 )= 6.60 pH = 14.00 – pOH = 14.00 – 6.60 = 7.40
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Chem2_Dr. Dura 10 HA (aq) + H 2 O (l) H 3 O + (aq) + A - (aq) Weak Acids (HA) and Acid Ionization Constants HA (aq) H + (aq) + A - (aq) K a = [H + ][A - ] [HA] K a is the acid ionization constant KaKa weak acid strength
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Chem2_Dr. Dura 11 NH 3 (aq) + H 2 O (l) NH 4 + (aq) + OH - (aq) Weak Bases and Base Ionization Constants K b = [NH 4 + ][OH - ] [NH 3 ] K b is the base ionization constant KbKb weak base strength Solve weak base problems like weak acids except solve for [OH-] instead of [H + ].
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I ONIZATION C ONSTANTS OF C ONJUGATE A CID - B ASE P AIRS Consider a weak acid (HA) and its conjugate base (A - ) in water: HA (aq) H + (aq) + A - (aq) K a = [H + ][A - ] / [HA] A - (aq) + H 2 O(l) HA(aq) + OH - (aq) K b = [HA][OH - ] /[A - ] K a x K b = [H + ][OH - ] = K w Chem 2_Dr. Dura 12
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IB S TANDARDS 18.2.1 Describe the composition of a buffer solution and explain its action. 18.2.2 Solve problems involving the composition and pH of a specified buffer system. 18.3 Deduce whether salts form acidic, alkaline or neutral aqueous solutions. 13 Chem2_Dr. Dura
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B UFFER S OLUTIONS 14 Chem2_Dr. Dura A buffer solution is a solution of: 1. A weak acid or a weak base and 2. The salt of the weak acid or weak base Both must be present!
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Chem2_Dr. Dura 15 The common ion effect is the shift in equilibrium caused by the addition of a compound having an ion in common with the dissolved substance. The presence of a common ion suppresses the ionization of a weak acid or a weak base. Example: A buffer consisting of CH 3 COONa (salt) and CH 3 COOH (weak acid). CH 3 COONa (s) Na + (aq) + CH 3 COO - (aq) CH 3 COOH (aq) H + (aq) + CH 3 COO - (aq) common ion
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16 A buffer solution has the ability to resist changes in pH upon the addition of small amounts of either acid or base. Add strong acid H + (aq) + CH 3 COO - (aq) CH 3 COOH (aq) Add strong base OH - (aq) + CH 3 COOH (aq) CH 3 COO - (aq) + H 2 O (l) How buffers work Consider an equal molar mixture of CH 3 COOH and CH 3 COONa Chem2_Dr. Dura
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17 Consider mixture of salt NaA and weak acid HA. HA (aq) H + (aq) + A - (aq) NaA (s) Na + (aq) + A - (aq) K a = [H + ][A - ] [HA] [H + ] = K a [HA] [A - ] -log [H + ] = -log K a - log [HA] [A - ] -log [H + ] = -log K a + log [A - ] [HA] pH = pK a + log [A - ] [HA] pK a = -log K a
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H ENDERSON -H ASSELBALCH EQUATION 18 pH = pK a + log [conjugate base] [acid] Chem2_Dr. Dura pH of buffers can be determined from Henderson- Hasselbalch Equation.
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19 Exercise: Which of the following are buffer systems? (a) KF/HF (b) KBr/HBr, (c) Na 2 CO 3 /NaHCO 3 (a) KF is a weak acid and F - is its conjugate base buffer solution (b) HBr is a strong acid not a buffer solution (c) CO 3 2- is a weak base and HCO 3 - is its conjugate acid buffer solution Chem2_Dr. Dura
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20 = 9.20 Problem: Calculate the pH of the 0.30 M NH 3 /0.36 M NH 4 Cl buffer system. What is the pH after the addition of 20.0 mL of 0.050 M NaOH to 80.0 mL of the buffer solution? NH 4 + (aq) H + (aq) + NH 3 (aq) pH = pK a + log [NH 3 ] [NH 4 + ] pK a = 9.25 pH = 9.25 + log [0.30] [0.36] = 9.17 NH 4 + (aq) + OH - (aq) H 2 O (l) + NH 3 (aq) start (moles) end (moles) 0.0290.001 0.024 0.0280.00.025 pH = 9.25 + log [0.25] [0.28] [NH 4 + ] = 0.028 0.10 final volume = 80.0 mL + 20.0 mL = 100 mL [NH 3 ] = 0.025 0.10 Chem2_Dr. Dura
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21 Acid-Base Properties of Salts Neutral Solutions: Salts containing an alkali metal or alkaline earth metal ion (except Be 2+ ) and the conjugate base of a strong acid (e.g. Cl -, Br -, and NO 3 - ). NaCl (s) Na + (aq) + Cl - (aq) H2OH2O Basic Solutions: Salts derived from a strong base and a weak acid. NaCH 3 COOH (s) Na + (aq) + CH 3 COO - (aq) H2OH2O CH 3 COO - (aq) + H 2 O (l) CH 3 COOH (aq) + OH - (aq)
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Chem2_Dr. Dura 22 Acid-Base Properties of Salts Acid Solutions: Salts derived from a strong acid and a weak base. NH 4 Cl (s) NH 4 + (aq) + Cl - (aq) H2OH2O NH 4 + (aq) NH 3 (aq) + H + (aq) Salts with small, highly charged metal cations (e.g. Al 3+, Cr 3+, and Be 2+ ) and the conjugate base of a strong acid. Al(H 2 O) 6 (aq) Al(OH)(H 2 O) 5 (aq) + H + (aq) 3+2+
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Chem2_Dr. Dura 23 Acid-Base Properties of Salts Solutions in which both the cation and the anion hydrolyze : K b for the anion > K a for the cation, solution will be basic K b for the anion < K a for the cation, solution will be acidic K b for the anion K a for the cation, solution will be neutral
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IB S TANDARDS 18.4.1Sketch the general shapes of graphs of pH and volume for titrations of strong and weak acids and bases and explain their important features. 18.5.1 Describe qualitatively the action of acid- base indicator. 18.5.2 State and explain how the pH range of an acid-base indicator relates to its pKa value. 18.5.3 Identify an appropriate indicator for a titration given the equivalence point of titration and the pH range of the indicator. 24 Chem2_Dr. Dura
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25 Titrations In a titration a solution of accurately known concentration is added gradually to another solution of unknown concentration until the chemical reaction between the two solutions is complete. Equivalence point – the point at which the reaction is complete Indicator – substance that changes color at (or near) the equivalence point Slowly add base to unknown acid UNTIL The indicator changes color (pink) Chem2_Dr. Dura
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26 Strong Acid-Strong Base Titrations NaOH (aq) + HCl (aq) H 2 O (l) + NaCl (aq) OH - (aq) + H + (aq) H 2 O (l) Chem2_Dr. Dura
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27 Weak Acid-Strong Base Titrations CH 3 COOH (aq) + NaOH (aq) CH 3 COONa (aq) + H 2 O (l) CH 3 COOH (aq) + OH - (aq) CH 3 COO - (aq) + H 2 O (l) CH 3 COO - (aq) + H 2 O (l) OH - (aq) + CH 3 COOH (aq) At equivalence point (pH > 7): Chem2_Dr. Dura
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28 Strong Acid-Weak Base Titrations HCl (aq) + NH 3 (aq) NH 4 Cl (aq) NH 4 + (aq) + H 2 O (l) NH 3 (aq) + H + (aq) At equivalence point (pH < 7): H + (aq) + NH 3 (aq) NH 4 Cl (aq) Chem2_Dr. Dura
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29 Acid-Base Indicators HIn (aq) H + (aq) + In - (aq) 10 [HIn] [In - ] Color of acid (HIn) predominates 10 [HIn] [In - ] Color of conjugate base (In - ) predominates Chem2_Dr. Dura
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30 The titration curve of a strong acid with a strong base. Chem2_Dr. Dura
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31 Which indicator(s) would you use for a titration of HNO 2 with KOH ? Weak acid titrated with strong base. At equivalence point, will have conjugate base of weak acid. At equivalence point, pH > 7 Use cresol red or phenolphthalein Chem2_Dr. Dura
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32 Acid-Base Properties of Salts Acid Solutions: Salts derived from a strong acid and a weak base. NH 4 Cl (s) NH 4 + (aq) + Cl - (aq) H2OH2O NH 4 + (aq) NH 3 (aq) + H + (aq) Salts with small, highly charged metal cations (e.g. Al 3+, Cr 3+, and Be 2+ ) and the conjugate base of a strong acid. Al(H 2 O) 6 (aq) Al(OH)(H 2 O) 5 (aq) + H + (aq) 3+2+ Chem2_Dr. Dura
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33 Acid-Base Properties of Salts Solutions in which both the cation and the anion hydrolyze: K b for the anion > K a for the cation, solution will be basic K b for the anion < K a for the cation, solution will be acidic K b for the anion K a for the cation, solution will be neutral Chem2_Dr. Dura
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34 Chemistry In Action: Antacids and the Stomach pH Balance NaHCO 3 (aq) + HCl (aq) NaCl (aq) + H 2 O (l) + CO 2 (g) Mg(OH) 2 (s) + 2HCl (aq) MgCl 2 (aq) + 2H 2 O (l) Chem2_Dr. Dura
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