8_Ch17_AqEquilibria_1

8_Ch17_AqEquilibria_1 - Chapter 17 Complex Aqueous...

Info iconThis preview shows page 1. Sign up to view the full content.

View Full Document Right Arrow Icon
This is the end of the preview. Sign up to access the rest of the document.

Unformatted text preview: Chapter 17 Complex Aqueous Equilibria Common Ion Effect Buffers Titra<ons buffers and <tra<ons 1 Common Ion Effect CH3COOH H+(aq) + CH3COO-(aq) Consider the equilibrium involving the weak acid at 298K shown above. What will happen to the pH if some solid CH3COONa is added while the temperature is held constant? 1. The pH will increase 2. The pH will decrease 3. The pH will not be effected. 4. There is not enough information to determine the effect on the pH. Common Ion Effect DEMO: CH3COOH H+(aq) + CH3COO-(aq) pH = Add CH3COONa: CH3COONa Na+ + CH3COO-(aq) pH = What happened to [CH3COO-]? [CH3COOH]? [H+]? buffers and <tra<ons 3 Mixture of an acid and its conjugate base CH3COOH H+(aq) + CH3COO-(aq) pH 0.1M CH3COOH(aq) Add HCl Add NaOH Mixture pH 0.1M CH3COOH 0.1M CH3COONa Add HCl Add NaOH How does the addi<on of strong acid or base effect the pH of the weak acid? of the mixture? Buffers A buffer is a soluDon that "resists" a change in pH: E.g. blood contains substances that keep its pH fixed at 7.3 - important for life funcDons Buffer soluDons consist of either: buffers and <tra<ons 5 Buffers If a small amount of hydroxide is added to an equimolar solu<on of HF in NaF, the HF reacts with the OH- to make F- and water. Similarly, if acid is added, the F- reacts with it to form HF and water. buffers and <tra<ons 6 Quan<ta<vely Calculate the pH of a solu<on that is 0.1M CH3COOH and 0.1M CH3COONa Buffer Calcula<ons Consider the equilibrium constant expression for the dissocia<on of a generic acid, HA: HA(aq) H+(aq) + A-(aq) This is the HendersonHasselbalch equa<on. buffers and <tra<ons 8 pH of Buffer Calculate the pH of a soluDon that is 0.1M NH3 and 0.2M NH4Cl. buffers and <tra<ons 9 Example: What is the pH of a buffer that is 0.12 M in lac<c acid, CH3CH (OH)COOH, and 0.10 M in sodium lactate? Ka for lac<c acid is 1.4 10-4. buffers and <tra<ons 10 When Strong Acids or Bases Are Added to a Buffer... Addi<on of Strong Acid or Base to a Buffer 1. Determine how the neutraliza<on reac<on affects the amounts of the weak acid and its conjugate base in solu<on. 2. Use the HendersonHasselbalch equa<on to determine the new pH of the solu<on. 12 buffers and <tra<ons Calcula<ng pH Changes in Buffers A buffer is made by adding 0.300 mol HC2H3O2 and 0.300 mol NaC2H3O2 to enough water to make 1.00 L of solu<on. The pH of the buffer is 4.74. Calculate the pH of this solu<on afer 0.020 mol of NaOH is added. buffers and <tra<ons 13 pH range of a buffer: range of pH over which the buffer acts effec<vely. op<mal pH: Buffer region buffers and <tra<ons 14 Buffer Capacity Buffer capacity: volume of acid or base that can be added before the pH of the buffer changes appreciably. Which soluDon will have the greatest buffer capacity? 1M CH3COOH/CH3COONa 0.1M CH3COOH/CH3COONa buffers and <tra<ons 15 Titra<on A pH meter or indicators are used to determine when the solu<on has reached the equivalence point: In this technique a known concentra<on of base (or acid) is slowly added to a solu<on of acid (or base). 16 buffers and <tra<ons Titra<on of a Strong Acid with a Strong Base buffers and <tra<ons 17 Acid Base TitraDons Finding the pH in an acid-base DtraDon is a two step process. 1. Neutraliza<on What is in solu<on afer neutraliza<on? 2. Find the pH A) strong acid/base B) weak/acid base equilibrium calcula<on buffer solu<on Example: Titra<on of a Strong Acid with a Strong Base Titrate 0.1M HCl with 0.2M NaOH star<ng with 50 mL of HCl solu<on. Calculate pH when the following quan<<es of NaOH are added: a. 0 mL NaOH, b. 20 mL NaOH, c. 25 mL NaOH, d. 30 mL NaOH buffers and <tra<ons 19 Example con<nued: Titra<on of a Strong Acid with a Strong Base Titrate 0.1M HCl with 0.2M NaOH star<ng with 50 mL of HCl solu<on. Calculate pH when the following quan<<es of NaOH are added: a. 0 mL NaOH, b. 20 mL NaOH, c. 25 mL NaOH, d. 30 mL NaOH buffers and <tra<ons 20 Titra<on of a Weak Acid with a Strong Base buffers and <tra<ons 21 Titra<on of a Weak Acid with a Strong Base At each point below the equivalence point, the pH of the solu<on during <tra<on is determined from the amounts of the acid and its conjugate base present at that par<cular <me. Remember: Buffer solutions are encountered during titrations! 22 buffers and <tra<ons Example: Titra<on of a Weak Acid with a Strong Base Titrate 0.1M CH3COOH (Ka = 1.8 x 10-5) with 0.2M NaOH starting with 50 mL of CH3COOH solution and calculate pH when the following quantities of NaOH are added: a. 0 mL NaOH b. 12.5 mL NaOH buffers and <tra<ons 23 Example Cont.: Titra<on of a Weak Acid with a Strong Base Titrate 0.1M CH3COOH (Ka = 1.8 x 10-5) with 0.2M NaOH starting with 50 mL of CH3COOH solution and calculate pH when the following quantities of NaOH are added: c. NaOH needed to reach d. 30 mL NaOH equivalence point buffers and <tra<ons 24 Titra<on of a Weak Base with a Strong Acid The pH at the equivalence point in these <tra<ons is < 7. indicator of choice is: 25 buffers and <tra<ons Titra<on of a Weak Acid with a Strong Base 26 buffers and <tra<ons pH at equivalence Titrating a strong acid (base) with a strong base (acid) pH at equivalence Titrating a weak acid with a strong base pH at equivalence Titrating a weak base with a strong acid pH at equivalence buffers and <tra<ons 27 Titra<ng a BASE with a Strong Acid buffers and <tra<ons 28 INDICATORS InH+ pH interval for Name color change Methyl violet 1 - 2 Methyl yellow 1.5 - 2.5 Methyl orange 2.5 - 4.0 Methyl red 4.2 - 6.3 Bromthymol blue 6.0 - 7.6 Thymol blue 8.0 - 9.6 Phenolphthalein 8.5 - 10 Alizarin yellow G 10.1-12.0 In + H+ Base color Violet Yellow Yellow Yellow Blue Blue Pink Red Some of the more common acid-base indicators Acid color Yellow Red Red Red Yellow Yellow Colorless Yellow buffers and <tra<ons 29 Titra<ons of Polypro<c Acids 30 buffers and <tra<ons Problem A buffer solution is 0.20M in HPO42- and 0.10M in H2PO4-. What is the [H+] in this solution? H3PO4: Ka1 = 7.5 x 10-3 Ka2 = 6.2 x 10-8 Ka3 = 1 x 10-12 A. B. C. D. E. 3.7 x 10-3 M 3.7 x 10-4 M 3.1 x 10-8 M 3.1 x 10-9 M 5.0 x 10-13 M buffers and <tra<ons 31 ...
View Full Document

{[ snackBarMessage ]}

Ask a homework question - tutors are online