Catalyst A Problem I have two beakers Beaker 1 1 L of pure water Beaker 2 1 L of human blood I pour 5 mL of NaOH in the pure water and the pH goes from 7 up to 13.2 I pour 5 mL of NaOH into the blood and it goes from a pH of 7.2 to 7.3 A Problem I have the same two beakers Beaker 1 1 L of pure water Beaker 2 1 L of human blood I pour 5 mL of HCl in the pure water and the pH goes from 7 up to 2.2 I pour 5 mL of NaOH into the blood and it goes from a pH of 7.2 to 7.1 Justify TPS Why does the pure water show such a drastic change in pH, but the human blood exhibits such a small change in pH? Lecture 7.7 Buffers and Weak Acid Titrations Todays Learning Targets LT 7.18 I can explain what a buffer is and how a buffer can be created in the laboratory. LT 7.19 I can calculate the concentration of compounds in a buffer solution following the addition of a strong acid/base. Furthermore, I can calculate the pH of this solution. LT 7.20 I can interpret the titration curve for weak acid/strong base titration and I can calculate the initial pH, half equivalence point, equivalence point and unknown concentrations from titration data. Buffers A buffer is any solution that resists changes in pH Two components of a buffer: 1. A component that neutralizes an acid 2. A component that neutralizes a base A weak acid or a weak base are capable of creating a buffer because they have both of these components, but a strong acid or base cannot create a buffer. Weak Acids/Bases Make Excellent Buffers A weak acid or base make an excellent buffer because they have a component that can react with an acid and a component that can react with a base. CH 3 COOH + H 2 O CH 3 COO - + H + Can react with a base! Can react with an acid! Therefore, reacting each component produces a part of the equilibrium and little change in pH is observed! How to Make a Buffer The best buffers have close to equal concentrations of the conjugate acid/base pair. We can think about the K a expression for a weak acid Therefore, pH is determined by the ratio of conjugate acid/base pair and the value of K a. As long as the change in ratio of [HA]/[A - ] is small, the change in pH will be small. Class Example You have a buffer system of methylamine (CH 3 NH 2 ) that has a K b of 5.0 x and a conjugate acid (CH 3 NH 3 + ) whose K a is 2.0 x You have 40 mL of buffer with a concentration of 0.50 M. You add 10 mL of M HCl to this buffer. Calculate the pH before adding the HCl and the pH after adding the HCl. Table Talk You have a solution of acetic acid (CH 3 COOH) that was created by adding moles of CH 3 COOH and moles of CH 3 COO- to enough water to make a L solution. You add in 5.0 mL of 4.0 M NaOH. Calculate the pH before adding in the NaOH and the pH after adding in the NaOH (K a for the solution is 1.8 x ) Adding Strong Acid/Base to Buffer Adding base to a buffer shows minimal change Adding base to a neutral solution causes huge changes Henderson Hasselbalch Equation When we have a buffer, we do not need to use ICE tables to determine the pH of the solution. We can use the Henderson-Hasselbalch equation in order to solve for the pH Class Example Calculate the pH of a buffer that is 0.12 M lactic acid and 0.10 M sodium lactate. The K a for lactic acid is 1.4 x 10 -4 Table Talk Calculate the pH of a buffer composed of 0.12 M benzoic acid and 0.20 M sodium benzoate. The K a for the solution is 6.4 x 10 -5 When Buffers Stop Working The pH at which any buffer works most effectively is when pH = pK a This is known at the equivalence point Buffers usually have a useable range within 1 pH unit of the pK a Pushing It to the Next Level! Green = Cake Walk Level Buffer Problems Yellow = Heating Up Level Buffer Problems Red = Expert Chemist Level Buffer Problems Each correct answer yields the following points: Green = 1 point Yellow = 5 points Red = 10 points Titrating a Weak Acid/Base with a Strong Acid/Base When we titrate a weak acid/base with a strong acid/base, the titration curve looks different than the ones previously studied. A strong acid/base titration goes to completion the instant we add the solution. A weak acid/base titration reacts and than reestablishes equilibrium Buffer Region Creating a Titration Curve 4 important points to plot: 1. Initial pH Equilibrium Problem 2. Half Equivalence Point Buffer Problem 3. Equivalence Point Weak Base Problem 4. Excess Strong Acid/Base strong acid/base pH problem Class Example You create a solution of mL of M HN 3 where K a = 1.9 x You titrate the weak acid with M NaOH. Create a titration curve for this titration. Table Talk You have 50.0 mL of M CH 3 COOH (K a = 1.8 x 10-5) with M NaOH. Create a titration curve for this titration using the 4 points. Collaborative Poster Create a poster with your group members solving the problem that you were given. You must explain and justify your answers using the following vocabulary/phrases: Buffer Conjugate acid/base pair Dominant species in solution The poster should be explained in such a way that another AP Chemistry student could learn about titration curves from your poster alone. Exit Ticket 1. You create a solution of 50.0 mL of M HN 3 where K a = 1.9 x You titrate the weak acid with M NaOH. Create a titration curve for this titration. 2. You have a solution of acetic acid (CH 3 COOH) that was created by adding moles of CH 3 COOH and moles of CH 3 COO- to enough water to make a L solution. You add in 5.0 mL of 4.0 M NaOH. Calculate the pH before adding in the NaOH and the pH after adding in the NaOH (K a for the solution is 1.8 x ) Closing Time Read: 17.1, 17.2, and 17.3 Homework: