Buffers. How can a solution neutralize both acids and bases? Beaker B: 100 ml of 1.00 M HCl. HCl (aq) + H 2 O H 3 O 1+ (aq) + Cl 1 (aq)

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1 Buffers How can a solution neutralize both acids and bases? Why? Buffer solutions are a mixture of substances that have a fairly constant ph regardless of addition of acid or base. They are used in medicine, industry and manufacturing to keep the ph of a solution stable regardless of other reactions that might be occurring. You have buffers in your blood, for example, because the proper function of enzymes is dependent on the ph of your blood staying in a narrow range. Eating, breathing and exercising can all alter the acidity or basicity of your blood, but the buffers in your blood help control the ph and keep all of your enzymes working properly. Model 1 Comparing Solutions Beaker A: 100 ml Pure Water H 2 O + H 2 O H 3 O 1+ (aq) + OH 1 (aq) Beaker B: 100 ml of 1.00 M HCl HCl (aq) + H 2 O H 3 O 1+ (aq) + Cl 1 (aq) Beaker C: 100 ml of 1.00 M HNO 2 K a = Beaker D: 100 ml of 1.00 M HNO 2 with 1.00 M NaNO 2 HNO 2 (aq) + H 2 O H 3 O 1+ (aq) + NO 2 1 (aq) HNO 2 (aq) + H 2 O H 3 O 1+ (aq) + NO 2 1 (aq) 1. For each beaker in Model 1 write SA (strong acid), SB (strong base), WA (weak acid), WB (weak base), CA (conjugate acid) or CB (conjugate base) under each molecule or ion in the reaction. 2. For each beaker in Model 1 write the final or equilibrium concentrations of the molecules or ions under each species in the reaction. Divide the work among group members. If calculations are required, show your work below. Buffers 1

2 3. Imagine that 1.00 M sodium hydroxide was added dropwise to each of the beakers in Model 1. List the species in each beaker that would react with this base and neutralize significant quantities (at least 0.05 mole) of it. If neutralization is not likely to occur, write none. Beaker A Beaker B Beaker C Beaker D 4. Imagine that 1.00 M hydrochloric acid was added dropwise to each of the beakers in Model 1. List the species in each beaker that could react with this acid and neutralize significant quantities (at least 0.05 mole) of it. If neutralization is not likely to occur, write none. Beaker A Beaker B Beaker C Beaker D 5. Which beaker in Model 1 was able to neutralize significant quantities of both acid and base? Read This! A buffer solution is one that can neutralize both small quantities of acid and small quantities of base. This is possible because the solution contains both a weak acid and its conjugate base species in equilibrium. By neutralizing any acid or base that is added, the solution can keep a fairly constant ph. 6. Which beaker in Model 1 contains a buffer? 7. One way to make a buffer solution is to mix a weak acid solution with a solution of a salt of its conjugate base. Explain why both the weak acid and the salt of its conjugate (common ion) are required to make a buffer. 8. Circle all the combinations below that would make a buffer solution when mixed in equal volumes. 1.0 M HCl and 1.0 M NaCl 1.0 M CH 3 COOH and 1.0 M NaCl 1.0 M HNO 2 and 1.0 M HNO M HF and 1.0 M NaF 1.0 M NH 4 Cl and 1.0 M NH M H 2 SO 4 and 1.0 M Na 2 SO 4 9. Choose one of the buffers you circled in Question 8 and describe how it would neutralize both acids and bases to keep the ph fairly constant. 2 POGIL Activities for AP* Chemistry

3 Model 2 ph of Buffer Solutions Beaker 1 Beaker ml 1.0 M NH 4 Cl and 100 ml 1.0 M NH ml 1.0 M HF and 100 ml 1.0 M NaF 1+ K a for NH 4 = K a for HF = NH 4 H 3 O 1+ NH 3 HF H 3 O 1+ F 1 Beaker 3 Beaker ml 1.0 M H 2 CO 3 and 100 ml 1.0 M NaHCO ml 1.0 M HF and 300 ml 1.0 M NaF K a for H 2 CO 3 = K a for HF = H 2 CO 3 H 3 O 1+ 1 HCO 3 HF H 3 O 1+ F Model 2 describes four buffer solutions made by mixing two solutions together a weak acid and the salt of its conjugate base. Write a net ionic equation for the chemical equilibrium working in each buffer in the space above each table in Model Use the tables to find the ph of each buffer solution in Model 2. Divide the work evenly among group members. Be sure to consider how the molarities of the solutions will change when the two parts of the buffer are mixed. Buffers 3

4 12. Do all buffers keep solutions at a neutral ph? Justify your answer with data from Model Calculate the pk a s for the weak acids in each of the buffer solutions described in Model 2 and list them in the model. 14. How are the pk a values of the weak acids related to the ph of the buffer system that is made when equal molarities of acid and conjugate base are combined? 15. When unequal molarities of acid and conjugate base are combined to make a buffer (as in beaker 4 of Model 2), does the ph change significantly from the pk a of the weak acid? Support your answer with data from Model An industrial process requires a constant ph of The weak acids available in the warehouse are benzoic acid (HC 7 H 3 O 2 ), K a = , nitrous acid (HNO 2 ), K a = , phosphorous acid (H 3 PO 3 ), K a = , and hydrocyanic acid (HCN), K a = a. Which of the four acids would be best to make a buffer for the industrial process? b. What other chemical(s) are needed to prepare the buffer? 4 POGIL Activities for AP* Chemistry

5 Extension Questions 17. Would either of the beakers you circled in Question 8 be able to neutralize unlimited quantities of acid or base? If no, explain what the limiting factor would be. 18. Circle all the combinations below that would make a buffer solution when mixed. Note: In all cases a neutralization reaction will occur. Consider what the products of the neutralization will be and how much of each species will be present when the reaction is completed. a. 100 ml of 1.0 M HCl with 100 ml of 1.0 M NaOH d. 100 ml of 1.0 M HCl with 50 ml of 1.0 M NaOH b. 100 ml of 1.0 M HNO 2 with 100 ml of 1.0 M NaOH c. 100 ml of 1.0 M sodium acetate with 100 ml of 0.50 M HCl e. 100 ml of 1.0 M HNO 2 with 50 ml of 1.0 M NaOH f. 100 ml of 1.0 M acetic acid with 100 ml of 0.50 M NaOH 19. A buffer solution is made by mixing equal moles of a weak acid and the salt of its conjugate ion. Would the resulting solution have a ph equal to, greater than, or less than the ph of the weak acid solution in water alone? Justify your reasoning. Buffers 5

6 20. One buffer system in the bloodstream of animals is the carbonic acid/bicarbonate buffer. H 2 CO 3 + H 2 O HCO H 3 O + The carbonic acid in the bloodstream is formed by the combination of carbon dioxide gas and water. This is a reversible process. CO 2 + H 2 O H 2 CO 3 Therefore, the whole buffer system is linked to the respiratory system of the animal and the concentration of dissolved CO 2 gas in the animal s bloodstream. a. What species in the blood buffer system neutralizes excess acid in the body? b. What species in the blood buffer system neutralizes excess base in the body? 6 POGIL Activities for AP* Chemistry

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