REPORT FORM ACID BASE EQUILIBRIA. Name

Transcription

1 REPORT FORM ACID BASE EQUILIBRIA Name Section A Write chemical equations to explain the results of the conductance experiments demonstrated by your instructor. Only write equations for conductive solutions. How will the conductivity change during the course (i.e. before, at, and after the equivalence point) of a titration between H 2 SO 4 and Ba(OH) 2? (Review your solubility rules!) Give the values of pk a for all weak acids and pk b for all weak bases used in this part (i.e., Part A) of the experiment. Section B Step 4. What observation supports the conclusion that the time for mixing to occur is much longer than the time required for the acid-base reaction itself? Describe the change you observed and explain the resulting color using Le Châtelier's principle. What is the ph, -log[h 3 O + ], of the mixture of 1 drop of indicator and 1 drop of 0.01 M HCl?

2 Step 6. Explain the color changes you saw in this step. How did you show that the reaction is reversible? Section C Briefly describe the indicator colors you saw: ph MO BCG BTB Phth What are the K a and pk a values for the four indicators used? (Recall the sig fig rule for log numbers.) K a pk a MO BCG BTB Phth Why are these acid dissociation constants only approximate? What experimental changes could be made to increase confidence in these values?

3 Section D Briefly describe the colors of the universal indicator that you saw in wells 1-12: ph UI Step 4. Identify your 5 solutions of unknown ph and report their measured ph values. Solution ph Section E-1 Attach the page from the manual showing Figure 14.2 with your graph of a) ph vs. drops of 0.01 M NaOH added for the HCl titration in Part E-1. b) ph vs. drops of 0.01 M NaOH added for the acetic acid titration in Part E-2. c) ph vs. drops of 0.01 M HCl added for the ammonia titration in Part E-8. Compare your graph with Figure 14.3 in the manual. Are there any differences? Describe them here. Write a net ionic chemical equation for the reaction that occurs in the titration of HCl with NaOH. In Figure 14.4, why is the ph rise for the titration of 1 x 10-5 M HCl with 1 x 10-5 M NaOH very small? Attach your copy of Figure 14.5, with the ph transition ranges written in for (1) methyl orange (MO), (2) bromcresol green (BCG), (3) bromthymol blue (BTB), and (4) phenolphthalein (Phth). Which indicator would be suitable for a titration of M HNO 3 with M KOH? Explain your choice.

4 What acid molarities did you calculate for Curve A? Curve B? Curve C? Curve D? Section E-2 Describe any differences between acetic and hydrochloric acids. Write a net ionic chemical equation for the reaction that occurs in the titration of acetic acid with NaOH. Section E-3 In Figure 14.7, what is the limiting reagent for the region before the endpoint? From Figure 14.7, what is the pk a of acetic acid? What is the corresponding K a? Section E-4 Drops of [CH 0.01 M NaOH 3COO ] [CH COOH ] 3 ph from curve ph from H-H Equation Section E-6 Attach Figure 14.8, showing the vertical line you drew at 2 drops of added NaOH. What is the value of the [CH 3 COO - ]/[CH 3 COOH] ratio at this stage of the titration? What are the ph values of the 5 solutions at this point?

5 Section E-7 Attach the page from the manual with Figure 14.9, showing the vertical lines you drew to indicate the regions of good buffer capacity. Section E-8 From Figure 14.11, determine the pk b of ammonia and the pk a of ammonium ion, and the corresponding K b and K a values. Write a net ionic equation for the reaction in this titration. Attach Figure 14.12, including the vertical lines you drew to indicate the regions of good buffer capacity. What is the effect of dilution on the buffer solutions (ph of the buffer, buffer capacity)? Are titrations of bases with K b values of less than 1 x 10-8 feasible in aqueous solutions? Explain. Using the graph, describe the proportions of the acid and conjugate base in a buffer solution of ph 8.2. Section F Attach the page from the manual with Figure 14.15, with your experimental titration curve added. What are the major ionic species present in the solution at any point between "A" (the start of the titration) and "C" (the first equivalence point)? What indicator would be most suitable for the detection of the first equivalence point? Which two anions form the buffer system in the buffer region between points "C" and "D" (the second equivalence point)? What is the value of the acid dissociation constant, K a2, for H 2 PO 4 -?

6 Why are the titration curves for the different reagent concentrations in Figure so similar? Using the graph, describe the content of a buffer with a ph of 2.70 with a very good buffer capacity. Section G - Interpreting Titration Curves All titration curves of weak polyprotic acids contain information regarding the nature of the acid and unknown concentration. Use Figure to extract all the information available regarding the weak diprotic acid, H 2 A. Include both quantitative and qualitative answers here. Use Figure to extract all the information available regarding the weak triprotic acid, H 3 A. Include both quantitative and qualitative answers here.