Oxidation reduction reaction

Transcription

1 P.O. Box 219 Batavia, Illinois f l i n f l i n n s c i. c o m Visit our website at: www. f l i n n s c i. c o m 2003 Flinn Scientific, Inc. All Rights Reserved. Your Safer Source for Science Supplies Oxidation Reduction Titrations AP Chemistry Laboratory #8 Catalog No. AP8815 Publication No A I n t ro d u c t i o n A common task in analytical ch e m i s t ry is the determ i n ation of the amount of a substance pre s e n t in a sample or product. If the product contains a substance that can be ox i d i ze d, then it is possibl e to determine the number of moles of that substance by titrating the sample with a solution of a s t rong oxidizing agent. In this lab, an oxidizing solution will be standard i zed and then use to determine the number of moles of oxalic acid, a reducing age n t. C o n c e p t s Oxidation reduction reaction Titration Half-reaction Equivalence point B a c k g ro u n d Oxidation reduction reactions occur by electron transfer. The balanced chemical reaction can be written as the combination of two half-reactions, representing the oxidation reaction and the reduction reaction, respectively. For example: If solid iron is placed in a solution of gold(iii) ions, the gold(iii) ions are reduced to solid gold and the iron oxidized to iron(iii) ions, according to the following half-reactions: Fe(s) Fe 3+ (aq) + 3e oxidation half-reaction Au 3+ (aq) + 3e Au(s) reduction half-reaction Fe(s) + Au 3+ (aq) Au(s) + Fe 3+ (aq) oxidation reduction reaction Reaction 1 In this ex p e ri m e n t, potassium perm a n ga n at e, K M n O 4, is used as the oxidizing agent. In an acidic s o l u t i o n, the MnO 4 ion is reduced from Mn(VII) to Mn(II) according to the fo l l owing half-re a c t i o n : 8H + (aq) + MnO 4 (aq) + 5e Mn 2+ (aq) + 4H 2 O(l) Half-Reaction 2 In Part 1, a solution of KMnO 4 is standardized by titration with a solution containing a known concentration of iron(ii) ions, (Fe 2+ ). C H E M - FAX...makes science teaching easier. I N A

2 Oxidation Reduction Ti t r a t i o n Page 2 P re-lab Questions In the corresponding ox i d ation half-re a c t i o n, the Fe 2 + ion is ox i d i zed to Fe 3+ : Fe 2+ (aq) Fe 3+ (aq) + e Half-Reaction 3 Combining half-reactions 2 and 3 and balancing the number of electrons tra n s fe rred gives the ove r- all reaction equat i o n : 8 H + ( a q ) + M n O 4 ( a q ) + 5 Fe 2 + ( a q ) M n 2 + ( a q ) + 5 Fe 3 + ( a q ) + 4 H 2 O ( l ) Reaction 4 The balanced equation shows that 5 moles of Fe 2 + a re re q u i red to react with 1 mole of MnO 4. For this re d ox titrat i o n, the equivalence point o c c u rs when the exact number of moles of Fe 2 + i o n s has been added to react completely with all the MnO 4 ions in solution. At this point: If the volume and molarity of the Fe 2 + solution are know n, t h e n : moles Fe 2+ = 5 (moles MnO 4 ) Equation 1 V Fe 2+M Fe 2+ = 5 V MnO4 M MnO4 Equation 2 R e a rra n ging Equation 2 yields the equation for the concentration of the potassium perm a n ga n at e s o l u t i o n. (V Fe 2+) (M Fe 2+) M MnO4 = 5 (V MnO4 ) The indicator for this titration is the MnO 4 ion itself. The MnO 4 ion is purple in solution. At the endpoint of the titrat i o n, the solution ch a n ges from light pink to colorl e s s. In Pa rt 2, the concentration of an oxalic acid solution is determined by titration with the perm a n- ga n ate solution standard i zed in Pa rt 1. In this case, the endpoint occurs when the pink color of the MnO 4 ion persists. The half-reaction for the ox i d ation of oxalic acid is: 2H 2 O(l) + H 2 C 2 O 4 (aq) 2H 2 CO 3 (aq) + 2H + (aq) + 2e Half-Reaction 5 The ox i d ation state of carbon ch a n ges from (+3) in H 2 C 2 O 4 to (+4) in H 2 CO 3. Experiment Overview The purpose of this lab is to standard i ze a solution of potassium perm a n ga n ate by re d ox titrat i o n with a standard solution of iron(ii) ions. A solution of oxalic acid is then titrated with the perm a n- ga n ate solution to determine the exact concentration of oxalic acid. 1. W rite the balanced net ionic equation for the reaction between MnO 4 ions and H 2 C 2 O 4 in acid s o l u t i o n. 2. H ow many moles of Fe 2 + ions can be ox i d i zed by moles of MnO 4 i o n s? g of iron ore is dissolved in an acidic solution. This solution is titrated to a pink endpoint with ml of a M KMnO 4 s o l u t i o n. a. H ow many moles of MnO 4 ions we re consumed? b. H ow many moles of Fe 2 + we re in the iron ore sample? c. Wh at is the percent of iron in the iron ore sample?

3 Oxidation Reduction Ti t r a t i o n Page 3 M a t e r i a l s B u re t, 50 - m L E rl e n m eyer fl a s k s, m L, 3 Hot plat e Th e rm o m e t e r Vo l u m e t ric pipet, 10 - m L Vo l u m e t ric pipet, 25 - m L B e a ke rs, m L, 3 G ra d u ated cy l i n d e r, 10 - m L Wash bottle Ring stand B u ret cl a m p Potassium perm a n ga n at e, K M n O 4, 0.02 M, 100 ml Fe rrous ammonium sulfat e, Fe ( N H 4 ) 2 S O 4 6 H 2 O, M, 50 ml Oxalic acid solution, H 2 C 2 O 4, 60 ml S u l f u ric acid, H 2 S O 4, 6 M, 50 ml M a n ganese sulfat e, M n S O 4 H 2 O, 1.0 M, 5 ml Wat e r, distilled or deionize d Safety Pre c a u t i o n s P ro c e d u re S u l f u ric acid (6 M) is corro s ive to eye s, s k i n, and other tissue; always add acid to wat e r, n ever the reve rs e. Potassium perm a n ga n ate solution may be a skin irritant. The oxalic acid solution is a skin and eye irritant; it is moderat e ly toxic by ingestion. The manganese sulfate solution is a body tissue irritant. Wear chemical splash goggles and ch e m i c a l - resistant gloves and ap ron. Wash hands t h o ro u g h ly with soap and water befo re leaving the lab o rat o ry. Part 1. Standardization of a Potassium Permanganate Solution 1. Obtain ap p rox i m at e ly 80 ml of the potassium perm a n ga n ate solution in a 100-mL beake r. Obtain 50 ml of the M Fe 2 + solution in another 100-mL beake r. Label both beake rs. 2. Set up a cl e a n, 50-mL bu ret in the ring stand and bu ret cl a m p. 3. Rinse the bu ret with ap p rox i m at e ly 10 ml of distilled or deionized water and then with two 5 ml portions of the MnO 4 s o l u t i o n. 4. Close the stopcock and fill the bu ret to ab ove the ze ro mark with MnO 4 s o l u t i o n. 5. Open the stopcock to allow any air bu bbles to escape from the tip. Close the stopcock when the liquid level is between the 0- and 10-mL mark s. 6. R e c o rd the precise level of the solution in the bu ret in the Pa rt 1 D ata Tabl e. This is the initial volume of the MnO 4 solution. (See Fi g u re 1 for reading bu ret leve l. ) 7. With the vo l u m e t ric pipet, t ra n s fer 10 ml of the M Fe 2 + solution to a clean 250-mL Erl e n m eyer flask. Record this vo l u m e in the Pa rt 1 Data Tabl e. 8. M e a s u re out 10 ml of the 6 M H 2 S O 4 into a clean 10-mL gra d u at e d cylinder and add this to the Erl e n m eyer flask. Swirl to mix. Figure 1.

4 Oxidation Reduction Ti t r a t i o n Page 4 P ro c e d u re 9. Position the flask under the bu ret so that the tip of the bu ret is within the flask but at least 2 cm ab ove the liquid surfa c e Ti t rate the Fe 2 + solution with the MnO 4 solution until the fi rst trace of pink color persists fo r 30 seconds. Remember to sw i rl the flask and to rinse the walls of the flask with distilled wat e r b e fo re the endpoint is re a ch e d R e c o rd final bu ret reading as the final volume of the MnO 4 solution in the Pa rt 1 Data Tabl e R ep e at the standard i z ation titration two more times. Part 2. Determination of Concentration of an Oxalic Acid Solution 1. Obtain ap p rox i m at e ly 60 ml of the oxalic acid solution in a clean 100-mL beake r. 2. With a 25-mL vo l u m e t ric pipet, t ra n s fer 25 ml samples of the oxalic acid solution to each of t wo 250-mL Erl e n m eyer flasks. Record the volume in the Pa rt 2 Data Tabl e. 3. A dd 5 drops of the 1.0 M MnSO 4 solution to each flask. (The Mn 2+ ion acts as a catalyst for the reaction.) 4. M e a s u re out 10 ml of 6 M H 2 S O 4 into a gra d u ated cylinder and add this amount to each of the 250-mL Erl e n m eyer flasks. A dd 20 ml of distilled water to each flask and sw i rl. 5. Wa rm the fi rst flask to about 85 C on the hot plat e. 6. I m m e d i at e ly titrate this solution with the standard i zed MnO 4 solution from Pa rt 1. Record both the initial and final bu ret readings in the Pa rt 2 Data Tabl e. 7. R ep e at steps 5 and 6 with the second fl a s k. D i s p o s a l Your instructor will provide disposal instru c t i o n s.

5 Oxidation Reduction Ti t r a t i o n Page 5 Data Ta b l e s Part 1 M o l a rity of Fe 2 + M Trial 1 Trial 2 Trial 3 Volume of Fe 2 + solution titrat e d m L m L m L Initial volume of MnO 4 s o l u t i o n m L m L m L Final volume of MnO 4 s o l u t i o n m L m L m L Volume of MnO 4 a dd e d m L m L m L Part 2 M o l a rity of MnO 4 solution M Trial 1 Trial 2 Volume of H 2 C 2 O 4 solution titrat e d m L m L Initial volume of MnO 4 s o l u t i o n m L m L Final volume of MnO 4 s o l u t i o n m L m L Volume of MnO 4 a dd e d m L m L M o l a rity of H 2 C 2 O 4 solution M Post-Lab Calculations 1. From the Pa rt 1 standard i z ation dat a, c a l c u l ate the molarity of the M n O 4 solution for each t rial. Ave rage the values and enter the ave rage in the Pa rt 2 Data Tabl e. 2. From the Pa rt 2 titration dat a, c a l c u l ate the molarity of the H 2 C 2 O 4 solution for each tri a l. Ave rage the values and enter the ave rage in the Pa rt 2 Data Tabl e. 3. H ow many moles of Fe 2 + ions and MnO 4 ions we re titrated in each Pa rt 1 tri a l? 4. H ow many moles of oxalic acid, H 2 C 2 O 4 we re titrated in each Pa rt 2 tri a l?