December 2012 - January 2013

Qualitative analysis of a redox reaction

Clyde Chamberlain, Marc Garneau Collegiate Institute, Don Mills ON

The reaction of iron(II) sulfate and silver nitrate has led to some interesting observations such as the formation of metallic silver and iron(III) sulfate/nitrate. This investigation of the reaction involves precipitation of sparingly soluble salts, formation of complex ions, conductivity measurements, filtration, centrifugation, redox and melting. The students prepare a fully labelled flowchart as a summary of their processes, reagents added, observations and the substances separated and identified. The students will conduct themselves through an analytical procedure following a preliminary flowchart that they develop from the laboratory instructions. The teacher will check over the procedure before the students begin and identify any safety considerations beforehand.

This lab replaces a qualitative lab, which uses mercury and lead. It relates to the grade 12 units on solubility, acid/base reactions and redox.

Materials

  • well plate (chemplate)
  • 0.1 M AgNO3, 0.1 M FeSO4, 0.1 M KSCN,
    0.1 M Ba(NO3)2, 0.1 M HClm 0.1 M HNO3
  • 0.05 M NaCl, 0.1 M NH3
  • crucible and clay triangle, Bunsen burner and tripod
  • multimeter, hotplate, centrifuge
  • 3-mL plastic funnel and 3-cm filter paper
  • 10 x 75 mm test tubes
  • distilled water
  • dropping pipet

Procedure

  1. Add 25 drops of 0.1 M silver nitrate with 25 drops of 0.1 M iron(II) sulfate in a 10 x 75 mm test tube. Swirl and let it sit for five minutes. Record your observations.
  2. Place the test tube in a centrifuge, balance and spin for 2 minutes at 5000 rpm. Stop the centrifuge and remove the test tube. Decant the solution into two dry wells of the chemplate. You may have to use a pipet.
  3. Keep the solid at the bottom of the test tube as sample A.
  4. To one well add 5 drops of 0.1 M potassium thiocyanate and record your observations. If a white precipitate forms then some silver ion must not have reacted in reaction one. Get the students to check the CRC Handbook to determine what the precipitate might be. If the solution turned red then go to step 5. If not then fold the small piece of filter paper in a fluted style, place it in a funnel and using a pipet transfer the mixture from the well to the filter paper, holding the funnel over another well. Once the filtering is complete wash the residue with 5 drops of water and add 5 more drops of 0.1 M potassium thiocyanate to the filtrate. The red complex ion should form. Record all observations.
  5. Take the second well with solution from reaction one and show it to the teacher. The teacher will withdraw it into a master chemplate and perform the barium test with 0.1 M barium nitrate. (Note: restricted substance in the Toronto District School Board). The teacher will add 5 drops of the barium nitrate to the well. If a white precipitate forms then you have a positive test for sulfate with the formation of barium sulfate, a sparingly soluble salt. Record your observations. What, then, were the product compounds formed in reaction one? The teacher will discard this solution with the barium in the proper disposal container.
  6. Add 20 drops of distilled water to sample A in the test tube. Stir it around to wash the solid, let it settle and then remove the wash with a pipet to waste.
  7. Add 15 drops of 0.1 M nitric acid to the test tube with the remaining solid residue. Turn on your hot plate and hold the test tube with tongs with its bottom close to the plate. Warm gently. Have the hot plate under the bench-top fume column. With continuous agitation and heating (remove if it steams or bumps) heat until the solid disappears. Point the test tube away from people. Then remove the warmed test tube from the hot plate and let it cool for 5 minutes. Bring the test tube to room temperature.
  8. Once the solution is back to room temperature add 10 drops of 0.05 M sodium chloride. If a white precipitate forms then we have a positive test for silver ion. Find out how nitric acid changes silver metal into silver ion. Record observations.
  9. Remove a small sample of the white precipitate into another well and add 10 drops of 0.1 M aqueous ammonia. If the precipitate becomes clear with shaking then a silver ammonium complex ion has formed. Get the students to investigate its formula. This confirms that the white precipitate was silver chloride. Add more if it doesn’t become clear. Record observations.
  10. Remove a few drops of the final solution in step 9 and add drops of 0.1 M nitric acid (maximum 15 drops) to the well until it turns white. What precipitate formed and why?
  11. The teacher will have made a larger sample A. It will be placed in a crucible and heated with a Bunsen burner at maximum temperature. Small shiny, metallic globules will form that confirms the production of silver in reaction one. Students should perform conductivity test on the globules. Record observations.

Analysis and discussion

1.   The suspected reaction step 1 is:

3FeSO4(aq) + 3AgNO3(aq)  →    3Ag(s) + Fe2(SO4)3(aq) + Fe(NO3)3(aq)

Calculate the theoretical yield of the silver product using a limiting reagent calculation? (Students can find the volume of one drop from the pipet by determining the number of drops needed to make 1.0 mL.)

2.   Draw a fully labelled flowchart showing the separation and identification processes, reagents, colours and tests. Include in the boxes the formulae. Beside the lines joining the boxes include the reagents and processes.

3.   Investigate the oxidation and reduction processes that appear to have taken place based on your results. Outline these reactions in your writeup and explain why they are redox.

4.   What is qualitative analysis?

5.   Identify and explain all of the separation techniques used in this investigation.

6.   How was solubility involved in this lab exercise?

7.   What equilibria may be present?

8.   What complex ions were formed? What is a complex ion?

9.   Construct a chart to summarize the results. Explain briefly what happened in each step. Columns should be as follows: step #, reactants, observations and a comment column of how this step helps you determine what happened in the reaction in step 1.

10.  Write all of the balanced equations for the reaction in each step. There are seven reactions. Identify the step # beside each reaction.

11.  Why would the white solid form before the FeSCN2+ ?
(Hint: Ksp, Keq)

Teacher notes

This lab provides the students with the opportunity to develop various microscale analytical skills, it familiarizes them with equipment; and it offers them a challenge where they can design some procedures. Separation and identification are key processes in inorganic analysis. These procedures certainly could be used to analyze ore samples, soil from a crime scene and water and air quality. There are some open-ended parts to which the student and teacher can develop further investigations. The small scale saves on equipment and chemicals.

The students liked designing their own flowchart and extending it where possible. They felt they were doing real research or acting like CSI agents and enjoyed the microscale aspects. As well, it was interesting for them to use several procedures they had learned from previous study.

The laboratory exercise written above is organized as information for teachers to design their own student lab sheets. These are not student instructions but can be massaged to become just that! Once the lab is completed, the students can prepare a formal lab report to be graded.

Answer to question 10

Steps with proposed reactions

1.   3AgNO3(aq) +  3FeSO4(aq) → Fe2(SO4)3(aq) +  3Ag(s) +  Fe(NO3)3(aq)

4.   Fe3+(aq) +  SCN-(aq)  →  FeSCN2+(aq)

5.   3Ba(NO3)2(aq) +  Fe2(SO4)3(aq)  → 2Fe(NO3)3(aq) +  3BaSO4(s)

7.   Based on their experimental results, students may come up with three possible balanced chemical equations. The silver likely reacts to become silver nitrate but the other products have not been verified. Given the experiment is microscale and dilute nitric acid is used, it may be difficult to test for these gases.

3Ag(s)  +  4HNO3(aq) --> 3AgNO3(aq)  +  NO(g)  +  2H2O(l)

2Ag(s)  +  2HNO3(aq)  →  2AgNO3(aq)  +  H2(g)

Ag(s)  +  2HNO3(aq)  -->  AgNO3(aq) +  NO2(g)  +  H2O(l)

[These three equations were proposed by the author and proofreaders with varying rationales.]

8.   AgNO3(aq) +  NaCl(aq)  →  AgCl(s) +  NaNO3(aq)

9.   AgCl(s) +  2NH3(aq)  →  Ag(NH3)2+(aq) +  Cl-(aq)

10.   Ag(NH3)2+(aq) +  2HNO3(aq) +  Cl-(aq)  → AgCl(s) +  2NH4NO3(aq)

More about December 2012 - January 2013