13 minutes reading time (2514 words)

    MICROSCALE CHEMISTRY 2000

    J Skinner
    The Royal Society of Chemistry

    The following four experiments have been reproduced (with permission from The Royal Society of Chemistry) from "Microscale Chemistry" published and distributed by the Royal Society of Chemistry.

    4. THE REACTION OF METALS WITH ACIDS

    In this experiment you will be looking at the reactions between various metals and some acids.
    Read the instructions before you start the experiment to make sure you understand the procedure.


    INSTRUCTIONS

    1. Cover the worksheet with a clear plastic sheet.
    2. Place a few copper turnings in each box in the copper row.
    3. Place one small piece of magnesium ribbon in each box in the magnesium row.
    4. Place a few zinc granules in each box in the zinc row.
    5. Place some iron filings in each box in the iron row.
    6. Finally, place a few tin granules in each box in the tin row.

    When all the pieces of metal are in place:
    7. Add two drops of dilute hydrochloric acid to each metal in the hydrochloric acid column.
    8. Add two drops of dilute nitric acid to each metal in the nitric acid column.
    9. Add two drops of dilute sulfuric acid to each metal in the sulfuric acid column.
    10. Finally, put one piece of copper turning in the box at the bottom and add two drops of concentrated nitric acid.

    COMMENTS
    As you do these experiments observe carefully and record your findings.

    QUESTION
    1. What do you observe? Give explanations for your observations.
    31. OXYGEN AND METHYLENE BLUE

    In this experiment you will be generating oxygen gas by reacting hydrogen peroxide and potassium manganate(VII) and testing for it using methylene blue solution
    You will be familiar with testing for oxygen using a glowing splint which re-lights in the gas. This microscale experiment provides an alternative test.

    INSTRUCTIONS

    ​1. Construct the gas generating apparatus by cutting the top off a plastic pipette that a piece of rubber tubing can be attached to the pipette as shown:


    2. Add methylene blue solution to a 10mL beaker until it is about half-full.
    3. Add 10 drops of hydrogen peroxide to the shortened pipette.
    4. Turn the pipette almost to the horizontal position and carefully put five drops of potassium manganate (VII) solution in the stem as shown:

    5. Attach the rubber tubing to the pipette, place the other end in the methylene blue solution and gently turn the pipette upright. The potassium manganate(VII) solution, held in the stem, should fall down into the hydrogen peroxide causing vigorous evolution of oxygen gas.

    Describe your observations.


    QUESTIONS

    1. The reaction below shows the structures of methylene blue in the reduced (colourless) and blue (oxidised) forms. Which structure is which? Can you give reasons for your answer?
    2. Can you write an equation for the reaction between potassium manganate(VII) and hydrogen peroxide?

    36. MEASURING THE AMOUNT OF VITAMIN C IN FRUIT DRINKS

    In this experiment you will be finding out how much vitamin C there is in a fruit drink. The chemical name for vitamin C is ascorbic acid.

    The basis of the experiment is as follows.

    A known amount of iodine is generated by the reaction between iodate, iodide and sulfuric acid:

    IO3-(aq) + 5I-(aq) + 6H+(aq) → 3I2(s) + 3H2O(l)

    A measured amount of fruit drink is added. The ascorbic acid in the drink reacts quantitatively with some of the iodine as the iodine is in excess:

    The excess iodine is then titrated against standard sodium thiosulfate solution:

    I2(aq) + 2S2O32-(aq) → S4O62-(aq) + 2I-(aq)

    From the titration results the amount of iodine that reacts with the sodium thiosulfate solution can be found. Since the total amount of iodine originally formed is known the amount that reacts with the ascorbic acid is found by difference. Therefore the amount of ascorbic acid that reacts with this amount of iodine can be found.


    INSTRUCTIONS

    1. Set up the microscale titration apparatus.
    2. Fill the apparatus with sodium thiosulfate solution.
    3. Using the glass pipette add 2mL of potassium iodate solution to the beaker.
    4. Measure, using the measuring cylinder, 3mL of potassium iodide solution, then add this to the beaker. (Note: the potassium iodide solution is added in slight excess).
    5. Add three drops of sulfuric acid. A yellow-brown colour appears due to iodine.
    6. Add a few drops of starch solution. A deep blue-black colour forms.
    7. Using the glass pipette add 1mL of the fruit drink to the beaker and swirl gently.
    8. Titrate the remaining iodine in the beaker against the sodium thiosulfate solution. (The beaker can be swirled very gently to mix the chemicals. Alternatively, the tip of a plastic pipette can be used as a mini stirring rod). The disappearance of the deep blue-black colour marks the end-point.
    9. Do a duplicate titration and check the agreement between the two titres. If it is acceptable take the mean value of the two titres and use it for your calculations.


    CALCULATIONS

    A specimen result and calculation is given below. Study this carefully and use it as a guide for working out the vitamin C content of your fruit drink.
    The volume of thiosulfate delivered during the titration = 0.74mL.

    The concentration of thiosulfate = 0.010M.

    Therefore the number of moles of thiosulfate =

    (0.74 x 0.010) / 1000 = 7.4 x 10-6

    Therefore the number of moles of iodine that this reacts with during the titration is 3.7 x 10-6.

    The total number of moles of iodine produced in the reaction between iodate, iodide and sulfuric acid based on using 2mL of iodate with a concentration of 0.0012M =

    (3.2 x 2 x 0.0012) / 1000 = 7.2 x 10-6

    Therefore the number of moles of iodine that reacts with the ascorbic acid is 7.2 x 10-6 - 3.7 x 10-6€ = 3.5 x 10-6.

    Since 1 mole of iodine reacts with 1 mole of ascorbic acid then the number of moles of ascorbic acid is also 3.5 x 10-6.

    The volume of the fruit juice used is 1mL. Therefore the number of moles of ascorbic acid in 1000mL = 3.5 x 10-3.

    The relative molar mass of ascorbic acid = 174.12 g. Therefore the mass of ascorbic acid (in 1000mL) = 174.12 x 3.5 x 10-3 = 0.609 g.

    Therefore the vitamin C content of the fruit drink - 61 mg per 100mL.

    59. THE TREATMENT OF OIL SPILLS

    In this experiment you will be looking at an unusual and interesting way of chemically treating a microsize oil spill.

    INSTRUCTIONS

    1. Half fill a 100mL beaker with water.
    2. Using your pipette, add some oil or paraffin to the beaker to give a thin layer on top of the water.
    3. Cut off the end of a pipette to form a scoop as shown below.

    Add two scoops of polymer powder to the beaker and stir with the end of the pipette.

    QUESTIONS

    1. What do you observe?
    2. How do you explain your observations?
    3. If you were to do this experiment on a large scale to try to deal with an oil slick at sea what would be the advantages of using this polymer powder and what difficulties might you encounter?

    TEACHER'S GUIDE

    4. THE REACTION OF METALS WITH ACIDS

    TOPIC

    Metals - reactions with acids; reactivity series.

    LEVEL

    Pre-16.

    TIMING

    20 min.

    DESCRIPTION

    in this experiment students observe the reactions between metals and acids

    APPARATUS (PER GROUP)

    • One student worksheet
    • One clear plastic sheet (eg. OHP sheet)
    • Magnifying glass.

    CHEMICALS (PER GROUP)

    • Solutions contained in plastic pipettes.
    • Hydrochloric acid 1M
    • Dilute nitric acid 1M
    • Concentrated nitric acid 5M
    • Sulfuric acid 1M
    • Magnesium ribbon
    • Zinc metal - small granules
    • Iron filings
    • Tin granules
    • Copper turnings

    OBSERVATIONS

    The magnesium ribbon reacts vigorously with each acid. The zinc and iron also react, but less vigorously. in each case hydrogen gas is produced as well as the metal salt. The reaction between iron and nitric acid eventually produces a red-brown rust colour (iron (III) oxide). Students could link this with corrosion and acid rain. Tin and copper do not react with the hydrochloric and sulfuric acids but a few bubbles may be seen (using the magnifying glass) with the nitric acid.
    The copper reacts with the 5M nitric acid to produce a blue solution and bubbles (of brown nitrogen dioxide).

    Students can write word and symbol equations for these reactions.

    SAFETY

    Students must wear eye protection.

    It is the responsibility of the teacher to carry out a risk assessment.

    31. OXYGEN AND METHYLENE BLUE

    TOPIC

    Organic chemistry, redox reactions, dyes and colour chemistry.

    LEVEL

    Pre-16 and post-16.

    TIMING

    15 min.

    DESCRIPTION

    In this experiment students generate oxygen gas by the reaction between hydrogen peroxide and potassium manganate(VII), and then test for the gas by bubbling it into a solution of the reduced form of methylene blue dye, turning the solution blue.

    APPARATUS (PER GROUP)

    • One student worksheet.
    • One 10mL beaker.
    • One plastic pipette (standard form).
    • One piece of rubber tubing, ca 10 cm long.
    • Scissors.

    CHEMICALS (PER GROUP)

    Solutions contained in plastic pipettes, see p. 2.
    • Hydrogen peroxide 5% solution
    • Potassium manganate(VII) 0.1M
    • Methylene blue solution (colourless, leuco form of dye).
    • Glucose.
    Dissolve 4g of potassium hydroxide pellets in 150mL of deionised water in a plastic bottle or stoppered 250mL conical flask. Allow to cool and add 5g of glucose powder. Add 3-4 drops of methylene blue solution (0.25g in 1L of deionised water or Aldrich cat. no. 31,911-2). The blue solution should become colourless on standing a few minutes but will turn blue when shaken.

    OBSERVATIONS

    This experiment is a little tricky to perform and students will need to practice it first!

    The hydrogen peroxide and potassium manganate (VII) react together vigorously to produce oxygen gas. The colourless solution of methylene blue should turn blue quickly when the oxygen gas is directed into it.

    Students are given the structures of the oxidised and reduced forms of methylene blue and are asked to say which is which. The oxidised (blue) form contains conjugated double and single bonds throughout the whole molecule whereas in the colourless form the delocalised electron systems are isolated from each other. The structures are given on the previous page.

    REFERENCE

    Barton, D., and Ollis, W.D., Comprehensive organic chemistry, vol 4, pp1102-1107. Oxford: Pergamon, 1979.

    This book gives an interesting account of the dibenzo-1,4-thizines, of which methylene blue is a member.

    SAFETY

    Students must wear eye protection.

    It is the responsibility of the teacher to carry out a risk assessment.

    36. MEASURING THE AMOUNT OF VITAMIN C IN FRUIT DRINKS

    TOPIC

    Food, scientific methodology. Quantitative chemistry/mole calculations.

    LEVEL

    Pre-16 and post-16.


    TIMING

    20 min.

    DESCRIPTION

    In this experiment students use the microscale titration technique to measure the amount of vitamin C (ascorbic acid) in fruit drinks. The basis of the measurement is as follows.

    A known excess amount of iodine is generated by the reaction between iodate, iodide and sulfuric acid.

    IO3-(aq) + 5I-(aq) + 6H+(aq) → 3I2(s) + 3H2O(l)

    A measured amount of fruit drink is added. The ascorbic acid in the drink reacts quantitatively with some of the iodine:


    The excess iodine is then titrated against standard thiosulfate solution:

    I2(aq) + 2S2O32-(aq) → S4O62-(aq) + 2I-(aq)

    CHEMICALS (PER GROUP)

    • Sodium thiosulfate.
    • Potassium iodate.
    • Potassium iodide.

    Solutions contained in plastic pipettes.
    • Starch solution (freshly made).
    • Sulfuric acid 1M.
    • Sample(s) of fruit juice.

    APPARATUS (PER GROUP)
    • One student worksheet.
    • Microscale titration apparatus.
    • One 1mL pipette (glass).
    • One 2mL pipette (glass).
    • Pipette filter.
    • One 25mL beaker
    • One 5mL measuring cylinder.
    • One 10mL beaker (for filling titration apparatus).

    STOCK SOLUTIONS

    1. Sodium thiosulfate solution 0.010M.
    2. Weigh out, accurately, ca 0.620 g of Na2S2O3.5H2O, dissolve in deionised water and make up to 250mL in a volumetric flask. Store this stock solution in a dark glass bottle.
    3. Potassium iodate solution 0.001M.
    4. Weigh out, accurately, ca 0.054 g of KIO3, dissolve in deionised water and make up to 250mL in a volumetric flask.
    5. Potassium iodide solution 0.005M.
    6. Weigh out 0.21 g of KI, dissolve in deionised water and make up to 250mL with deionised water.

    NOTE

    The reaction to generate the iodine is based on using an accurately known volume of the potassium iodate solution (the concentration of which is accurately known). The potassium iodide solution and the sulfuric acid are added in slight excess and thus the concentrations of these solutions is not critical.

    OBSERVATIONS

    The titre volume should be in the range 0.5-1mL, the disappearance of the blue- black colour marking the end-point.

    This experiment offers possibilities for assessing students' abilities in following instructions and/or processing results.

    A survey of a range of fruit drinks (and maybe other products containing vitamin C) could form the basis of a class project or as an activity for a school or college chemistry club.

    SPECIMEN RESULT AND CALCULATION

    Volume of thiosulfate delivered during the titration = 0.74mL.

    Concentration of thiosulfate = 0.010M.

    Therefore number of moles thiosulfate =

    (0.74 x 0.010) / 1000 = 7.4 x 10-6

    Therefore the number of moles of iodine that this reacted with during the titration = 3.7 x 10-6.

    The total number of moles of iodine produced in the reaction between iodate, iodine and sulfuric acid based on using 2mL of iodate with a concentration of 0.0012M =

    (3.2 x 2 x 0.0012) / 1000 = 7.2 x 10-6

    Therefore the number of moles of iodine which reacted with the ascorbic acid - 7.2 x 10-6 - 3.7 x 10-6 = 3.5 x 10-6.

    Since 1 mole of iodine reacts with 1 mole of ascorbic acid then the number of moles of ascorbic acid is also 3.5 x 10-6.

    The volume of the fruit juice used was 1mL. Therefore the number of moles of ascorbic acid in 1000mL = 3.5 x 10-3.

    The relative molar mass of ascorbic acid = 174.12 g. Therefore mass of ascorbic acid (in 1000mL) = 174.12 x 3.5 x 10-3 = 0.609g.

    The vitamin C content of the fruit drink = 61 mg per 100mL.

    REFERENCE

    J. Chem. Ed. 1992, 69, A213-4.

    NOTE

    Instead of generating the iodine in situ, it is possible to use standard iodine solution in this procedure. This would need to be diluted to give an aliquot containing 7.2 x 10-6 moles of iodine (see above) for each determination.

    SAFETY

    Students must wear eye protection.

    It is the responsibility of the teacher to carry out a risk assessment.

    59. THE TREATMENT OF OIL SPILLS

    TOPIC

    Pollution control Polymers - uses of intermolecular bonding.

    LEVEL

    All

    TIMING

    10 min.

    Description

    In this experiment, oil or paraffin is added to some water in a beaker to simulate an oil spill. A special powdered polymer is then sprinkled on top. On stirring, the polymer absorbs the hydrocarbon molecules and a rubbery solid is formed which can then be scooped up. The experiment is quite fun to do and provides several interesting points for follow-up discussion in both theoretical and applied chemistry (pollution and its control).

    APPARATUS (PER GROUP)

    • One student worksheet.
    • One 100mL beaker.
    • Plastic pipette.
    • Scissors.

    CHEMICALS (PER GROUP)

    • Soil-moist hydrocarbon polymer (see below).
    • Oil or paraffin.

    OBSERVATIONS

    1. On adding the polymer, and stirring, a rubbery solid is formed very quickly, and the layer of oil/paraffin disappears.

    NOTE

    The essential ingredient in this experiment is the powdered polymer which can be obtained from Educational Innovations Inc, Cos Cob, Connecticut, USA at a price of $5 per oz. With careful use 1oz should provide enough for many experiments! The polymer itself is a copolymer of acrylamide and hydroxymethylmethacrylate, crosslinked and dehydrated. A similar substance is produced commercially by BP under the trade name Rigidoil.

    SAFETY

    Students must wear eye protection.
    It is the responsibility of the teacher to carry out a risk assessment.  
    IN SEARCH OF MORE SOLUTIONS 99
    MICROSCALE CHEMISTRY
     

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