11 minutes reading time (2269 words)

    MICROSCALE CHEMISTRY

    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.

    3. OBSERVING CHEMTCAL CHANGES

    In this experiment you will be observing the changes that occur when you mix solutions of chemicals on the grid shown.
    Instructions

    Cover the worksheet with a clear plastic sheet.

    1. Put two drops of barium nitrate solution into box 1 (at the top of the middle column). Add two drops of sodium sulfate solution to the drops of barium nitrate solution.
    2. Put two drops of lead nitrate solution into box 2. Add two drops of potassium iodide solution to the drops of lead nitrate solution.
    3. Put two drops of iron (III) nitrate solution into box 3. Add one drop of potassium thiocyanate solution to the iron (III) nitrate solution.
    4. Put two drops of copper (II) sulfate solution into box 4. Add two drops of ammonia solution to the copper(II) sulfate solution.
    5. Put two drops of ammonium vanadate(V) solution into box 5. Add one drop of hydrochloric acid, then a small piece of zinc metal to the ammonium vanadate(V) solution.
    6. Put two drops of iron (II) sulfate solution into box 6. Add two drops of sodium hydroxide solution to the iron (II) sulfate solution.
    7. Put two drops of potassium manganate (VII) solution into box 7. Add two drops of iron(II) sulfate solution to the potassium manganate(VII) solution.
    8. Put two drops of barium nitrate solution into box 8. Add two drops of sodium hydroxide to the barium nitrate solution. Observe, and record any changes over the next 10 min.
    9. Put one drop of silver nitrate solution into box 9. Add one drop of iron(II) sulfate to the silver nitrate solution. observe closely using a magnifying glass.
    10. Put two drops of copper(II) sulfate solution into box 10. Add a small piece of zinc metal to the copper sulfate solution.

    INFORMATION SHEET FOR EXPERIMENT 24
    These experiments all involve generating small amounts of gas inside a petri dish. Then, using test solutions or solid powders placed around the edge of the dish to test for the gas, you observe any colour changes that take place.

    Only very small amounts of gas are produced in each experiment (in most cases you will not even see any bubbles of gas!). Provided the instructions are followed carefully, all the gas is contained within the petri dish. These microscale experiments therefore enable you to do chemical reactions that would be difficult and hazardous to do on a larger scale.

    Before you start you will need to cut the bottom off a plastic pipette using a pair of scissors. The bottom piece of the pipette will be your reaction vessel in your experiments.

     At the end of each experiment mop up the chemicals inside the petri dish and the reaction vessel with tissue paper. The dish and vessel can then be re-used.


    24. SOME REACTIONS OF AMMONIA

    Instructions

    1. Cover the worksheet with a clear plastic sheet.
    2. Place the base of the petri dish directly over the circle below. Place the reaction vessel in the centre.
    3. At the corners of the triangle add drops of the test solutions only as indicated below (care: Nessler's reagent is toxic - it contains mercury compounds - make sure that you do not get any on your skin. If you do, wash it off quickly with water).
    4. Put three drops of ammonia solution into the reaction vessel and quickly replace the lid on the petri dish.
    5. Record all your observations over the next 15 min.

    Question

    1. What explanations can you give for your observations?

    50. THE OXIDATION OF ALCOHOLS

    In this experiment you will be testing various alcohols to see whether they can be oxidised by a solution of acidified potassium dichromate.

    Instructions

    ​1. Put 10 drops of acidified potassium dichromate solution into each of the wells A1- A5 and B3 (see diagram).
    2. Add two drops of alcohol to the wells as follows:

    Do not put any alcohol into well B3 - this well is used as a control.
    3. Observe the wells over the next 15 minutes and record any changes you see.

    Questions

    1. How do you explain any colour changes you see?
    2. Which alcohols have been oxidised?
    3. Can you find a connection between the ease of oxidation of an alcohol and its structure?

    54. THE ANALYSIS OF ASPIRIN TABLETS

    In this experiment you will be finding out how much 2-hydroxybenzoic acid (salicylic acid) is present in 2-ethanoyloxybenzenecarboxylic acid (aspirin) tablets.
    2-Hydroxybenzoic acid (salicylic acid) is formed in the following reaction:

    Instructions Part A The preparation of standard solutions

    In this part of the experiment you will be preparing a set of standard solutions with different colour intensities from the standard 2-hydroxybenzoic acid (salicylic acid) solution. You will be using these to match the intensity of the colour produced from the 2-ethanoyloxbenzenecarboxylic acid (aspirin) solution and so find out how much 2- hydroxybenzoic acid (salicylic acid) there is in your 2-ethanoyloxybenzenecarboxylic acid (aspirin) tablet.

    Taking your 24-well plate, add drops of solutions as indicated below:

    Part B The analysis of 2-ethanoyloxybenzenecarboxylic acid (aspirin) tablets

    1. Record the mass of a 2-ethanoyloxybenzenecarboxylic acid (aspirin) tablet and place it in a 100mL beaker.
    2. Add 10mL of the 50% ethanol-water mixture (from a measuring cylinder) and swirl the mixture. The tablet will begin to disintegrate.
    3. Using the microscale filtration method (p. 5), filter the mixture into a 25mL volumetric flask. Wash the beaker with a small quantity of the ethanol-water mixture and add to the flask. Make up to the mark, stopper and mix.
    4. Add 50 drops of this 2-ethanoyloxybenzenecarboxylic acid (aspirin) solution to well B3 followed by five drops of the iron(III) nitrate solution.
    5. Match the colour to that of one of the standard solutions.


    Calculations

    Calculate the percentage of 2-hydroxybenzoic acid (salicylic acid) in the 2- ethanoyloxybenzenecarboxylic acid (aspirin) tablet as follows.

    1. Identify the standard well that matches the colour intensity of the 2- ethanoyloxybenzenecarboxylic acid (aspirin) sample well.
    2. The mass of 2-hydroxybenzoic acid (salicylic acid) (in 25mL) in the solution from this standard well is therefore the same as the mass of 2-hydroxybenzoic acid (salicylic acid) in the 25mL of solution of your 2-ethanoyloxybenzenecarboxylic acid (aspirin) tablet solution.
    3. Divide this mass (mg) by the mass of your 2-ethanoyloxybenzenecarboxylic acid (aspirin) tablet (mg) and multiply this value by 100 to give a percentage by mass.


    Questions

    By considering the equation for the formation of 2-hydroxbyenzoic acid (salicylic acid) from 2-ethanoyloxybenzenecarboxylic acid (aspirin), are there any differences in how much 2-hydroxybenzoic acid (salicylic acid) is present in both old and new bottle of 2-ethanoyloxybenzenecarboxylic acid (aspirin) tablets?

    TEACHER'S GUIDE
    OBSERVING CHEMICAL CHANGES

    Topic

    Displacement, redox and precipitation reactions. Chemistry and colour.

    Level

    Any

    Timing

    20 min.

    Apparatus (per group)

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

    Chemicals (per group)

    Solutions contained in plastic pipettes
    • Barium nitrate 0.2M
    • Sodium sulfate 0.5M
    • Lead nitrate 0.5M
    • Ammonia solution 3M
    • Ammonium vanadate(V) 0.2M (acidified with sulfuric acid)
    • Hydrochloric acid 1M
    • Sodium hydroxide 1M
    • Potassium manganate(VII) 0.01M
    • Silver nitrate 0.1M
    • Copper(II) sulfate 0.2M
    • Iron(III) sulfate 0.2M
    • Potassium thiocyanate 0.2M
    • Zinc metal granules.


    Observations

    1. A dense white precipitate of barium sulfate forms. Barium sulfate is used as a barium meal in medicine since it is opaque to X-rays. Because it is very insoluble it is non-toxic, unlike other, soluble, barium compounds.
    2. A bright yellow precipitate of lead nitrate forms. Lead nitrate is a very effective pigment but it is toxic.
    3. A deep red colour is produced due to iron(III) thiocyanate ions. This reaction is used to test for the presence of iron.
    4. A deep blue colour of tetra-amminocopper(II) forms. There may also be some light blue precipitate of copper(II) hydroxide.
    5. Bubbles (of hydrogen) are seen. The yellow colour of the ammonium vanadate gradually changes (as the vanadium is reduced)to blue owing to the formation of the vanadium(IV) ion (VO2+). The colour then changes to green due to the vanadium(III) ion (V3+) and finally to lilac due to the vanadium(II) ion (V2+). The changes in oxidation states of vanadium salts have been investigated for applications in battery technology.
    6. A greenish precipitate of iron(II) hydroxide forms. This gradually changes to the brown iron(III) hydroxide as the iron is oxidised.
    7. The deep purple colour of the potassium manganate(VII) gradually fades first to the brown manganese(IV) dioxide and then to the pale pink manganese(II) ion (Mn2+). Manganese(II) compounds in solution usually appear virtually colourless. However, a solid manganese(II) salt is pink.
    8. Barium hydroxide forms. This is soluble so nothing is seen at first. Barium hydroxide is alkaline and gradually absorbs carbon dioxide from the air to form the insoluble barium carbonate. The drop takes on a hazy appearance as a skin of barium carbonate forms on the surface.
    9. A glittering of metallic silver forms as the iron(III) reduces the silver nitrate. This is seen clearly using a magnifying glass.
    10. The surface of the pieces of zinc turn red-brown as copper metal deposits via a displacement reaction. The blue colour of the copper(II) sulfate solution fades.


    Note

    Both procedure 9 and procedure 10 involve the displacement of a valuable, but less reactive, metal using a less valuable, but more reactive, metal. This could be used as a topic for discussion.

    Safety

    Students must wear eye protection. It is the responsibility of the teacher to carry out a risk assessment.

    24. SOME REACTIONS OF AMMONIA

    Topic Gases.

    Level Pre-16 and post-16.


    Timing 

    20 min.

    Apparatus (per group)

    Student information sheet and worksheet

    • One clear plastic sheet (eg. OHP sheet)
    • One 9cm plastic petri dish (base + lid)
    • One plastic pipette
    • Scissors


    Chemicals (per group) Solutions contained in plastic pipettes, see p. 2

    • Ammonia solution 3M.
    • Full range indicator solution diluted 1:1 with deionised water
    • Copper(II) sulfate solution 0.2M.
    • Nessler's reagent (an alkaline solution of mercury iodide containing the complex ion
    • HgI4-).


    Method
    Evaporation of ammonia gas from ammonia solution:
    NH3(aq) → NH3(g)

    Tests

    1. Full-range indicator solution turns blue-green.
    2. Copper(II) sulfate solution turns hazy and then develops deep blue streaks as the tetra-amminocopper(II) ion is formed.
    3. Nessler's reagent turns first yellow then brown. This is a very sensitive test for ammonia. The compound formed has the formula (OHg2NH2)I and consists of covalent metal-non-metal bonds which might provide an interesting point for subsequent class discussion.


    Safety

    Students must wear eye protection. Nessler's reagent is toxic and contains mercury. It is the responsibility of the teacher to carry out a risk assessment.


    50. THE OXIDATION OF ALCOHOLS
    Topic Organic chemistry - alcohols.
    Level Post-16

    Timing 

    20 min.

    Description 

    In this experiment students look for colour changes when drops of various alcohols are added to acidified potassium dichromate solution in a well-plate. The experiment has advantages over traditional methods in that only very small amounts of each alcohol are needed and no heating is required.

    Apparatus (per group)

    • Well-plate (24 wells) - eg. Sigma ref: M9655.
    • Chemicals (per group)
    • Methanol
    • Ethanol
    • Propan-1-ol
    • Propan-2-ol
    • 2-Methylpropan-2-ol (or other tertiary alcohol)
    • Solution contained in plastic pipette, see p. 2
    • Acidified potassium dichromate. (To prepare a stock solution: dissolve 2g of potassium dichromate in 80mL of deionised water and carefully add 10mL of concentrated sulfuric acid).

    Observation

    1. With the primary alcohols (methanol, ethanol and propan-1-ol) the dichromate solution starts to turn green after a few minutes indicating that oxidation is taking place although the methanol usually seems to be slower than the other two. With propan-2-ol (secondary alcohol) the colour change occurs, but is rather slow.
    2. With 2-methylpropan-2-ol no oxidation occurs and the dichromate solution does not change colour.

    Safety

    Students must wear eye protection. It is the responsibility of the teacher to carry out a risk assessment.


    54. THE ANALYSIS OF ASPIRIN TABLETS

    Topic Organic chemistry, chemical analysis.
    Level Post-16.

    Timing 

    20 min.

    Description 

    In this experiment students measure the amount of free 2-hydroxybenzoic acid (salicylic acid) in 2-ethanoyloxybenzenecarboxylic acid (aspirin) tablets. 2-hydroxybenzoic acid (salicylic acid), being a substituted phenol, reacts with Fe3+ ions to produce a purple colour. The colour is matched against that produced by a set of standard solutions of 2-hydroxybenzoic acid (salicylic acid) in a well-plate.

    Apparatus (per group)

    • One 24-well plate.
    • One 100mL beaker.
    • Cotton wool.
    • One plastic pipette (standard form, eg. Aldrich ref:Z13, 500-3)
    • Two plastic pipettes (fine tip, eg. Aldrich ref:Z13, 503-8)
    • Sheet for microscale filtration technique.

    Chemicals (per group)

    • Various 2-ethanoyloxybenzenecarboxylic acid (aspirin) tablets
    • Solutions contained in plastic pipettes (fine tip).
    • Iron(III) nitrate solution
    • 2-Hydroxybenzoic acid (salicylic acid) (working) solution
    • Deionised water.


    1. Stock 2-hydroxybenzoic acid (salicylic acid) solution (0.1% w/v)
    Dissolve 0.100 g of 2-hydroxybenzoic acid (salicylic acid) in ca 20mL of a 1:1 mixture of ethanol and deionised water in a 100mL beaker. Make up to 100mL in a volumetric flask.

    2. Working 2-hydroxybenzoic acid (salicylic acid) solution (0.0025g 2-hydroxybenzoic acid (salicylic acid) /25mL)
    Dilute 2.5mL of the stock solution to 25mL in a volumetric flask with a 1:1 ethanol/water mixture.

    3. Iron(III) nitrate solution, 0.1M.


    Observations

    1. The set of standard solution should give a range of intensities of a bluish colour. Students should be careful to add the correct number of drops as indicated. The experiment works best with old tablets containing some free 2-hydroxybenzoic acid (salicylic acid). New tablets with minimal free acid do not give any blue coloration but merely the colour of iron(III) in solution (yellow) so they do not fit into the range of standards.
    2. The equation by which 2-hydroxybenzoic acid (salicylic acid) is formed is:

    Reference
    This experiment is based on a similar procedure given in the publication G. Rayner- Canham and A. Slater, Microscale chemistry - laboratory manual. Don Mills, Ontario: Addison-Wesley, 1994.

    Safety 

    Students must wear eye protection. It is the responsibility of the teacher to carry out a risk assessment. 

    MICROSCALE CHEMISTRY 2000
    Light Scattering by a Colloid (The Tyndall Effect)...
     

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