4 minutes reading time (849 words)

    The Ammonium Dichromate Volcano

    Description

    A small conical heap of orange ammonium dichromate is ignited. It sparks and produces a large volume of green chromium (III) oxide as well as steam, resembling a volcano.

    Apparatus

    • Bunsen burner, heatproof mat.
    • Metal tray such as a large tea tray.
    • Watch glass.
    • Bell jar (optional).
    • One 1L conical flask (optional).
    • One 250mL conical flask (optional).

    Chemicals
    The quantities given are for one demonstration.
    • 10g of ammonium dichromate (VI) (ammonium dichromate, (NH4)2CR2O7).
    • Wooden spill.
    • A little ethanol.
    • One piece of blue cobalt chloride paper.
    • A little glass wool or mineral wool (optional).
    • A few grams of silica gel granules (optional).
    • Access to a fume cupboard (optional).

    Method
    The Demonstration
    Working in a fume cupboard, place a conical pile of about 10g of ammonium dichromate on a heat proof mat standing on a tray to collect the chromium oxide that shoots into the air. Soak about 3cm length of wooden spill in ethanol and stick this into the top of the pile so that about 2cm protrudes to act as a wick. Light the wick. As the wick burns down to the ammonium dichromate, the compound begins to spark and decompose leaving behind a cone of green chromium (III) oxide that has a considerably larger volume than the original compound. Some of the oxide shoots into the air. The volcano burns for between 30 seconds and one minute. Hold a watch glass above the volcano; this becomes steamed up with water from the decomposition. Confirm that this is water with blue cobalt chloride paper.

    The reaction may also be started by pointing a roaring Bunsen flame at the top of the pile of ammonium dichromate.

    Visual tips
    A portable fume cupboard gives all round vision. If it is desirable to do the demonstration without the fume cupboard, place a large bell jar over the reaction. However this soon steams up. Place matchsticks or something similar under the rim of the bell jar to allow the nitrogen produced in the reaction to escape.Teaching tipsIf appropriate, some students could be asked to predict the products given the formula of ammonium dichromate.

    The demonstration could be used to enliven a lesson on thermodynamics (post-16) in which case students could be asked to calculate ΔH, ΔS and hence ΔG for the reaction. The values they should obtain are:

    ΔH = -478 kJ mol-1
    ΔS = +217 J mol-1 K-1
    ΔG = -543 kJ mol-1


    They should be able to predict qualitatively that there is an entropy increase, The data required are given in the table. 

     Post-16 students could also be asked to balance the equation using oxidation numbers.

    Extensions
    An alternative way of doing this experiment without a fume cupboard is as follows.

    Place about 3g of ammonium dichromate in a 1L conical flask. Place a loose plug of glass wool or mineral wool in the mouth of the flask to prevent loss of chromium (III) oxide. Start the reaction by heating the flask on a Bunsen burner with the tip of the roaring flame pointing at the pile of ammonium dichromate. Once the reaction has started, place the flask on a heatproof mat in view of the audience. The flask will steam up somewhat, a little steam will escape. To confirm that this is a decomposition reaction rather than a combustion reaction, flush the flask with nitrogen from the cylinder and repeat the reaction. It will be unaffected.

    It is possible to modify this method to suggest that a gas is formed. Replace the glass wool or mineral wool plug with a loose sandwich of glass wool and silica gel granules to absorb any steam (see Figure 2). Weigh the flask before and after the reaction. A weight loss will be observed suggesting loss of gas (although it is difficult to ensure that no steam escapes).

    Calculations show that 2.5g of ammonium dichromate should produce about 240mL of nitrogen. React 2.5g of ammonium dichromate in a 250mL conical flask with a loose glass wool or mineral wool plug. Immediately the reaction has finished, place a lighted taper in the flask. The nitrogen will cause it to go out. Compare with a lighted tampered taper in an air filled flask of the same size. It will burn for several seconds.

     Further details

    The value for the standard molar entropy of ammonium dichromate has been estimated by Latimer's rule, which states the entropy of each atom of each element in a compound in JK-1 mol-1 is given by: ΔS0 = (1.5R ln Ar) - 3.92

    (where Ar is the relative atomic mass of the element and R the gas constant) and that the entropy of a compound is the sum of the elements in the compound.

    For further details see W.M.Latimer, J Am. Chem. Soc., 1921, 43, 818.

    This rule may be found useful for estimating the entropies of other compounds, which are not readily available in the literature.

    Safety
    Wear eye protection. Dispose of the residue in a sealed plastic bag placed in the dust bin.

    It is the responsibility of teachers doing this demonstration to carry out an appropriate risk assessment.

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