Decomposition into the elements: a demonstration/experiment
Compounds which are easily formed and can be broken up easily by heating are useful for showing the nature of chemical combination, the difference between elements and compounds, stoichiometry. Mercury(II) oxide played an important role in the early days of chemistry in this way. Lead(II) iodide is a useful example which is easily made by mixing lead(II) nitrate and potassium iodide solutions, in a molar ratio of 1:2. (CAUTION: lead compounds are toxic) The starting materials are colourless and lead(II) iodide is bright yellow and insoluble. A compound has obviously formed and potassium nitrate can be recovered from the filtrate, if required.
Pb(NO3)2(aq) + 2KI(aq) —› PbI2(s) + 2KNO3(aq)
Filter off the solid and dry the precipitate.
Weigh out about 1g of lead(II) iodide into a hard-glass test-tube and heat in a bunsen flame. Purple fumes of iodine come off before the solid melts and when these stop a bead of metallic lead is left. Cool and reweigh the test-tube. The mass of lead left enables the stoichiometry of the compound to be checked/determined. The iodine vapour can be condensed on a cold test-tube.
The pure elements and the compound can be displayed to show the differences.
N.B. I found this reaction out by accident while trying to melt lead(II) iodide to measure its conductance. If I’d thought about it I should have predicted its instability: both metal and non-metal are ‘soft’, easily polarisable and the compound will be appreciably covalent and unstable. The fluoride and chloride will be more ionic and more stable than the iodide. To measure the m.pt. of lead(II) iodide one would have to measure it under pressure to prevent decomposition i.e. an application of Le Chatelier’s principle.
PbI2(s) —› heat Pb(s) + I2(s)
Pressure increases will favour the backward reaction and thus prevent decomposition below the m.pt.
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Demonstrations for inter-cert science
From Leo O’Donoghue, Tubber, Co.Clare
Instability of hydrogen peroxide in sunlight
Set up two experiments as shown below using 20 vol. hydrogen peroxide. The control is covered with an opaque cover e.g. a bucket and both are placed in sunlight. The sunlight decomposes the peroxide giving oxygen, and this can lead into the preparation of oxygen and the use of manganese(IV) oxide as a catalyst to speed up the decomposition.
H2O2(aq) —› H2O + ½O2(g)
The chemistry of respiration
The apparatus below can be used to sample a reaction producing gas and test for the product by sucking the gas through a test solution. In this case we test for CO2 by sucking the gases through lime-water.
The ‘weetabix’ is soaked in ethanol to make it burn better – this is slightly cheating, but ethanol is also consumed by humans. Set fire to the weetabix and suck the gases through the apparatus. The cobalt chloride paper in the glass tube will change colour as water is given off and the lime-water will go cloudy. The glass tube will get hot showing heat is evolved.
Since we know that oxygen is needed for combustion we can write a simple equation:
Food + oxygen —› CO2 + H2O + heat
which is a simple respiration formula, showing how food is ‘burnt’ in the body.
(See p.3, issue 2 for another way of testing for evolved gases using a plastic syringe.)
Lightness of hydrogen gas
Demonstrating that hydrogen is lighter than air is often difficult if the gas flow/pressure is not high enough to inflate a balloon or plastic bag. An old idea is to bubble the hydrogen through soap solution: bubbles are formed which are lighter than air and also burn!
