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9: Lead/TEL Chemistry and Toxicity

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    40258
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    To understand this section fully, you need to be familiar with the principles of intermolecular properties, thermochemistry and thermodynamics and periodic properties of the periodic groups as described in your chemistry text. This section amplifies on those principles in describing lead and TEL chemistry and toxicity.

    Chemistry of Lead From the Earth's Crust

    Lead is an inert, heavy metal, density 11.4g/cc, positioned in Group 14 in the periodic table - in the series that includes carbon, silicon, germanium and tin. As we know, as we proceed down the periodic table metallic properties - the ability to form ionic compounds and to be electrically conductive - increases. Nevertheless it is the remaining trace of covalent, nonionic properties of lead that gives us the story of tetraethyllead!

    Lead is found as the mineral galena, lead sulfide, and the metal is obtained from the galena by the following processes:

    \[2PbS + 3O_2 \rightarrow 2PbO + 2SO_2 \label{1}\]

    and

    \[2PbO + C \rightarrow 2Pb + CO_2 \label{2}\]

    To understand lead production in the context of the Second Law of Thermodynamics, let's be guided by the thermodynamic properties of the substances involved. We know that the spontaneity of a process comes from considering the Free Energy (G) and that for a process to be spontaneous, the value of delta G in the equation below must be negative:

    \[\Delta G^o = \Delta H^o - T \Delta S^o\]

    We find at the NIST site, values for the standard enthalpies and entropies for the substances.

    delta H0 kJ/mol S0 J/mol*K
    Pb

    0

    +65
    PbO -219 +60
    PbS -98 +91
    C 0 6
    CO -110 +198
    CO2 -394 +214
    SO2 -297 +248

    We can calculate that reaction \(\ref{1}\), the conversion of lead sulfide to lead oxide has an extremely large negative free energy, -391kJ/mol PbS.

    A similar calculation for reaction \(\ref{2}\) shows spontaneity in the standard state also with a negative free energy of -9kJ/mol. Some text books show the reaction to make lead as producing CO, carbon monoxide:

    \[PbO + C \rightarrow Pb + CO \label{3}\]

    So let us look at this reaction more closely. The standard free energy for Equation \(\ref{3}\) is positive however, about +67kJ/mol.

    But it is likely that reaction \(\ref{3}\) would have to be be the first step in the conversion of PbO to Pb. After all think of the likelihood of two individual particles - a carbon atom and a lead oxide molecule, coming together with enough energy to react (Equation \(\ref{3}\) ) as compared to equation \(\ref{2}\) in which TWO molecules of PbO and a carbon must react at the same time. Does it not appear more likely that a collision between two rather than three objects is more likely?

    And under actual reaction conditions - high temperatures and varying concentrations differing far from the standard conditions of one atmosphere pressure and 298 degrees Kelvin reaction \(\ref{4}\) can take place.

    Finally, since we know \(CO_2\) is found in the process, we expect that Equation \(\ref{4}\) follows \(\ref{3}\), again requiring collision between two, not three molecules:

    \[PbO + CO \rightarrow Pb + CO_2 \label{4}\]

    The sequence of reactions \(\ref{3}\) and \(\ref{4}\) exemplify a reaction pathway leading to the overall process as described in Equation \(\ref{2}\). Hess's Law, tells us the thermodynamic properties of any process are the sum of the thermodynamics of additive processes.

    \[PbO + C \rightarrow Pb + CO \label{3a}\]

    \[PbO + CO \rightarrow Pb + CO_2 \label{4a}\]


    2PbO + C --> 2Pb + CO2 (2)

    Reaction delta G0 kJ/mol
    (2) -9
    (3) +67
    (4) = (2)-(3) -76

    From this analysis we see that the spontaneity lies primarily in the oxidation of carbon monoxide to carbon dioxide.

    Intermolecular Forces and Properties of a Liquid

    Although tetraethyllead (TEL):

    < face="Arial" size="2">Pb(C2H5)4
    Tetraethyllead (TEL)

    has a molecular weight of 323 g/mol, it is a low viscosity liquid and it is soluble in other liquids such as hexane that have very low intermolecular forces. TEL has a density of 1.7g/cc and boils with decomposition at about 200 degrees Celsius. Furthermore TEL can be volatilized to a gas when dissolved in auto fuel, it burns and acts as an antiknock by modifying the flame in the cylinder. The more familiar lead compounds, lead sulfide and oxide, the metal itself and lead sulfate that we see in auto batteries are all crystalline solids.

    We expect a metal with its mobile electrons to form a crystalline solid structure with a regular pattern of atomic disposition within the crystal. We expect metals to have strong intermolecular forces that keep the materials as solids. But lead falls in Group 14, below carbon, silicon and tin. Carbon is ubiquitous for forming strong covalent bonds with itself and with silicon, and it forms bonds with lead.

    The TEL molecule is in the form of a tetrahedron with the four (\(C_2H_5\)) groups directed towards the apices of this regular geometric figure. The first carbon in each ethyl group is bonded covalently to the lead. Little in the way of intermolecular forces exist between molecules completely surrounded by hydrocaarbon residues such as the ethyl group. And so, each of the lead atoms is shielded from the lead atom in the adjacent molecule and strong forces cannot be developed. So TEL is a liquid, is soluble in gasoline even though on a weight basis more than 60% of the weight of TEL is the lead.

    Leadville

    In central Colorado, near the town of Leadville, gold, silver, lead and zinc have been mined for more than 125 years. As with most underground deposits gold is found pure and unreacted while lead, zinc and silver are found as sulfides.

    As we will see, lead sulfide, PbS is converted to PbO in a highly spontaneous process. Deposits of PbS, left unconstrained in a moist, oxidative atmosphere, dissolve and carry the metal into the streams and other environment. The result of mining activities - whether to obtain the lead or separate it from gold and silver - has been to leave a monumental environmental problem in the Arkansas River basin now undergoing cleanup. Leadville is a Superfund site in which extraordinarily expensive measures are being carried out to contain the heavy metal pollutants in this town.

    One author (MEH) has personal experience in Leadville. On many trips he has noted banks of grey black mine tailing along the roadsides of the town. These deposits have diminished over the years as the cleanup continues.

    Lead: Chemical Explanation of Toxicity

    Lead compounds slowly poison humans and other animals. Low doses over a long period of time or large concentration exposures for short periods are both dangerous. Lead accumulates in the bones and blood of its victims. Children are particularly sensitive to lead poisoning and suffer hearing loss, neurological effects and perhaps reduced intelligence.

    The level of lead in the blood is the usual measure of poisoning. The mean value for lead in the blood of Americans is about 0.2 parts per million (ppm) and at a level of 0.6-0.8ppm health authorities consider an individual to be poisoned with lead. Lead levels of 21st century humans are much higher than those of early man. The long term use of lead in paints and tits use as a fuel additive have left a residue of lead salts on the earth - some of which makes its way into the human food chain. the elimination of lead from these two sources has resulted in a general reduction in human lead concentrations.

    The accumulation of lead in bones and blood is not surprising. Bone contains high calcium levels in the form of salts - it is not surprising that lead ions would be picked up in a salt producing system of the body. Of greater importance is the oxygen transfer system in the body that depends upon hemoglobin. At the core of the heme molecule is an atom of iron that plays an irreplaceable role in the efficient transfer of oxygen around our systems. Lead interferes with heme synthesis - just by taking the place of the iron atoms.


    9: Lead/TEL Chemistry and Toxicity is shared under a not declared license and was authored, remixed, and/or curated by ChemCases.

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