William Kovarik
Radford University
and
Matthew E. Hermes
Kennesaw State University

Fuels and Society B: 9. Lead/TEL Chemistry and Toxicity

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1. Chemistry of Lead From the Earth's Crust:

2. Tetraethyllead - Surprising Properties of a Liquid

3. Lead: Chemical Explanation of Toxicity

1. 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 covelent, 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 + 3O₂ --> 2PbO + 2SO₂ (1)

and

2PbO + C --> 2Pb + CO₂ (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 G⁰ = delta H⁰ - T delta S⁰

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

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

A similar calculation for reaction (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 --> Pb + CO (3)

So let us look at this reaction more closely.

The standard free energy for (3)is positive however, about +67kJ/mol.

But it is likely that reaction (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 3) as compared to equation (2) in which TWO molecules of PbO and a carbon must react at the same time. Doesn't it 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 (3) can take place.

Finally, since we know CO₂ is found in the process, we expect that (4) follows (3), again requiring collision between two, not three molecules:

PbO + CO --> Pb + CO₂ (4)

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

PbO + C --> Pb + CO (3)

PbO + CO --> Pb + CO₂ (4)

2PbO + C --> 2Pb + CO₂ (2)

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

2. Tetraethyllead - Intermolecular Forces and the Surprising Properties of a Liquid: Although tetraethyllead (TEL):

Pb(C₂H₅)_{4
Tetraethyllead (TEL)}

_{has a molecular weight of 323 it is a liquid with low viscosity. 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 crstalline 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} (C₂H₅) 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.

3. Lead: Chemical Explanation of Toxicity: Lead compounds sllowly 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, neurobilogical 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 authoriities 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 irreplaceqable role in the efficient transfer of oxygen around our systems. Lead interferes with heme synthesis - just by taking the place of the iron atoms.

You can read references on specific lead toxicity at emedicine