Logo IMG
HOME > PAST ISSUE > Article Detail


The Thermodynamic Sinks of this World

What would an elemental soup cook up to?

Roald Hoffmann

Thermodynamic Sinks

Let’s use some simple chemistry to get a feeling for the thermodynamic sinks of this world. We are looking for compounds with the most negative heats of formation. A heat of formation is defined as the enthalpy change (under standard terrestrial conditions, P = 1 atmosphere, T = 298 kelvin) for the process

A + B + C ↔ ABnCm,

where A, B and C are the elements and ABnCm a compound. Here it is a ternary, but the generalization is obvious.

Let’s start with carbon and oxygen. The well-known oxides of carbon are CO, CO2, and carbon suboxide, C3O2. Other oxides, CnO2, n = 2–7; CnO, n > 2; and CnOn, n = 2–6, are also experimentally known in small amounts, trapped in an inert matrix at low temperatures. The heats of formation of the oxides available in quantity are –110 (CO, [g]), –394 (CO2, [g]), –122 (C3O2, [l]) kJ/mol, respectively. Note the negative heats of formation, indicating stability with respect to the elements. Here is the first principle of stability, one we have already seen in the reaction forming water: Form oxides. This is confirmed for every element, except the noble gases. And gold. Incidentally, quartz, SiO2, is “better” than CO2, for the heat of formation of the former is a whopping –911 kJ/mol.

One oxide of carbon, CO2 , is much more stable than the others. This can be checked by calculating from the above heats of formation the energetics of all possible interconversion reactions, for example, CO2 ↔ CO + ½ O2, 3CO2 ↔ C3O2 + 2 O2 and so forth, and finding them all positive, endothermic.

But … if one introduced a lot of Ca in the environment, the Ca would react with CO2 (enough heat supplied to overcome all barriers) to give CaCO3, calcium carbonate, limestone, a real thermodynamic sink:

Ca (s) + ½ O2 (g) + CO2 (g) ↔ CaCO3 (s) ΔH = –813 kJ/mol

Is this going to go on? That is, are we going to get in a mix of 100 or so elements (let’s not worry about the late actinides) a most stable compound of “multinary” composition, AxByCzDw…? I don’t think so. Metastable molecules with more than 10 elements are known, and solid state compounds of 9 elements at least. But I think both experience and intuition indicate that such multielement compounds are likely to be unstable with respect to disproportionation or reaction with oxygen.

comments powered by Disqus


Of Possible Interest

Spotlight: Making the Cut

Spotlight: Briefings

Feature Article: The Rising Cost of Resources and Global Indicators of Change

Subscribe to American Scientist