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Limits of Thermodynamics

Thermodynamics is a powerful approach toward understanding chemical reactions, but only provides part of the picture. Specifically:

  1. Thermodynamics only points the way
  2. Thermodynamics says nothing about how long it takes to get there
  3. The stoichiometric equation for the reaction says nothing about its mechanism

Thermodynamics only points the way: Chemical change is driven by the tendency of atoms and molecules to rearrange themselves in a way that results in the maximum possible dispersion of thermal energy into the world. The observable quantity that measures this spreading and sharing of energy is the free energy of the system. As a chemical change takes place, the quantities of reactants and products change in a way that leads to a more negative free energy. When the free energy reaches its minimum possible value, there is no more net change and the system is said to be in equilibrium.

The beauty of thermodynamics is that it enables us to unfailingly predict the net direction of a reaction and the composition of the equilibrium state even without conducting the experiment; the standard free energies of the reactants and products, which can be independently measured or obtained from tables, are all we need.

Note

When the free energy reaches its minimum possible value, there is no more net change and the system is said to be in equilibrium

Thermodynamics says nothing about how long it takes to get there: It is worth noting that the concept of "time" plays no role whatsoever in thermodynamics. But kinetics is all about time. The "speed" of a reaction — how long it takes to reach equilibrium — bears no relation at all to how spontaneous it is (as given by the sign and value of ΔG°) or whether it is exothermic or endothermic (given by the sign of ΔH°). Moreover, there is no way that reaction rates can be predicted in advance; each reaction must be studied individually.

Note

The concept of "time" plays no role whatsoever in thermodynamics.

The stoichiometric equation for the reaction says nothing about its mechanism: The term "mechanism" refers to, "who does what to whom". Think of a reaction mechanism as something that goes on in a "black box" that joins reactants to products. The inner workings of the black box are ordinarily hidden from researchers, are highly unpredictable, and can only be inferred by indirect means.

Note

The stoichiometric equation for the reaction says nothing about its mechanism!

Three reactions that look alike, but are different

Consider, for example, the gas-phase formation reactions of the hydrogen halides from the elements. The thermodynamics of these reactions are all similar (they are all highly exothermic), but their dynamics (their kinetics and mechanisms) could not be more different.

\[ H_{2(g)} + I_{2(g)} \rightarrow 2 HI_{(g)} \tag{1a}\]

Careful experiments, carried out over many years, are consistent with the simplest imaginable mechanism: a collision between the two reactant molecules results in a rearrangement of the bonds.

\[H_{2(g)} + Br_{2(g)} \rightarrow 2 HBr_{(g)} \tag{1b}\]

One might be tempted to suppose that this would proceed in a similar way, but experiments reveal that the mechanism of this reaction is far more complex. The reaction takes place in a succession of steps, some of which involve atomic H and Br.

\[H_{2(g)} + Cl_{2(g)} \rightarrow 2 HCl_{(g)} \tag{1c}\]

The mechanism of this reaction is different again. Although the first two reactions reach equilibrium in minutes to an hour or so at temperatures of 300 to 600 K, a mixture of hydrogen and chlorine will not react at all in the dark, but if you shine a light on the mixture, it goes off with a bang as the instantaneous reaction releases heat and expands the gas explosively.

 

What is particularly noteworthy is that these striking differences cannot be reliably predicted from theory; they were revealed only by experimentation.

Contributors

Stephen Lower, Professor Emeritus (Simon Fraser U.) Chem1 Virtual Textbook