More on Standard States

This reaction does not proceed at commercially useful rates at these low temperatures. Commercially, it is carried out over a catalyst at 265°C and 70 atm [7] (see Problem 12.26). 

In principle, all chemical reactions are reversible if we wait long enough they come to some equilibrium state at which some amount of reactants are in equilibrium with some amount of products. However, as the following example shows, in practice, some reactions seem irreversible, with practically complete consumption of the reactants. 

Example 12.6 Compute the equilibrium concentration of hydrogen and oxygen to be expected when hydrogen and oxygen react to form water, at 25°C and 1 bar by 

Using values of the Gibbs energies of formation in Table A.8 we have 

The calculated equilibrium concentration of unreacted hydrogen is far below the limit of detection by any known analytical method, so we are safe in calling it zero. If a reaction consumes all the starting product, we consider it an irreversible reaction. This reaction, if carried to thermodynamic equilibrium would be such a reaction. Many combustion reactions have such high values of (—Ag°) and thus such high values of K that at equilibrium they would be nearly complete and be considered irreversible. Real combustion reactions come close to equilibrium, but are limited by kinetics. So, in principle, there are no irreversible reactions, reactions in which none of the reactants remain at equilibrium. But, in practice, many reactions, mostly combustion reactions, have such low equilibrium concentrations of the original reactants that they appear to be irreversible. 

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THE SELECTION OF THERMOCHEMICAL DATA AND MORE ON STANDARD STATES... 

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