Coupling in the same direction

The fact that pressure and temperature influence the chemical potential of substances and therefore the drive for transformations can be considered a result of coupling of mechanical, thermal, and chemical quantities. With rare exceptions, volume V or entropy S and the amounts of substance , are coupled in the same direction V tt and S tt However, the couplings between V and as well as between S and exhibit no directional preference because starting and final substances can exchange their roles according to which conversion formula is chosen. 

We could have used the concept of coupling without performing any calculations to correctly predict the qualitative behavior, i.e., to say whether heating causes the drive to increase or decrease or the equilibrium to shift forward or backward. Let us imagine an endotropic reaction. S and would then be coupled in the same direction (S tt )- If increases, S does as well and when S increases, so does If an equilibrium exists it will shift forward when heating takes place (increase of S). 

An example of coupling in the same direction ( i "T 2) would be a small amount of undissolved PbCl2 in a beaker with water that dissolves when KNO3 is added ( salting-in effect ). The first substance s rise in potential can be used to measure the strength of reciprocal action caused by the second substance. This is the so-called displacement coefficient (9/opposite effect, the displacement of the second substance by the first, which we describe by dpi/dn2)j p is just as great, as we can see by applying the flip rule ... 

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