Some General Statements About Equilibrium

We know that in a system at equihbrium there can be no spontaneous process occurring within the system. This leads to three simple, nonmathematical statements. 

we know that if a system has temperature differences in it, then there will be spontaneous change as heat flows from hot to cold. Thus, for any system to be at equilibrium with its surroundings, it must be at the same temperature as its surroundings and must have the same uniform internal temperature. In many situations we have systems in which the temperature is not the same at every point in the system, but it is constant in time at every point that is. 

This is not an equilibrium situation. Rather it is the steady state, which forms the basis of much of irreversible thermodynamics. If we could find a perfect heat insulator, then we could have a system at equilibrium with two parts at 

Physical and Chemical Equilibrium for Chemical Engineers, Second Edition. Noel de Nevers. 2012 John Wiley Sons, Inc. Published 2012 by John Wiley Sons, Inc. 

FIGURE 3.1 If two parts of a system are at different temperatures and are in thermal contact with each other, then there will be a spontaneous change as heat flows from hot to cold. So this is not an equilibrium system. 

---

Polyphosphates do not precipitate metal ions from solution, but hold them in solution as soluble complexes, provided the ratio of cations to phosphates does not exceed some value related to a metastable solubility product. It should be obvious that a true solubility product does not exist for the same reasons a true solubility does not exist. If polyphosphates become overwhelmed, a precipitate will form and condensed phosphates function as orthophosphates in removing metal ions from solution. Rather than attempting to include an in-depth study of complexing agents and stability constants, a general statement will be made and readers are referred to published literature for a more comprehensive view. Perhaps the most perplexing issue about complexing, and equilibrium constants used to study and represent it, is the number of different constants used to represent a system. 

---