Gibbs Free Energy Changes in Laboratory Conditions

7 GIBBS FREE ENERGY CHANGES IN LABORATORY CONDITIONS 

Under conditions of constant T, P, the criterion for general Gibbs free energy changes AG can be stated as 

Consider a simple 2-phase system consisting of some number of moles of solid and vapor, expressed in terms of mole fractions xsol, xvap. The following is a schematic graph of the various energetic quantities G, H, TS as functions of xvap (with pure solid as the reference 0 )  

For given l P, will the sample completely vaporize, completely condense, or something intermediate To answer this question, let us first consider the separate variations of enthalpy and entropy at any particular vvap. As shown in the graph, the overall enthalpy variation is monotonically positive, so AH = A//(xvap) is always positive  

Consider a sample of rainwater from a cold cloud that is initially at temperature T= — 10°C and pressure P= 1 atm as it comes to earth. Will it freeze We can pose the question in terms of the initial state (water) and final state (ice) of the system  

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For the important conditions of constant pressure and constant temperature (such as reactions carried out in an open beaker on the laboratory bench), a sum known as the Gibbs free energy G totals these entropy changes. And as every good chemistry student knows, reactions proceed spontaneously only when G for the initial state is greater than G for the final, and at equilibrium the change in G is zero. 

He realized that changes in this function could predict whether or not a process is spontaneous under conditions of constant pressure and temperature. This constraint is not that demanding because many laboratory processes occur under these conditions (or approximately so). Now, if we focus on the change in the Gibbs free energy for a process at constant temperature, we have the following result ... 

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