Introduction to mixtures (more in 3rd year) Thermodynamics handout Introduction to mixtures (more in 3rd year) Deal only with homogeneous mixtures of a solvent (major component) and solute (minor component). Introduce partial molar property: contribution that a substance makes to overall property. V = nAVA + nBVB Page

The chemical potential, m We can extend the concept of partial molar properties to state functions, such as Gibbs energy, G. This is so important that it is given a special name and symbol, the chemical potential, m. G = nAGA + nBGB G = nAmA + nBmB

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Thermodynamics handout Chemical Equilibria DG = DH – TDS In a spontaneous change at constant temperature and pressure, the Gibbs energy decreases A reaction mixture, at constant T and p, adjusts its composition until its Gibbs energy is a minimum Page

Thermodynamics handout Reaction Gibbs energy For a general reaction: aA + bB  cC + dD DrG = (cmC +dmD)-(amA + bmB) Note error in eqn 7.1c in textbook At equilibrium DrG = 0 Page

Variation of DrG with composition Thermodynamics handout Variation of DrG with composition Can derive that: mJ = mJ° + RT ln aJ Page

Variation of DrG with composition Thermodynamics handout Variation of DrG with composition mJ = mJ° + RT ln aJ DrG = (cmC +dmD)-(amA + bmB) =DrG° Page

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Spontaneity again

Thermodynamics handout Summary The chemical potential of a substance is a measure of the ability of the substance to bring about change. Reaction Gibbs energies represent the slope of a plot of Gibbs energy versus composition, and represents the tendency for a process to approach equilibrium. Reaction Gibbs energies are zero at equilibrium, and the standard Gibbs energy change is related to the equilibrium constant. Spontaneous processes possess negative standard Gibbs energy changes and have K greater than 1. Page