Chapter 19

Dissolution of NaCl in Water

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Gibbs Free Energy

G = H - TS

For reactions at constant temperature:

ΔG = ΔH - TΔS

ΔG , ΔH, and ΔS refer to "products m inus reactants" or"final m inus initial" per the usual energy conventions.

"Constant tem perature" m erely m eans that if reactionchanges tem perature we need to bring it back to

original T before m aking final m easurem ents (reviewstate functions).

Gibbs Free Energy

ΔG = ΔH - TΔS is operative form of G ibbs free energyequation we will use in this course .

Any process for which ΔG is negative is spontaneous(prefers to go in the "forward" direction) and any

process for which ΔG is positive prefers to go in thereverse direction (ie . is nonspontaneous).

G is the "instability level" of the system . Nature triesto m inim ize G by as m any different schem es as it can.

Each successful schem e reduces G and therebym akes ΔG (final m inus initial) negative .

Gibbs Free Energy

What contributes to instability gain/loss (ΔG )?

1. Enthalpy (H). Loss of heat (negative ΔH) fromsystem contributes favorably to process.Negative ΔH m eans heat balance in system isnegative because system has dum ped heat intothe surroundings. This causes entropy ofsurroundings to increase .

2. System entropy (ΔS). Gain of entropy (positiveΔS) in system contributes favorably to a process.To put system entropy change (ΔS) into sam eunits as the ΔH which causes surroundingsentropy change (com parison purposes) wem ultiply ΔS by T (ie . TΔS).

Gibbs Free Energy

ΔH: heat energy organized from surroundings intosystem (unfavorable).

TΔS: Δisorganization of m atter in system expressed inenergy units (favorable).

ΔG expresses balance betweenorganization/disorganization of energy and

m atter by a system .

ΔG = ΔH - TΔS

When chem ists use term "energy" generically, as in"unstable species like to lose energy to generatem ore stable species," they usually im plicitly m ean

G ibbs free energy.

Gibbs Free Energy

For ΔG to be negative (favorable) e ither ΔH m ust benegative (disorganize energy) or TΔS m ust be

positive (disorganize m atter) or both.

How does this relate to m olecular changes?

Typical reaction results in bonding changes andstructure m aking/breaking.

Making shorter bonds releases H (favorable); longerbonds absorb H (unfavorable).

Making fewer bonds or m ore fragm ents results inincreased entropy (m ore broken structure ,

favorable) and vice -versa.

Gibbs Free Energy

HH

O

O

HH

ΔH - TΔS

HH

O

OHH

ΔG:

ΔG and Concentration

At any moment in time during a reaction at anynonstandard set of concentrations if you measure theinstabilities of the product and reactant molecules and

calculate the energy difference (product moleculesminus reactant molecules) you get:

ΔG = ΔG° + RT ln Q

The higher the Q value (the more concentratedproduct is relative to reactant) the higher the

instability of product relative to reactant.