Chapter 19
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Transcript of Chapter 19
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Chapter 19
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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).
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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 .
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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).
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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.
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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.
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Gibbs Free Energy
HH
O
O
HH
ΔH - TΔS
HH
O
OHH
ΔG:
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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.
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