Equilibrium

• Equilibrium Constant, K (or Keq) describes conditions AT equilibrium

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Equilibrium/ Reversibility

• Anything at equilibrium is theoretically undergoing forward and reverse reactions:

• A + B ↔ C– A + B C AND C A +B

• Equilibrium has 2 criteria:– Reaction does not appreciably change in time– Perturbation of that equilibrium will result in a

return to the equilibrium

Assessing equilibrium

Q reaction quotient, aka Ion Activity Product (IAP) is calculated from knowing activity of all components of a reaction

K aka Keq, we get from thermodynamic data – it is one number defined AT EQUILIBRIUM

Equilibrium for any reaction is when Q = K

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log Keq

• CaCO3(calcite) = Ca2+ + CO32- -8.48

• CO2(g) + H2O = H2CO30 -1.47

• H2CO30 = H+ + HCO3

- -6.35

• H+ + CO32- = HCO3

- +10.33

CaCO3(calcite) + CO2(g) + H2O = Ca2+ + 2 HCO3- -5.97

Equations can be ‘added’ together, equilibrium constants also get ‘added’ together!

Convenient way to rewrite reactions (to look at more appropriate reactions or to use things you’ve more directly measured….)

Where do K’s come from?

• Measure directly – experimental determination of conditions at equilibrium

• Use thermodynamic data – K is directly related to free energy of reaction – GR

Hydroxylapatite

• Ca5(PO4)3(OH) = OH- + 3 PO43- + 5 Ca2+

• Log K = -59.0351 at 25ºC

Thermodynamics

• “the branch of science that deals with energy levels and the transfer of energy between systems and between different states of matter”

Thermodynamics• Thermodynamics answers the following question:

• For any reaction - defined by a set of reactants and products set in exactly defined conditions (temperature, pressure, concentration, etc.) will that reaction go forward spontaneously or not??

• It can address ANY geochemical reaction, if thermo says NO, rest assured the reaction will not proceed, if thermo says yes, then we can progress to the next question how fast?

What is Energy???• “It is important to realize that in physics today, we have no

knowledge of what energy is. We do not have a picture that energy comes in little blobs of a definite amount. It is not that way.” –Richard Feynman

• HOWEVER Feynman goes on to elaborate that energy has meaning as a way to define, and quantify, changes which bring about changes – between systems, energy levels, or states of matter – i.e. for any reaction

• “How seriously must we take the physical existence of this energy? No more and no less than any other bookkeeping practices.” –Richard Feynman