1 Chemical Equilibrium GLY Lecture 5 Fall, 2014

Chemical Reaction A chemical reaction may be defined as one or more substances, known as reactants, combining chemically to form one or more different substances, known as products. 2

3 Reactants to Products

4 Reaction Completeness How far can the reaction proceed?  Almost complete  Negligible

5 Speed of Reaction How fast does the reaction proceed?  Very slow, unless ignited  This is a kinetic problem, not a thermodynamic one

6 Law of Mass Action Formulation First formulated by two Norwegian chemists, Cato Maximilian Guldberg & Peter Waage When the driving forces of the forward and back reactions become equal, equilibrium has been achieved

7 Forward Reaction Driving force of the forward reaction = k f [NaCl]

8 Back Reaction Driving force of the back reaction = k b [Na + ][Cl - ]

9 Equilibrium k f [NaCl] = k b [Na+][Cl-] K is called the equilibrium constant for the equation as written

10 Law of Mass Action The rate of a reaction is directly proportional to the concentration of each reacting substance (Konrad Krauskopf) Concentration is a better term than mass

11 Problems Changing the equation requires rewriting the equilibrium expression K f and k b are not physically measurable

12 Disturbance of Equilibrium If the system is at equilibrium, and some component B is added, so that[B] increases, what happens to the system? The disturbance can also be a change in temperature or pressure, since K is a function of both

13 Henri Louis Le Chatelier French chemist, Formulated the principle now named in his honor

14 Le Chatelier’s Principle “Any system in stable chemical equilibrium, subjected to the influence of an external cause which tends to change either its temperature or its condensation (pressure, concentration, number of molecules in unit volume), either as a whole or in some of its parts, can only undergo such internal modifications as would, if produced alone, bring about a change of temperature or of condensation of opposite sign to that resulting from the external cause.”

15 Le Chatelier’s Principle Le Chatelier restated his principle as “Every change of one of the factors of an equilibrium occasions a rearrangement of the system in such a direction that the factor in question experiences a change in a sense opposite to the original change”

16 Formation of Water Introducing a spark generates free radicals

17 Thermodynamic Criteria for Equilibrium If ΔG < 0, the reaction will take place spontaneously, although the rate may be extremely small If ΔG = 0, the reaction is at equilibrium If ΔG > 0, the reaction cannot take place without energy being supplied from outside the system

18 Kinetic Barrier High kinetic barriers lead to metastable systems, which are not at equilibrium

19 Metastable Situations in Geology Many high pressure minerals, formed at great depths within the earth, do not immediately revert to the lower pressure polymorphs when they reach the surface  Diamond and graphite  Closer to the surface, aragonite and calcite

20 Generalized Equilibrium Constant The equilibrium constant, K, may be defined for a general system as follows:

21 Formation of Water - 1 K is specific to a reaction

22 Formation of Water - 2 Another formulation, with a new equilibrium constant

23 Formation of Water - 3 Still another formulation

24 Solubility Product A solubility product (K sp ) is a special type of equilibrium constant Very useful for environmental studies What is the concentration of a solid? It is effectively a constant, so we may set it equal to one. Then:

25 Solubility The solubility in moles/liter is equal to [Pb 2+ ] or [SO 4 2- ], since one ion of each is produced when the PbSO 4 molecule dissolves Similarly, the solubility product for galena, PbS, is

26 Common Ion Effect What happens if galena (PbS) is added to a saturated solution of anglesite (PbSO 4 )? X = solubility of galena

27 Using an Approximation This could be solved as a quadratic equation However, X will be less than the solubility of pure galena In the presence of anglesite, the solubility of galena is greatly reduced

28 Check the Approximation With X = 2.4 x , is the above equation true?