M which is an approximation of the activity of ch 3

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}/1M which is an approximation of the activity ofCH3COO-in the solution and the activity of the solvent (water) equals 1.A more general expression of K must be used for more complex chemical reactionssuch as when some stoichiometric coefficients have values different than 1 or whenthe reaction includes gases.For the generic reactionA +BαβγC +δDK = aCγ.aDδ/aAα.aBβ42
WithaA= the activity of reactant A at equilibriumFor the reactionCH3COOH(aq) + H2O(l)CH3COO-(aq) + H3O+(aq)Ka= aCH3COO-(aq).aH3O+(aq)/aCH3COOH(aq).aH2O(l)Using the following approximations of the activitiesPure solida =1Pure liquid, solventa =1Solutea = [solute]/1MGasa = pgas/ 1 barKa =[CH3COO-].[H3O+]/[CH3COOH][CH3COOH] indicates the concentration of CH3COOH divided by 1M at equilibriumThe equilibrium constant K is a particular value of the reaction quotient Q.Thismeans that Q, like K is the ratio of products over reactants. The difference is that Qcan take any value and depends only of what amount is present at a given time whileK has only one possible value for a given reaction at a given temperature.Comparing Q and K is very useful because a reaction away from equilibrium willalways tend to go toward equilibrium which means modify the ratio of products overreactants in order for Q to reach K.By comparing Q and K we can figure out thedirection in which the reaction will proceed spontaneously.Thermodynamically speaking the value of K is related toG°rxn , the change ofΔstandard Gibbs FreeEnergy for the reaction where the reactants in standardcondition are entirely transformed into products in standard conditionsK = e-∆G°/RT.As explained in more details in the lecture in order to figure out the direction inwhich the reaction will proceed spontaneously the value ofG must be calculated.ΔG of a reaction is the change of Gibbs Free Energy for a reaction occurring in nonΔstandard condition and is expressed by an equation where a term expressing thedeviation from standard conditions is addedG =G° + RTlnQΔΔEquilibria can be simple or multiple as well a homogeneous and heterogeneous. Wewill study a few simple homogeneous equilibria and see how the reaction quotientalways moves toward the equilibrium value. By comparing Q to its equilibrium value(K) it is possible to find in which direction the reaction will proceed. A similarconclusion can be reached with the calculation of the change of Gibbs Free Energyfor the reaction (∆G).43
The behavior of a reaction away from its equilibrium was described qualitatively inthe XIXth century by the French engineer Henri Louis Le Châtelier: ”if an externalstress is applied to a system at equilibrium the system will adjust in such a way thatthe stress is partially offset as the system reaches a new equilibrium”.Interestingly the observation applies not only to concentration and pressure but alsoto volume and temperature. It is however not useful in quantitative analysis becauseit lacks the theoretical background required for calculations.

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