Properties of Systems in Chemical Equilibrium

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Expt 20 Properties of Systems in Chemical Equilibrium Le Chatelier s Principle NOTE Advance Study Assignment included at end of lab handout do it as part of the prelab When working in the laboratory one often makes observations that at first sight are surprising and hard to explain One might add a reagent to a solution and obtain a precipitate Addition of more of that reagent to the precipitate causes it to dissolve A violet solution turns yellow on addition of a reagent Subsequent addition of another reagent brings back first a green solution and then the original violet one Clearly chemical reactions are occurring but how and why they behave as they do is not at once obvious In this experiment we will examine and attempt to explain several observations of the sort we have mentioned Central to our explanation will be recognition of the fact that chemical systems tend to exist in a state of equilibrium If one disturbs the equilibrium in one way or another the reaction may shift to the left or right producing the kinds of effects we have mentioned If one can understand the principles governing the equilibrium system it is often possible to see how one might disturb the system such as by adding a particular reagent of heat and so cause it to change in a desirable way Before proceeding to specific examples let us examine the situation in a general way noting the key principle that allows us to make a system in equilibrium behave as we wish Consider the reaction Where A B and C are molecules or ions in solution If we have a mixture of these species in equilibrium it turns out that their concentrations are not completely unrelated Rather there is a condition that those concentrations must meet namely that A aq B aq C aq B x C Kc A 1 2 where Kc is a constant called the equilibrium constant for the reaction For a given reaction at any given temperature Kc has a particular constant value When we say that Kc has a particular value we mean just that For example we might find that for a given solution in which Reaction 1 can occur when we substitute the equilibrium values for the molarities of A B and C into Equation 2 we get a value of 10 for Kc Now suppose we add more of species A to that solution What will happen Remember Kc can t change If we substitute the new higher molarity of A into Equation 2 we get a value that is smaller than Kc This means the system is not in equilibrium and must change in some way to get back to equilibrium How can it do this It can do this by shifting to the right producing more B and C and using up some A It must do this and will until the molarities of C B and A reach values that on substitution into Equation 2 equal 10 At that point the system is once again in equilibrium In the new equilibrium state B and C are greater than they were initially and A is larger than its initial value but smaller than if there had been no forced shift to the right The conclusion you should reach on reading the last paragraph is that one can always cause a reaction to shift to the right by increasing the concentration of a reactant An increase in concentration of a product will force a shift to the left By a similar argument we find that a decrease in reactant concentration causes a shift to the left a decrease in product concentration causes a shift to the right This is all true because Kc does not change unless you change the temperature The changes in concentration that one can produce by adding particular reagents may be simply enormous so the shifts in the equilibrium system may also be enormous Much of the mystery of chemical behavior disappears once you understand this idea Another way one might disturb an equilibrium system is by changing its temperature When this happens the value of Kc changes It turns out that the change in Kc depends upon the enthalpy change H for the reaction If H is positive greater than zero endothermic reaction Kc increases with increasing T If H is negative exothermic reaction Kc decreases with an increase in T Let us return to our original equilibrium between A B and C where Kc equals 10 Let us assume that H for Reaction 1 is 40 kJ If we raise the temperature Kc will go down H 0 say to a value of 1 This means that the system will no longer be in equilibrium Substitution of the initial values of A B and C into Equation 2 produces a value that is too big 10 instead of 1 How can the system change itself to regain equilibrium It must of necessity shift to the left lowering B and C and raising A This will make the expression in Equation 2 smaller The shift will continue until the concentrations of A B and C on substitution into Equation 2 give the expression a value of 1 From the discussion in the previous paragraph you should be able to conclude that an equilibrium system will shift to the left on being heated if the reaction is exothermic H 0 Kc goes down It will shift to the right if the reaction is endothermic H 0 Kc goes up Again since we can change temperatures very markedly we can shift equilibria a long long way An endothermic reaction that at 25 C has an equilibrium state that consists mainly of reactants might at 1000 C exist almost completely as products The effects of concentration and temperature on systems in chemical equilibrium are often summarized by Le Chatelier s principle If you attempt to change a system in chemical equilibrium it will react in such a way as to counteract the change you attempted If you think about the principle for a while you will see that it predicts the same kind of behavior as we did by using the properties of Kc Increasing the concentration of a reactant will by the principle cause a change that decreases that concentration that change must be a shift to the right Increasing the temperature of a reaction mixture will cause a change that tends to absorb heat that change must be a shift in the endothermic direction The principle is an interesting one but does require more careful reasoning in some cases than the more direct approach we employed For the most part we will find it more useful to bse our arguments on the properties of Kc In working with aqueous systems the most important equilibrium is often that which involves the dissociation of water into H and OH ions H2O H aq OH aq Kc Kw H OH 1 x 10 14 3 In this reaction the concentration of water is very high and is essentially constant at about 55 M it is incorporated into Kc The value of Kc is very small …

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