Equilibrio Quimico

Exp. 5 EQUILIBRIUM

CHEMISTRY 206 Experiment 5: EXPERIMENTS IN CHEMICAL EQUILIBRIUM
Instructor’s Informal Preamble
In these experiments, you will investigate several systems where reactions do not proceed to completion. In reality, this is true of all chemical processes. However, in many cases, the reaction proceeds so nearly to completion that for stoichiometric calculations it can safely beassumed that the limiting reactant is entirely consumed. In reactions that seem to stop before all the limiting reactant is consumed, what happens is that the products begin to react as they are formed, to give back the original reactants. Remember that reaction rates depend on concentration: as more product forms, the backward reaction becomes faster, and at the same time, the forward reactionslows down since reactant has been consumed. Eventually, an equilibrium situation is reached where the forward and backward reaction rates are equal. Equilibrium is reached quickly for reactions with large rate constants (with small activation energies, Ea, or at high temperature, T), but slowly for reactions with small rate constants (with large Ea, or at low T). When a chemical system reachesequilibrium, the system will appear from the outside to be unchanging, because if substances are being consumed but also produced at the same rate, then their concentrations will not actually change. Students often misinterpret this phenomenon as the reaction “stopping”, but in fact, at the molecular level the situation is still dynamic, with molecules colliding and reacting non-stop. However, theunchanging nature of the concentrations at equilibrium is quite useful, because it allows us to determine whether or not a reaction has reached equilibrium. If we see that the concentrations in a chemical system are unchanging, we can make two hypotheses: 1) the activation energy for the reaction is so large that the reaction cannot proceed in one or both directions under these conditions (i.e.,temperature is too low…); 2) the system is at equilibrium (this one is more likely!). To test these hypotheses, we can add more of one of the substances and see if this causes the reaction to “start up” again, which can only happen if hypothesis #1 is not correct. If the system is at equilibrium, then if we add more reactant, the forward reaction will suddenly be faster than the reverse reaction.Therefore, we will notice a net forward reaction for a while, until the forward and reverse reaction rates balance out again and the reaction reaches a new equilibrium situation and appears to “stop”. We can predict this kind of outcome using Le Châtelier’s Principle, the handy rule that states that if a system at equilibrium is disturbed (by adding or removing a substance, or changing the pressureor temperature), it will respond in a way that counteracts the disturbance. To understand why this rule works, you must think about how each type of disturbance affects the rate of both the forward and reverse reactions, like we just did. 5-1

Exp. 5 EQUILIBRIUM

CHEMISTRY 206 Experiment 5: EXPERIMENTS IN CHEMICAL EQUILIBRIUM
Introduction
If a system is at equilibrium, the concentrations ofthe reactants and products will be unchanging, and they will be related to each other by a constant called the equilibrium constant, Keq. For a reaction: aA + bB we express Keq as: cC + dD Keq = [C]c[D]d / [A]a[B]b

The equilibrium constant for a particular reaction has a constant value under a particular set of conditions. The value of Keq for a reaction will vary, however, if the temperatureis changed, because the rate constants (k) for the forward and reverse reactions will each change, and not by the same amount (recall Arrhenius’ equation: k = Ae–Ea/RT). Therefore, when studying reactions at equilibrium and/or their response to disturbances, it is necessary to maintain a constant (and known) temperature; we will perform all of our reactions at room temperature. It is important to...