STRESSES AND LE CHATELIER’S PRINCIPLE
Once a reaction has reached equilibrium, any external stress that is imposed results in a shift of the equilibrium and a subsequent change in the concentrations of the reactants and products involved. The direction of the shift—right (or product side) or left (or reactant side)—is in accordance with Le Chatelier’s principle, which states that if a stress is imposed on a reaction at equilibrium, the reaction will shift either right or left in the way or direction that accommodates or relieves the stress.
There are five categories of stress:
- • The reactant concentration or product concentration is increased or decreased.
- • The temperature of the reaction is increased or decreased.
- • The volume of the reaction vessel is increased or decreased.
- • The pressure of the reaction is increased or decreased (in any one of three different ways).
- • A catalyst is added to the reaction.
Consider the following reaction already at equilibrium.
The enthalpy or heat of reaction is AH° = -84.3 kJ.
Predict what will happen—increase, decrease, or no change—to the equilibrium concentration of CCl4 when each of the following stresses is imposed:
A. Cl2 is added.
B. S2Q2 is removed.
C. The temperature is increased.
D. The volume of the reaction vessel is decreased.
E. The pressure is decreased by increasing the volume of the reaction vessel.
F. A catalyst is added.
Use Le Chatelier’s principle to answer each part.
A. Adding Cl2 will cause the reaction to shift to the right, increasing [CCl4].
B. Removing S2Q2 will cause the reaction to shift to the right, increasing [CCh].
C. To answer this part, the meaning of AH° must be understood. Recall the explanation provided in Section 1.9.9. This is the amount of heat energy either liberated or absorbed in the reaction and may be expressed in either kilojoules (kJ) or kilocalories (kcal). If heat energy is liberated, the reaction is exothermic, and the sign of AH is negative by convention. If heat energy is absorbed, the reaction is endothermic, and the sign of AH is positive. Here, the sign is negative, so heat is produced. Hence, since raising the temperature adds heat and heat behaves as a product, increasing the temperature will shift the reaction to the left, decreasing [CCLJ.
D. In the balanced reaction, 4 molecules of reactant are reacting to form 2 molecules of product. Increasing the volume of the vessel favors the reactant or left side of the reaction, while decreasing the volume favors the product or right side. Hence, decreasing the volume will cause the reaction to shift to the right, increasing [CCLJ. In the special case where the two sides of a reaction contain equal numbers of molecules, changing the volume has no effect.
E. If the pressure is decreased by increasing the volume, the effect can be evaluated as a volume stress. As discussed in D, increasing the volume will cause the reaction to shift to the left and decrease
Note, however, that if the pressure is increased or decreased by the addition of a reactant or product (discussed in A or B), the effect should be evaluated as a stress of the type discussed in A or B...
Also, note that if the pressure is increased by the addition of an inert or unreactive gas, there is no effect on the equilibrium.
F. A catalyst speeds up the rate of reaction. Since the reaction has already reached equilibrium, adding a catalyst will have no effect.
DEPENDENCE OF Kc ON TEMPERATURE
The exact dependence of Kc (or Kp) on temperature is given by the van’t Hoff equation:
where Kc(1) is the equilibrium constant value at absolute temperature 7j and Kc(2) is the value at T2, AHrx is the enthalpy (or heat) of reaction, and R is the gas constant in units of either joules per mole or calories per mole, consistent with the units of AHRX. The similarity of the van’t Hoff equation to the Clausius-Clapeyron equation, given as equation 1.18, is noteworthy.