Question

3. (15 Points, due Thurs 07 Nov) For the following reaction: A(s)+2B(8) = C(g), Kp <<1.0 @298 K, AH<0.0 What would be the effect on the final equilibrium amount of C(g) if a gaseous mixture originally at equilibrium with a large excess of A(s) at 298 K is subjected to the following changes: a. More B(g) is added b. A catalyst is added c. The mixture is transferred to a reaction vessel of greater volume d. C(g) is removed e. An inert gas is added, which leads to an increase of pressure f. The temperature is lowered to 250 K. g. A small amount of A(s) is removed.

Answer 1

3. According to the Le Chatelier's principle, if a system at equilibrium is subjected to a stress (like change in concentration/pressure of reactants/products, temperature etc.) then it will move in a direction so as to offset the change to restore the equilibrium.

a. If more B(g) is added, then as B is a reactant and we are essentially increasing the concentration of reactant B, so the system will move in forward direction to reduce the amount of B(g) resulting in an increase in amount of C(g) at equilibrium.

b. A catalyst increases the rate of both forward and backward reaction to the same extent, so if we add a catalyst to the system, it will have no effect on the final equilibrium amount of C(g).

c. Concentration is given as amount of a substance per unit volume. So an increase in volume means a decrease in concentration. It also means a decrease in partial pressures of reactants and products. Thus an increase in volume always makes the reaction move to the side where there are number of moles of gas.

If the mixture is transferred to a reaction vessel of greater volume, it will move to backward direction (as there are 1+2=3 moles of gas on reactant side) resulting in a decrease in amount of C(g) at equilibrium.

d. If C(g) is removed, this means a decrease in concentration of product, so the system will move in forward direction to offset the decrease by producing more of C. So this will increase the amount of C(g) at equilibrium.

e. An inert gas added at a constant volume has no effect on equilibrium as though it increases the total pressure of the system, but it doesn't affect the partial pressure of reactants and products. So this doesn't affect the amount of C(g) at final equilibrium.

f. As enthalpy change $\Delta H<0.0$ , i.e. the reaction is exothermic (as the sign of enthalpy change is negative), so if we lower the temperature to 250 K, the reaction will move in forward direction so as to offset the decrease in temperature, resulting in production of heat and thus it moves in direction of product C(g). So the amount of C(g) will be more at the final equilibrium.

g. As A(g) is in excess, removing a small amount of A(g) has no effect on equilibrium and hence on amount of C(g) at equilibrium.