Question

Part A:

Consider the following decomposition reaction of dinitrogen tetroxide: N₂O₄ ⇄ 2NO₂

After 180 seconds of reaction at 100°C, a dynamic equilibrium is established in which the partial pressures of N₂O₄ and NO₂ were experimentally found to be 0.22 atm and 1.56 atm, respectively. At this point, which statement best describes the reaction rates of the forward and reverse reactions? Explain.

a. The forward reaction is occuring at a greater rate than the reverse reaction.

b. The reverse reaction is occuring at a greater rate than the forward reaction

c. The reaction rate for the forward and reverse reaction are equal.

Part B:

From the same equilibrium reaction seen in Part A, knowing ΔH° for this reaction is +57.2 kJ. Use Le Chatelier's principle to predict the firection of equilibrium shift to counteract the follwing changes to the equilibrium system.

Disturbance Direction

a. Compression ← or →

b. Add N₂O₄ ← or →

c. Increase Temperature ← or →

Answer 1

Part A

Let us write the reaction taking place

N₂O₄ ⇄ 2NO₂

At equilibrium : Partial pressure of N₂O₄ :0.22 atm

                      Partial pressure of NO₂ : 1.56 atm

Rate of forward reaction is given by

Rf = Kf(P_No2 )²   = Kf 1.56² = Kf 2.4336             ..........(1)

Rate of backward reaction is given by

Rb = Kb P_N2O4 = Kb 0.22             ..........(2)

Now though it might look that the forward reaction rate is higher than backward reaction rate, But the question says that the system has reached to a stat of Dynamic equilibrium .

At Dynamic equilibrium , The rate of reactant getting converted to product and the rate of product getting converted to reactant is equal. Hence the rate of forward and backward reaction is same

Hence , Correct answer option (c): The reaction rate for the forward and reverse reaction are equal. since a dynamic equilibrium has reached

Part B

N₂O₄ ⇄ 2NO₂         ΔH°=+57.2 kJ

a) Compression

          Now compression means increase in pressure. As per Le chatelier's principle the system will try to negate the change that has occurs. Since the pressure has been increased the system will to shift in   direction where there is less pressure, that is less number of moles , that is backwards ( 1 mole N₂O₄)

Hence on compression the reaction will shift in backward direction

b) adding N₂O₄

N₂O₄ is added to the system. As per Le chatleier's principle , the system will try to negate this change by reacting the reactant and forming more product.

Hence on adding reactant to the system , the equilibrium will shift in forward direction

c) Increase Temperature

The reaction given is endothermic. On the increasing the temperature, the system will try to decrease this increase in temperature. Since the reaction is endothermic, the system absorbs the heat, and in doing to it reduces the temperature of the system.

Hence, on increasing temperature for an endothermic reaction, the equilibrium will move in forward direction to absorb the increase in temperature and form more product