Solutions to Web Experiment: Chemical Equilibrium II

Web Experiment: Chemical Equilibirum II
Answers to Questions





  1. A (4.0 mol/L) and B (4.0 mol/L) are allowed to react and the equilibrium molarities of A, B, and C are obtained from the plots of molarity versus time.



    The initial and equilibrium molarities of A, B, and C are entered in the equilibrium table.



    Note that the entries in the table are molarities (mol/L).

    Since


    then


    and


    Note that changes in the molarities of A, B, and C are related by mole ratios.


  2. After 8 seconds the molarities of A, B, and C no longer change with time. The reaction is at equilibrium.


  3. When the initial molarities of A, B, and C are 1.0, 1.0, and 3.0 mol/L at 25ºC, the molarities do not change with time. The system is at equilibrium.



  4. Since Kc (3.0054 1/M) > 1, the equilibrium is said to lie to the right.


  5. In exercise #1 the equilibrium molarities of A, B, and C are 1.0, 1.0, and 3.0 mol/L at 25ºC. The addition of 1.0 mol/L of B to this equilibrium results in a decrease in the molarities of A and B and an increase in the molarity of C (see plots of molarity versus time). After 7 seconds the system is again at equilibrium and the new equilibrium molarities of A, B, and C are 0.7, 1.7, and 3.3 mol/L. When 1.0 mol/L of B is added to the equilibrium, the reaction proceeds to the right to reestablish the equilibrium. Moles of A and B are consumed and moles of C are produced.



  6. When 1.0 mol/L of C is added to the equilibrium in exercise #1, the molarities of A and B increase and the molarity of C decreases with time (see plots of molarity versus time). After 7 seconds the system is again at equilibrium and the new equilibrium molarities of A, B, and C are 1.1, 1.1, and 3.9 mol/L. When 1.0 mol/L of C is added to the equilibrium, the reaction proceeds to the left to reestablish the equilibrium. Moles of C are consumed and moles of A and B are produced.



  7. When 1.0 mol/L of A and 1.0 mol/L of C are added to the equilibrium in exercise #1, the molarities of A and B decrease and the molarity of C increases with time (see plots of molarity versus time). After 7 seconds the system is again at equilibrium and the new equilibrium molarities of A, B, and C are 1.8, 0.8, and 4.2 mol/L. When 1.0 mol/L of A and 1.0 mol/L of A are added to the equilibrium, the reaction proceeds to the right to reestablish the equilibrium. Moles of A and B are consumed and moles of C are produced.



  8. When heat is added to the equilibrium in exercise #1 and the temperature is raised to 386 K, the molarities of A and B increase and the molarity of C decreases with time (see plots of molarity versus time). After 3 seconds the system is again at equilibrium and the new equilibrium molarities of A, B, and C are 1.4, 1.4, and 2.6 mol/L. In this case the value of Kc changes. When the temperature is increased, the value of Kc decreases and the equilibrium shifts to the left. Moles of C are consumed and moles of A and B are produced.



  9. When a system is in equilibrium, a change in any one of the factors upon which the equilibrium depends will cause the equilibrium to shift in such a way as to diminish the effect of the change. To diminish the effect of the added heat, the reaction proceeds in the direction that consumes heat. Since moles of C were consumed and moles of A and B were produced, heat is consumed when the reaction goes in the reverse direction,
    i.e. heat is required for C to dissociate into A and B. Therefore, heat is released when the A and B combine to form C and the forward reaction
    is exothermic.