Question

Consider the following system at equilibrium where Kc = 5.10×10-6 and H° = 268 kJ/mol at 548 K. NH4Cl (s) NH3 (g) + HCl (g) The production of NH3 (g) is favored by: Indicate True (T) or False (F) for each of the following: 1. decreasing the temperature. 2. decreasing the pressure (by changing the volume). 3. decreasing the volume. 4. adding NH4Cl . 5. adding HCl .

Answer 1

The equilibrium reaction is given as

NH₄Cl (s) <======> NH₃ (g) + HCl (g)       ΔH⁰ = 268 kJ/mol

The equilibrium constant for the reaction is given as

K_c = [NH₃][HCl] = 5.10*10^-6

(concentration of solid NH₄Cl is considered as unity).

1) The enthalpy change for the reaction, ΔH⁰ is positive; therefore, heat must be supplied to make the reaction work. Thus, heat can be considered a hypothetical reactant in the reaction. Decreasing the temperature of the equilibrium system decreases ΔH⁰ and thus, the hypothetical reactant. K_c is a thermodynamic equilibrium constant and must remain constant a particular temperature. Therefore, in order to counter the effect of decreased temperature, the dissociation of NH₄Cl is suppressed; infact, the reaction proceeds in the reverse direction to produce more NH₄Cl. Thus, the given statement is false.

2) The reaction produces gaseous products. Since gases are produced from solid NH₄Cl, hence, the number of moles of the product increases. Assuming that the gases behave ideally, the increase in the number of moles of the gases leads to an increase in volume as per the ideal gas law

P*V = n*R*T

where n = number of moles of the gases and V = volume of the gases. Since n increases, the volume increases.

Again, the volume and the pressure of the gas are related as

P*V = constant.

An increased volume (V) will lead to a reduction in pressure (P). Therefore, the decrease in pressure favors the formation of NH₃ and the given statement is true.

3) It has already been shown above that the volume of the gas is directly proportional to the number of moles. Since, the volume is reduced, the number of moles of the gases tend to decrease. The reactant side is characterized by having fewer to no gas molecules. Therefore, the decreased volume will favor the reverse reaction, i.e, the formation of NH₃ is suppressed. Thus, the given statement is false.

4) NH₄Cl is the reactant, though it doesn’t appear in the K_c expression. Since K_c is constant at a particular temperature and the concentration of solid NH₄Cl doesn’t appear in the expression, hence, increasing the concentration of NH₄Cl at the same temperature will have no effect on the concentration of NH₃.

5) HCl is a product of the reaction. Increasing the amount of HCl increases the concentration and therefore, tends to increase K_c. However, in order to keep K_c constant, HCl recombines with NH₃ to afford more NH₄Cl, i.e, the reverse reaction is favored. Consequently, the given statement is false.