Question

In the equation below,

PCl₃(g) + Cl₂(g) « PCl₅(g)

the forward reaction is first order in both PCl₃ and Cl₂ and the reverse reaction is first order in PCl₅.

(a)   Suppose that 2 moles of PCl₃ and 1 mole of Cl₂ are mixed in a closed 1.0 L container at constant temperature. Draw a graph that shows how the concentrations of PCl₃, Cl₂, and PCl₅ change with time until after equilibrium has been firmly established. Indicate the numerical values on the concentration axis.

(b)   Give the initial rate law for the forward reaction.

(c)   Provide a molecular explanation for the dependence of the rate of the forward reaction on the concentrations of the reactants.

(d)   Provide a molecular explanation for the dependence of the rate of the forward reaction on temperature. Remember to include the Maxwell-Boltzmann graph in your answer.

Answer 1

(a) exact values of concentrations can not be calculated because the value of K_eq is not given . See the graph below.

(b) Initial rate of forward reaction = k_f[PCl₃][Cl₂]

= k_f (2M)(1M) = 2k mol/L.sec^-1

(Where k_f is rate constant of forward reaction).

(c)

As PCl₃ and Cl₂ molecules collide , it might form the product PCl₅. The number of collisions would depends on the number density of that particular molecules. Hence higher the number of molecules of reactants per unit volume , higher will be number of collisions and higher would be the rate of reaction. Hence rate of reaction would depends upon the concentration of both PCl₃ and Cl₂ .

(d) As the temperature of reactants is increased the average kinetic energy (& total energy) of molecules also increases . The fraction of molecules having energy higher that activation energy also increases . Hence number of successful collision would also increases . See the picture below for Maxwell Boltzmann distribution of speed graph.