Topic 5.2: How Fast? The Rate of Chemical Change Questions

In an experiment, the concentration of a product increases from 0 to 0.48 mol dm⁻³ in the first 24 s.

Calculate the average rate of reaction over this interval, including its units. [2]

Using collision theory, explain why increasing the concentration of a reactant increases the rate of reaction, and state the two requirements for a collision to be effective. [3]

Explain, in terms of the Maxwell-Boltzmann distribution, why increasing the temperature increases the rate of a reaction. [3]

A reaction has the experimentally-determined rate equation rate = k[A].

(a) State the order with respect to A and the overall order.

(b) Explain why the species B, which appears in the balanced equation, does not appear in the rate equation, with reference to the rate-determining step.

(c) Deduce the units of k.

The reaction P + Q → products was studied at constant temperature.

Doubling [P] (with [Q] constant) increased the initial rate by a factor of four; doubling [Q] (with [P] constant) doubled the initial rate.

When [P] = 0.20 mol dm⁻³ and [Q] = 0.10 mol dm⁻³ the initial rate was 4.8 × 10⁻³ mol dm⁻³ s⁻¹.

(a) Deduce the order with respect to P and to Q, and write the rate equation.

(b) Calculate the rate constant k, including its units.

The rate constant for the hydrolysis of an ester was measured at several temperatures.

A graph of ln k against 1/T gave a straight line of gradient −7.20 × 10³ K and y-intercept 18.5.

(a) Calculate the activation energy, Eₐ, in kJ mol⁻¹.

(b) Determine the value of the Arrhenius factor, A. (R = 8.31 J K⁻¹ mol⁻¹.)

A reaction has a rate constant of 3.0 × 10⁻³ s⁻¹ at 290 K and 1.2 × 10⁻² s⁻¹ at 310 K.

(a) Calculate the activation energy, Eₐ, in kJ mol⁻¹, using the two-point form of the Arrhenius equation.

(b) State and explain what would happen to Eₐ if a catalyst were added. (R = 8.31 J K⁻¹ mol⁻¹.)

A catalyst is added to a reaction at constant temperature.

(a) Explain, in terms of activation energy and the Maxwell-Boltzmann distribution, how the catalyst increases the rate.

(b) State why the enthalpy change of the reaction is unaffected. [3]