Question
When ClNO2 reacts with NO an equilibrium is established.
\(ClNO_{2}(g) + NO(g) \rightleftharpoons NO_{2}(g) + ClNO(g)\)
In each ClNO2 molecule the nitrogen atom is bonded to the chlorine atom and bonded to each of
the oxygen atoms separately.
(a) Draw a ‘dot-and-cross’ diagram for the ClNO2 molecule.
(b) The reaction between ClNO2 and NO is first order with respect to each reactant.
(i) Write the rate equation for this reaction.
(ii) Deduce the units of the rate constant, k, when the concentrations of both gases are
measured in moldm-3 and the rate is measured in moldm-3 s-1
(iii) State and explain whether or not the reaction could take place in a single step.
(c) An experiment is carried out in which the initial [ClNO2] is 2.0 × 10-4moldm-3. A large excess
of NO is used. The initial rate of reaction is 1.0 × 10-4moldm-3 s-1. The rate of the reaction is
assumed to be constant for the first 0.20 seconds.
(i) Draw a graph on the grid to show how the concentration of ClNO2 varies for the first
0.20 seconds.
(ii) Deduce the concentration of the NO2 product at 0.20 seconds.
(iii) After 20 seconds the concentration of ClNO2 remains constant.
Explain this observation.
Answer/Explanation
Answer: (a)
(b)(i) (rate =) k[ClNO2][NO]
(b)(ii) mol-1 dm3 s-1
(b)(iii) Yes AND number of moles of reactants in overall equation is the same as order in rate equation
(c)(i) • straight line with a negative gradient • starting at 2.0 × 10-4 • reaches at 1.8 × 10-4 at 0.2 seconds
Award 1 mark for two points, award 2 marks for all three points
(c)(ii) 2 × 10-5 (mol dm-3)
(c)(iii) The reaction has reached equilibrium