NJC 7 Kinetics H1 Tutorial Ans

1 Reaction Kinetics Discussion Questions 1 The initial rate of the reaction X + Z ⎯→ P + Q was measured as a function of the concentration of each reactant. The following results were obtained: Expt [X] / mol dm−3 [Z] / mol dm−3 Initial rate / mol dm−3 s−1 1 0.20 0.30 9.0  10−3 2 0.20 0.40 12.0  10−3 3 0.40 0.40 48.0  10−3 (a) Determine the order of reaction with respect to X, Y and Z. Comparing expt 1 and 2 where [X] and [Y] are the same. When [Z]expt2 [Z]expt1 = 0.40 0.30 = 4 3 , Rate2 Rate1 = 12 9 = 4 3 Hence 1st order wrt Z. Comparing expt 2 and 3 where [Y] and [Z] are the same. When [X]expt3 [X]expt2 = 2 , Rate3 Rate2 = 4  Rate  [X]2 Hence 2nd order wrt X (b) Write the rate equation. Rate = k[X]2[Z]

2 2 The kinetics of the reaction between iodide and peroxodisulfate can be investigated by varying the volume of the reactants used. The two reactants are mixed in the presence of a known amount of Na2S2O3 and a little starch. The time taken for an intense blue colour to be observed is then determined. Experiment Volume used/cm3 t/s 1.0 mol dm−3 KI 0.040 mol dm−3 Na2S2O8 H2O 1 10.0 5.0 25.0 170 2 15.0 5.0 20.0 113 3 15.0 10.0 15.0 57 4 20.0 20.0 0.0 x What is the value of x? A 21 B 28 C 85 D 63 Rate ∝ 1 𝑡𝑖𝑚𝑒 𝑡𝑎𝑘𝑒𝑛 Expt Volume used/cm3 t/s 1 𝑡𝑖𝑚𝑒 𝑡𝑎𝑘𝑒𝑛 1.0 mol dm−3 KI 0.040 mol dm−3 Na2S2O8 H2O 1 10.0 5.0 25.0 170 1 170 = 0.00588 2 15.0 5.0 20.0 113 1 113 = 0.00885 3 15.0 10.0 15.0 57 1 57 = 0.01754 4 20.0 20.0 0.0 x Comparing expt 1 and 2 where [Na2S2O8] is the same. When [KI] × 1.5, rate also × 1.5. Hence it is 1st order wrt KI. Comparing expt 2 and 3 where [KI] is the same. When [Na2S2O8] × 2, rate also × 2. Hence it is 1st order wrt Na2S2O8 Rate = k[KI][ Na2S2O8] Comparing Expt 1 and 4, When [KI] × 2, and [Na2S2O8] × 4, rate is 2×4 = 8 times of expt 1. Rate of expt 4 = 8 × 0.00588 = 0.0470 1 𝑥 = 0.0470, x = 21.3 s Answer is A

3 3 The following reaction is monitored by measuring the changes in total pressure during the reaction, with initial pressure of O2 at 300 kPa. 3O2(g) → 2O3 (g) Determine the order of reaction with respect to oxygen As shown in the graph, t1/2 (1) is 55s (from 300 kPa to 150 kPa) t1/2 (2) is 59s (from 250 kPa @ t =14 to 125 kPa @ t=73) As the reaction proceeds, PO2 decreases and hence [O2] decreases. It is observed that the rate of reaction decreases with a constant half-life of 57s. Hence it is first order w.r.t O2.

4 4 Explain, using collision theory, how the following changes can affect the initial reaction rate of a gaseous system. Illustrate your answer with the Boltzmann distribution curve for parts (ii) and (iii). (i) increasing the pressure There is greater no of particle per unit volume leading to greater collision frequency therefore frequency of effective collision increases leading to increase in rate.