PRE-SPAINCHEMISTRY 4 (SNP20304)

TUTORIAL 1- HOW FAST? RATES

1. For the reaction: 4NH3(g) + 3O2(g) → 2N2(g) + 6H2O(g) the rate of decrease of NH3 is 0.82 mol dm-3 s-1. What is the rate of decrease of O2 and the rate of formation of N2 and H2O?

2. In the reaction X → 2Y, the concentration of X at t = 28.0 s and t = 117.5 s is 2.15 mol dm-3 and 1.08 mol dm-3 respectively. What is the average rate of the reaction during this time interval?

3. In the reaction W → products, the concentration of W at 10.0 min after the reaction has started is 1.00 mol dm-3. If the rate of dissociation of W at this point is 8.6 x 10-3 mol dm-3 min-1 and assuming that the rate remains constant, what is the concentration of W at 25.0 min after the reaction has started?

4. In the reaction 2NH3 (g) → N2(g) + 3H2(g), the initial pressure of NH3(g) in the vessel of the constant volume V dm3 is 36.0 kPa.

a) What will be the total pressure when the reaction has gone to completion?b) What is the average rate of decomposition of NH3 if the time taken for the reaction to

complete is 30.0 min?

5. Several hypothetical reactions are given below. For each reaction, what is the order with respect to each of the reactants and what is the overall reaction order?What is the unit of the rate constant, k for each of the reaction if the unit for the rate of reaction is mol dm-3 s-1?

a) P + Q → PQ ; Rate = k[Q]b) 3P + Q → P3Q ; Rate = k[P][Q]

6. For the reaction 2X2(g) + C2(g) → 2CX2(g), the rate equation was found to beRate = k[X2][C2]2

At temperature T, the rate of reaction is r. how does the reaction rate change ifa) Only the partial pressure of X2 is doubled?b) The partial pressure of X2 is halved and C2 is doubled?

7. The rate equation for the reaction A2(g) + X2(g) → 2AX(g) isRate = k [A2] [X2]

If the rate of reaction is x at 300K, what is the rate of reaction when the volume of the vessel is halved?

8. The initial rate of the reaction, 3P + Y → P3Y is measured for a series of different initial concentrations of P and Y. The data obtained is as follows.

ExperimentInitial concentration (mol dm-3)

Initial rate (mol dm-3 min-1)P Y

1 0.15 0.75 4.7 x 10-3

2 0.15 2.25 4.2 x 10-2

3 0.60 0.75 1.9 x 10-2

Determine the rate law for this reaction.

9. Hydrogen peroxide reacts with acidified iodide ions, liberating iodine. In investigations of this reaction, the following results were obtained when the experiments were carried out at T °C.

H2O2 + 2H+ + 2I- → I2 + 2H2O

ExperimentInitial concentration (mol dm-3)

Initial rate (mol dm-3 s-1)H2O2 H+ I-

1 0.20 0.20 0.20 0.0082 0.60 0.20 0.20 0.0243 0.60 0.20 0.40 0.0484 0.60 0.40 0.40 0.048

a) What is the rate equation for the reaction above?b) What is the rate of reaction when the concentrations of H2O2, H+ and I- are 0.55 mol

dm-3, 0.30 mol dm-3 and 0.25 mol dm-3 respectively?

10. At 400 °C, an organic compound A decomposes to form other organic compounds. The initial rate data are given in the following table.

ExperimentInitial [A]

(mol dm-3)

Initial rate of decomposition of A

(mol dm-3 s-1)1 1.8 x 10-2 9.0 x 10-4

2 3.0 x 10-2 1.5 x 10-3

a) Calculate the rate constant.b) Calculate the rate of decomposition of A when the concentration of A is 6.8 x 10-3 mol

dm-3

11. The decomposition of a compound A in its products is a zero order reaction. The rate constant at 380K is 0.083 mol dm-3 min-1 and the initial concentration of A is 3.50 mol dm-3.

a) Calculate the concentration of A after 10 minutes.b) How long will it take for the concentration of A to drop to 1.00 mol dm-3?

12. At 180 °C, the decomposition of a gaseous compound XO2 is a first order reaction with the half-life 38.6 min. The initial pressure of XO2 is 372.5 kPa.

a) What is the rate of decomposition of XO2 after 1 hour?b) What is the partial pressure of XO2 after 3 hours?

13. The gas phase decomposition of nitrogen dioxide is a second order reaction.2NO2(g) → 2NO(g) + O2(g)

If the rate constant, k at T K is 0.033 mol-1 dm3 min-1 and the initial concentration of NO2 is 8.00 mol dm-3,

a) What is the concentration after the reaction time of 30 mins?b) What is the time when the concentration of A is 0.08 mol dm-3?

14. The rate constants at different temperatures for the decomposition of an organic compound X are as shown.

Temperature (K) 320 385k (s-1) 5.70 x 10-6 2.15 x 10-3

a) Calculate the activation energy for the reaction.b) Determine the rate constant at 500 K.

15. The first order reaction X2 → products, has a half-life of 880 min at 260 K and 63.6 min at 520 K. Calculate the activation energy of this reaction.