Exam Qs use the arrow keys to move forward click any link below for a specific question click the home button top right to return Rates of Reaction (Kinetics) 1 Initial rates data and the rate equation 2 The Maxwell-Boltzmann distribution You are advised to work through these questions a screen at a time, writing your answers down on paper and then clicking the Answer link to check.

1 (a) The following data were obtained in a series of experiments on the rate of the reaction between compounds X and Y at a constant temperature. (i) Show with reasoning how the data in the table can be used to deduce that the reaction is first-order with respect to X. (ii) Deduce with reasoning the order with respect to Y. (iii) State the rate equation and overall order for this reaction. (5 marks) ExperimentInitial concentration of X/mol dm –3 Initial concentration of Y/mol dm –3 Initial rate/ mol dm –3 s – x 10 – x 10 – x 10 –6 Question page 1 Answer 1 (a)

1 (a) (i) Show with reasoning how the data in the table can be used to deduce that the reaction is first-order with respect to X. For expts 1 and 2: initial [Y] unchanged and so does not affect reaction rate, initial [X] and initial rate both double therefore first order wrt X (ii) Deduce with reasoning the order with respect to Y. For expts 2 and 3: initial [X] doubles, first order wrt X and so initial rate doubles, initial [Y] doubles and initial rate increases x 8 of which x 2 is due to X. So when initial [Y] doubles initial rate increases x 4, therefore second order wrt Y (iii) State the rate equation and overall order for this reaction. rate = k [X] [Y] 2 overall = 1+2 = 3 third order (5 marks) ExperimentInitial concentration of X/mol dm –3 Initial concentration of Y/mol dm –3 Initial rate/mol dm –3 s – x 10 – x 10 – x 10 –6 Answer page 1 Question 1 (a)

1 (b) The following data were obtained in a second series of experiments on the rate of the reaction between compounds M and N at a constant temperature. The rate equation for this reaction is rate = k [M] 2 [N] ExperimentInitial concentration of M/mol dm –3 Initial concentration of N/mol dm –3 Initial rate /mol dm –3 s – x 10 – To be calculated Question page 2 (i) Use the data from Experiment 4 to calculate a value for the rate constant, k, at this temperature. State the units of k. Value for k Units of k (ii) Calculate the value of the initial rate in Experiment 5. (4 marks) Answer 1 (b)

1 (b) The rate equation for this reaction is rate = k [M] 2 [N] ExperimentInitial concentration of M/mol dm –3 Initial concentration of N/mol dm –3 Initial rate/mol dm –3 s – x 10 – To be calculated Answer page 2 (i) Use the data from Experiment 4 to calculate a value for the rate constant, k, at this temperature. State the units of k. Value for k re-arrange and substitute the values from expt 4: k = rate / [M] 2 [N] = 3.10 x / ( x 0.50) = 9.92 x Units of k third order overall therefore mol -2 dm 6 s -1 (ii) Calculate the value of the initial rate in Experiment 5. substitute the values from expt 4 and for k : initial rate = 9.92 x x x 0.20 = 3.17 x mol dm -3 s -1 * remember rate has units (4 marks) Question 1 (b)

Question page 3 2 (a) State what is meant by the term activation energy of a reaction. (1 mark) (b) Explain how a catalyst increases the rate of a chemical reaction. (2 marks) Answers 2 (a) and (b)

Answer page 3 2 (a) State what is meant by the term activation energy of a reaction. (1 mark) The minimum energy needed for reaction to happen. (b) Explain how a catalyst increases the rate of a chemical reaction. A catalyst that provides an alternative reaction path with a lower activation energy. (2 marks) Questions 2 (a) and (b)

Number of molecules E mp E a Energy Question page 4 2 (c) The Maxwell–Boltzmann distribution of energies, at a constant temperature, for a molecular gas is shown below. The most probable molecular energy at this temperature is indicated by E mp and the activation energy by E a. Consider the following changes: (i) The number of molecules is increased at constant temperature. (ii) The temperature is decreased without changing the number of molecules. (iii) A catalyst is introduced without changing the temperature or number of molecules. For each change state how, if at all, the following would vary: the value of the most probable energy, E mp the number of molecules with the most probable energy, E mp the area under the molecular energy distribution curve the number of molecules with energy greater than the activation energy, E a (12 marks) Answer 2 (c) i iiiii

Number of molecules E mp E a Energy Answer page 4 2 (c) Consider the following change: (i) The number of molecules is increased at constant temperature. State how, if at all, the following would vary: the value of the most probable energy, E mp no change the number of molecules with the most probable energy, E mp increases the area under the molecular energy distribution curveincreases the number of molecules with energy greater than the activation energy, E a increases (4 marks) Question 2 (c) i

Number of molecules E mp E a Energy Answer page 5 2 (c) Consider the following change: (ii) The temperature is decreased without changing the number of molecules. State how, if at all, the following would vary: the value of the most probable energy, E mp decreases the number of molecules with the most probable energy, E mp increases the area under the molecular energy distribution curveno change the number of molecules with energy greater than the activation energy, E a decreases (4 marks) Question 2 (c) ii

Number of molecules E mp E a Energy Answer page 6 2 (c) Consider the following change: (iii) A catalyst is introduced without changing the temperature or number of molecules. State how, if at all, the following would vary: the value of the most probable energy, E mp no change the number of molecules with the most probable energy, E mp no change the area under the molecular energy distribution curveno change the number of molecules with energy greater than the activation energy, E a increases (assuming this refers to E a catalysed) (4 marks) Question 2 (c) iii

References An illustration of how exam questions and model answers might be presented for use in the classroom or independently. Steve Lewis for the Royal Society of Chemistry