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IB Chemistry Kinetics Exam Questions and Answers

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Introduction

Kinetics Review Questions 1. Define rate of reaction 2. What are the units of the rate of reaction? 3. Outline procedures by which you could obtain a value for the rate of reaction 4. Draw a graph that shows the concentration of products with time as a reaction goes to completion. Explain the shape of the graph. 5. Draw a graph that shows the concentration of reactants with time as a reaction goes to completion. Explain the shape of the graph. 6. Outline the main features of collision theory 7. Give two reasons why a collision would not result in a reaction 8. The reaction between nitrogen and oxygen in the atmosphere under normal conditions is extremely slow. Which statement best explains this? A. The concentration of oxygen is much lower than that of nitrogen B. The molar mass of nitrogen is less than that of oxygen C. The frequency of collisions between nitrogen and oxygen molecules is lower than that between nitrogen molecules themselves D. Very few nitrogen and oxygen molecules have sufficient energy to react 9. List 4 factors that affect reaction rates and explain why using the principles of collision theory. 10. Define activation energy 11. Sketch a Maxwell-Boltzmann Energy Distribution Curve for two different temperatures. 12. Does activation energy change with temperature? 13. How does the number of molecules with the required activation energy change at higher temperatures? Use energy distribution to explain. 14. Give two reasons why a temperature increase, increases reaction rate and identify the more important reason. 15. Will the following changes increase the reaction rate of the following reaction when 50 cm3 of 1.0 mol dm-3 HCl is added to 1.0 g calcium carbonate? CaCO3(s) + 2 HCl (aq) --> CaCl2 + CO2(g) + H2O(l) a. increasing volume of HCl b. increasing mass of CaCO3 c. decreasing size of CaCO3 particles d. decreasing amount of CO2 present e. ...read more.

Middle

2) This is nearly the same answer as C. B. The molar mass of nitrogen is less than that of oxygen This doesn't really matter C. The frequency of collisions between nitrogen and oxygen molecules is lower than that between nitrogen molecules themselves This may be true but doesn't explain why the reaction between nitrogen and oxygen is slow, only why its slower than a reaction between two nitrogen molecules. Also, it's pretty much the same answer as A. D. Very few nitrogen and oxygen molecules have sufficient energy to react This is the best explanation 9. List 4 factors that affect reaction rates and explain why using the principles of collision theory. 1. concentration: higher conc. means higher reaction rate because there are more chances for molecules to collide in a given amount of time. (on IB exam, they really want you to talk about the RATE of collisions, not just the number of collisions) 2. Surface area: the more surface area, the more chances for a collision in the same amount of time. 3. Temperature: the higher the temperature the faster the molecules are moving. This does two thing which BOTH increase reaction rate. So you get more out of increasing temperature than other changes. i. More collisions per unit time ii. More molecules have the required activation energy so more of the collisions successfully result in a reaction. 4. Catalysts: increase reaction rate by make more collisions successful in a given amount of time. i. Holding molecules in the right orientation for more successful collisions per unit time ii. Lowering activation energy so that more molecules, at a given temperature, have enough energy to have a successful collision 10. Define activation energy Minimum amount of energy required for a molecule to react 11. Sketch a Maxwell-Boltzmann Energy Distribution Curve for two different temperatures. x-axis: velocity/speed or energy y-axis: number of molecules Higher temperature has a lower peak, shifted to the right and distribution is spread out 12. ...read more.

Conclusion

This results in formation of the activated complex. Eventually the bond between the Br and the C break and the bond between the OH and the C strengthens. 33. Draw an energy diagram for the above reaction. Assume the reaction is exothermic. 34. Show on your diagram, what happens when a catalyst is added. 35. The following reaction occurs by the following mechanism CH3CH2Cl + OH-1 --> CH3CH2OH + Cl-1 1) CH3CH2Cl + OH-1 --> HO-(CH3)CH2-Cl 2) HO-(CH3)CH2-Br --> CH3CH2OH + Cl-1 a. If the first step is the rate determining step, what is the rate law? Rate =k[CH3CH2Cl][OH-] b. What is the overall order of the reaction? 2 c. What is the molecularity of the reaction? 2, bimolecular 36. The following reaction occurs by the following mechanism (CH3)3CCl + OH-1 --> (CH3)3COH + Cl-1 1) (CH3)3CCl --> (CH3)3C+1 + Cl-1 2) OH-1 + (CH3)3C+1--> (CH3)3COH a. If the first step is the rate determining step, what is the rate law? Rate = k[(CH3)3CCl] b. What is the overall order of the reaction? 1 c. What is the molecularity of the reaction? 1, unimolecular 37. Define molecularity The number of molecules that need to collide in the rate determining step 38. A reaction occurs by the following mechanism 1) AC --> A + C 2) A + B --> AB 3) AB + C --> ABC 4) ABC + A --> A2 + BC a. What is the overall reaction? 2AC + 2B + --> A2 + 2BC b. If the rate law is first order with respect to A and first order with respect to B, what is the rate law? Rate = k[A][B] c. What is the rate-determining step? The second step d. How do you know? The rate determining step is always the slowest step in the reaction mechanism. The rate law is a mathematical representation of the rate. Since the slowest step is what determines the rate, than the rate law must be based on the slowest step. So the reactants that affect the rate must be involved in the rate-determining step. ...read more.

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