Chemical Kinetics

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  • Topic: Chemical kinetics, Rate equation, Chemical reaction
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Tro’s Chemistry Chapter 13 – Chemical Kinetics

Page 1 of 13

Acknowledgements: Many of the images are adopted from Tro’s textbook, the only purpose of which is to enhance student learning. Key terms, concepts, skills: Refer to pp 599 – 601. Review questions: 3 – 24. Suggested problems: 25, 27, 33, 39, 43, 53, 57, 59, 69, 73, 75, 81, 93, 103. 13.1 & 2 Introduction to the Rate of a Chemical Reaction • kinetics is the study of the factors that affect the speed of a reaction and the mechanism by which a reaction proceeds. • experimentally it is shown that there are 4 factors that influence the speed of a reaction: nature of the reactants, temperature catalysts concentration • • rate of a chemical reaction is generally measured in terms of how much the concentration of a reactant decreases in a given period of time or product concentration increases for reactants, a negative sign is placed in front of the definition

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as time goes on, the rate of a reaction generally slows down because the concentration of the reactants decreases. at some time the reaction stops, either because the reactants run out or because the system has reached equilibrium.

Reaction Rate and Stoichiometry • in most reactions, the coefficients of the balanced equation are not all the same H2(g) + I2(g) → 2 HI(g) • for the above reaction, for every 1 mole of H2 used, 1 mole of I2 will also be used and 2 moles of HI made, therefore the rate of change will be different • in order to be consistent, the change in the concentration of each substance is multiplied by 1/coefficient ∆[H 2 ] ∆[I 2 ] ⎛ 1 ⎞ ∆[HI] Rate = − =− = +⎜ ⎟ ∆t ∆t ⎝ 2 ⎠ ∆t • the average rate is the change in measured concentrations in any particular time period, linear approximation of a curve • the larger the time interval, the more the average rate deviates from the instantaneous rate • the instantaneous rate is the change in concentration at any one particular time, i.e., the slope at a given point of the curve • determined by taking the slope of a line tangent to the curve at that particular point, first derivative of the function. • Refer to Example 13.1 and Practice 13.1 for problem solving help and hint.

Tro’s Chemistry / Chapter 13 – Chemical Kinetics / Page 2 of 13

Q# 25. Consider the following reaction: 2 HBr(g) → H2(g) + Br2(g) a. Express the rate of the reaction with respect to each of the reactants and products. b. In the first 15.0 s of this reaction, the concentration of HBr dropped from 0.500 M to 0.455 M. Calculate the average rate of the reaction in this time internal. c. If the volume of the reaction vessel in part b was 0.500 L, what amount of Br2 (in moles) was formed during the first 15.0 s of the reaction?

Q# 27. For the reaction 2 A(g) + B(g) → 3 C(g) a. Determine the expression for the rate of the reaction with respect to each of the reactants and products. b. When A is decreasing at a rate of 0.100 M/s, how fast is B decreasing? How fast is C increasing?

Q# 33. Consider the following reaction: H2(g) + Br2(g) → 2 HBr(g). The graph below shows the concentration of Br2 as a function of time. a. Use the graph to calculate the following I. The average rate of the reaction between 0 and 25 s. II. The instantaneous rate of the reaction at 25 s. III. The instantaneous rate of formation of HBr at 50 s. b. Make a rough sketch of a curve representing the concentration of HBr as a function of time. Assume that the initial concentration of HBr is zero.

Tro’s Chemistry / Chapter 13 – Chemical Kinetics / Page 3 of 13

Measuring Reaction Rate • measure the concentration of at least one component in the mixture at many points in time • there are two ways of approaching this problem (1) for reactions that are complete in less than 1 hour, it is best to use continuous monitoring of the concentration, or (2) for reactions that happen over a very long time, sampling of the mixture at various times can be used when sampling is used, often the reaction in the...
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