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Chapter 13 Chemical Kinetics

Chapter 13 Chemical Kinetics. Factors that affect reaction rates How to express reaction rates Rate Laws Effects of temperature reaction rates Mechanisms of reactions Catalysis. Kinetics. Studies the rate at which a chemical process occurs.

jolene-rasmussen
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Chapter 13 Chemical Kinetics

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  1. Chapter 13Chemical Kinetics • Factors that affect reaction rates • How to express reaction rates • Rate Laws • Effects of temperature reaction rates • Mechanisms of reactions • Catalysis

  2. Kinetics • Studies the rate at which a chemical process occurs. • Besides information about the speed at which reactions occur, kinetics also sheds light on the reaction mechanism (exactly how the reaction occurs).

  3. Factors That Affect Reaction Rates • Physical State of the Reactants • In order to react, molecules must come in contact with each other. The more rapidly reactant collide with each other, the more rapidly they react • Most of reactions we consider are homogenous, involving either gases or liquid solutions • Concentration of Reactants -As the concentration of reactants increases, so does the likelihood that reactant molecules will collide. • Temperature • At higher temperatures, reactant molecules have more kinetic energy, move faster, and collide more often and with greater energy. • Presence of a Catalyst • Catalysts speed up reactions by changing the mechanism of the reaction. • Catalysts are not consumed during the course of the reaction. On the molecular level, reaction rates depend on the frequency of collisions between molecules. The higher the frequency of collision, the faster the reaction

  4. Reactant Product If at time t1 the concentrations are A1 M; B= B1 M If at a later time t2 the concentrations are A1 M; B= B1 M Over a specific period of time Dt (t2-t1), the average rate of the reaction is Reaction Rates • Reaction rate is the change in concentration of reactants or products per unit time. Usually expressed in units of M/s. Let us consider a hypothetical reaction where • Rates of reactions can be determined by monitoring the change in concentration of either reactants or products as a function of time.

  5. Reactant Product An Example For the time interval between t1 = 0 sec and t2 = 20 sec For the time interval between t1 = 20 sec and t2 = 40 sec Practice Exercise: calculate the average rate of the above reaction over the time interval between 0 and 40 second.

  6. [C4H9Cl] t Change of Rate with Time Consider the following actual reaction, C4H9Cl(aq) + H2O(l) C4H9OH(aq) + HCl(aq) Butyl chloride Butyl alcohol The average rate of the reaction over each interval is the change in concentration divided by the change in time: In this reaction, the concentration of butyl chloride, C4H9Cl, was measured at various times. Average rate = -

  7. Change of Rate with Time C4H9Cl(aq) + H2O(l) C4H9OH(aq) + HCl(aq) • Note that the average rate decreases as the reaction proceeds. • This is because as the reaction goes forward, concentrations of reactants decrease and therefore there are fewer collisions between reactant molecules.

  8. Change of Rate with Time C4H9Cl(aq) + H2O(l) C4H9OH(aq) + HCl(aq) • The slope of a line tangent to the curve at any point is the instantaneous rate at that time. • All reactions slow down over time. Therefore, the best indicator of the rate of a reaction is the instantaneous rate at the beginning, i.e. the initial rate. What is the instantaneous rate at t = 600 s? In what follows, the term “rate” means “instantaneous rate”, unless indicated otherwise What is the instantaneous rate at t = 0 s?

  9. 1 2 [HI] t Rate = − = [I2] t -[C4H9Cl] t Rate = = [C4H9OH] t Reaction Rates and Stoichiometry C4H9Cl(aq) + H2O(l) C4H9OH(aq) + HCl(aq) • In this reaction, the stoichiometric ratio of C4H9Cl to C4H9OH is 1:1. • Thus, the rate of disappearance of C4H9Cl is the same as the rate of appearance of C4H9OH. What if the stoichiometric ratio is not 1:1? 2 HI(g) H2(g) + I2(g) For example 2 moles 1 mole 1 mole Rate of disappearance of HI = twice the rate of production of I2 or H2 Therefore,

  10. aA + bB cC + dD = = Rate = − = − 1 b 1 d 1 a 1 c [A] t [B] t [C] t [D] t Reaction Rates and Stoichiometry • To generalize, then, for the reaction

  11. Answer (b) Solving the equation from part (a) for the rate at which O3 disappears, –[O3]t we have: Check: We can directly apply a stoichiometric factor to convert the O2 formation rate to the rate at which the O3 disappears: SAMPLE EXERCISE: Relating Rates at Which Products Appear and Reactants Disappear (a) How is the rate at which ozone disappears related to the rate at which oxygen appears in the reaction (b) If the rate at which O2 appears, [O2]t, is 6.0  10–5M/s at a particular instant, at what rate is O3 disappearing at this same time, –[O3]t?

  12. PRACTICE EXERCISE The decomposition of N2O5 proceeds according to the following equation: If the rate of decomposition of N2O5 at a particular instant in a reaction vessel is 4.2  10–7 M/s, what is the rate of appearance of (a) NO2, (b) O2? Answers:(a) 8.4  10 –7M/s,(b) 2.1  10 –7M/s

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