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5-3-2011

5-3-2011. Chemical Kinetics. Chemical Kinetics.

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5-3-2011

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  1. 5-3-2011

  2. Chemical Kinetics

  3. Chemical Kinetics The science studying the rate of chemical reactions is referred to as chemical kinetics. We will see later that thermodynamics tells about the spontaneity of chemical reactions but not their speeds. Therefore, both chemical kinetics and thermodynamics are essential to describe a chemical reaction.

  4. There are some factors that influence the speed of a chemical reaction including: • The nature of reactants and products where some reactions are fast while others are slower • The concentration of reactants where higher concentrations result in faster reactions due to the fact that it will be easy for molecules of reactants to collide resulting in products • The temperature is an important factor affecting reaction speed where usually higher temperatures result in faster reactions • The presence of a catalyst

  5. Reaction Rates and Their Measurement The rate of reaction can be defined as the change in concentration per unit time where: Rate = D Concentration (mol L-1)/time (s) For instance, in the reaction: A g B

  6. We may write: Rate of reaction is either rate of disappearance (consumption) of A or rate of appearance of B: Rate = - D[A]/Dt = D[B]/Dt The minus sign is because [A] is decreasing with time.

  7. From the graphics above • A decreases with time, but at the same time B increases with time • The rate of disappearance of A (or appearance of B) does not appear to be linear with time • The reaction appears to be much faster at the beginning and will slow down with time

  8. Reaction Rates Consider the chemical reaction: AB t = 40. min 2.0 mol A 8.0 mol B t = 20. min 5.0 mol A 5.0 mol B Time = 0. 10. mol A

  9. Reaction Rates • The average rate of appearance of B during the second 20 minutes of the reaction: Avg. rate = 8.0 mol B – 5.0 mol B 40. min – 20. min = 0.15 mol/min Notice that the average rate of reaction decreases over the course of the reaction.

  10. Reaction Rates Example:Given the following data, what is the average rate of the following reaction over the time interval from 54.0 min to 215.0 min? CH3OH (aq) + HCl (aq) CH3Cl(aq) + H2O (l) Time (min) [HCl] (M) 0.0 1.85 54.0 1.58 107.0 1.36 215.0 1.02

  11. Given: [HCl]54 min = 1.58 M [HCl]215 min = 1.02 M Find: avg. rate of disappearance of HCl Avg. rate = - D [HCl] D t = - (1.02 M - 1.58 M) 215 min - 54 min = 0.0035 M / min

  12. When a complex reaction is considered, the rates of disappearance of reactants and appearance of products are not equal. Look at the following reaction: Hydrogen is disappearing 3 times faster than nitrogen while ammonia is formed twice as fast as nitrogen disappears

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