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Chem 2 - Chemical Kinetics I: Introduction and Factors Affecting Reaction Rates

Chem 2 - Chemical Kinetics I: Introduction and Factors Affecting Reaction Rates

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Chem 2 - Chemical Kinetics I: Introduction and Factors Affecting Reaction Rates

  1. 1. Chemical Kinetics (Pt. 1) Introduction and Factors Affecting Reaction Rates By Shawn P. Shields, Ph.D. This work is licensed by Shawn P. Shields-Maxwell under a Creative Commons Attribution-NonCommercial-ShareAlike 4.0 International License.
  2. 2. What is Chemical Kinetics? Chemical kinetics is the study of chemical reaction rates. Kinetics can be more simply described as how the concentration of a given reactant or product changes in time during a chemical reaction.
  3. 3. What is Chemical Kinetics? In kinetics experiments, reaction conditions are changed in a systematic way. The effect on the rate of the reaction (i.e., speed of reaction) is measured. Reaction rates can be measured using a variety of spectroscopic methods.
  4. 4. Kinetics and Mechanism Kinetics provides a way to study complicated reactions and determine how they proceed. Once we know how a reaction proceeds, we may be able to learn to control it. Kinetic studies can also provide evidence and support for a theory about how a reaction works (its mechanism), but can never “prove” the mechanism.
  5. 5. Macroscopic vs Microscopic Aspects of Kinetics The macroscopic aspect involves how fast the reaction runs (overall). Rate laws are determined by experiment. Rate laws show the dependence on concentrations and/or partial pressures of reactants.
  6. 6. Macroscopic vs Microscopic Aspects of Kinetics The microscopic aspect of reactions involves how the reaction proceeds at the molecular level. A reaction mechanism is determined, which includes a series of elementary steps in converting reactants to products.
  7. 7. Kinetics and Mechanism Example: SN1 Reaction When t-butyl bromide is boiled in methanol, the reaction produces methyl-t-butyl ether. This looks like a very specific reaction, but it can actually be placed in a large category of reactions called “First-Order Nucleophilic Substitution” (or SN 1). t-butyl bromide methanol methyl-t-butyl ether
  8. 8. Step 1 (slow): Step 2 (fast): Step 3 (fast): Mechanism for SN1 Reaction
  9. 9. Collision Theory and Reaction Rate Collision theory offers the basic framework for understanding chemical kinetics. The basic assumption is that molecules must come into contact (collide) to react. Therefore, (forward) reaction rates are affected by several factors.
  10. 10. Factors Affecting Reaction Rates 1) Concentration of the reactants (higher concentration leads to a faster rate because there are more collisions) 2) Temperature (higher T leads to more collisions and with greater energy, which leads to a faster rate)
  11. 11. Factors Affecting Reaction Rates 3) The physical state of the reactants: Molecules must mix to react (solutions provide more opportunities for mixing) The amount of surface area on a solid available for reaction Nanoparticles have more surface area than larger particles. More surface area; faster rate
  12. 12. Factors Affecting Reaction Rates 4) The addition of a catalyst (usually involves a different mechanism of reaction) Trypsin Active Site by Fdardel (Own work) [CC BY-SA 3.0 (http://creativecommons.org/licenses/by-sa/3.0)], via Wikimedia Commons Enzymes, such as Trypsin, act as catalysts in chemical reactions.
  13. 13. Next up… Reaction rates and how they change with concentration (Pt. 2)

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