Chemistry · MU Sofia

3.1. Chemical kinetics.

Covers the definition of chemical kinetics and how reaction rate is defined, both average and instantaneous. It then explains collision theory and the rate law, the rate constant k, and reaction order with zero, first and second order examples. Molecularity, the rate-determining step with an N2O5 decomposition example, and the effect of temperature via the Arrhenius equation and Van't Hoff rule follow. The note closes with other factors such as catalysts, pressure, solvent, and surface area, plus a list of biological significance and high-yield points.

Inside: 18 sections · 10 labelled pictures · 25 flashcards · 10 min

Topic 8 of 42 in the MU Sofia syllabus for Chemistry (Medicine, Year 1). The free AceMed note covers it in 18 sections, about 1,407 words (10 min of reading), with 25 flashcards.

In this note

  1. Chemical Kinetics
  2. Concentration dependence on the reaction rate
  3. Kinetic equation. Rate constant.
  4. Reaction order. Zero, first and second order reactions.
  5. Molecularity of the reaction.
  6. Rate - determining step
  7. Temperature dependence on the rate of a reaction :
  8. Arrhenius equation
  9. Other main factors that can affect the reaction rate of a chemical reaction and Biological significance :
  10. Biological Significance:
  11. High-Yield Points (Kinetics)
  12. Arrhenius Equation
  13. Van’t Hoff Rule (Temperature Coefficient Q₁₀)
  14. Activation Energy – Definition
  15. Catalyst Effects
  16. Pressure & Concentration Effects
  17. Diffusion-Controlled Reactions
  18. Biological Significance

Chemical Kinetics

  • Definition: Chemical kinetics is a branch of chemistry that studies the rate and mechanism of chemical reactions and the factors influencing reaction rates.

Rate of chemical reaction

  • Rate of Chemical Reactions
  • Rate Definition: The rate of a chemical reaction is the number of elementary acts occurring per unit time per unit volume (for homogeneous reactions) or per unit surface area (for heterogeneous reactions).
  • Expressing Rate: It's expressed as the change in concentration of reactants or products per unit time per unit volume
  • (or area) of the system.
  • Average Rate: Given by the change in concentration of any substance over a specified time interval.
  • Instantaneous Rate: Represents the rate at a specific moment and is expressed by a differential equation.
  • Different Substances in a Reaction: Rates of substances with different stoichiometric coefficients are related to 1 mole of substance.
  • Example:
  • For the reaction: $aA+bB→yY+zZ$ The rate is given by:

Concentration dependence on the reaction rate

  • Collision Theory:
  • Chemical reactions occur when particles of reactants collide.
  • Not all collisions lead to a reaction; only effective collisions trigger product formation.
  • These effective collisions are called elementary acts of the reaction.
  • Concentration and Collision Frequency:
  • More concentrated solutions have more particles per unit volume.
  • This leads to a higher frequency of collisions between reactant particles.
  • Increased collision frequency directly translates to a higher probability of effective collisions.
  • Rate Proportional to Concentration Product:
  • The rate of the reaction (number of effective collisions per unit time) is proportional to the product of the molar concentrations of the reactants.
  • This relationship is often expressed mathematically through the rate law equation, which incorporates the reaction's stoichiometry and specific rate constant.
  • The provided graph illustrates how the number of collisions increases with increasing concentration.

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