Chm1205 (3)

• Chemical Kinetics


  • Investigates how different experimental conditions can influence the speed of a chemical reaction
  • Yields information about the reaction's mechanism and transition states
  • Can lead to the construction of mathematical models that can describe the characteristics of a chemical reaction

  
• Rate of reaction


  The change in concentration of a product or reactant per unit time
• Arrhenius Equation


  A simple, but remarkably accurate, formula for the temperature dependence of the rate constant, and therefore, rate of a chemical reaction

  
• Experimental methods to determine rate of reaction

  • Gas collection method
  • Colorimetric method
  • Precipitation (disappearing cross) method
  • Titrimetric method
• Arrhenius Equation


  • First proposed in 1884 by the Dutch chemist J. H. van't Hoff
  • In 1889, the Swedish chemist Svante Arrhenius provided a physical justification and interpretation for it
• Factors affecting Rate of Reaction

  • Concentration
  • Temperature
  • Catalysts
  • Pressure
  • Surface area
• Rate Constant


  Frequency Factor (A) is dependent on: Frequency of collisions, Orientation of particles
• Collision Theory


  Based on kinetic theory and supposes that particles must collide with both the correct orientation and with sufficient kinetic energy if the reactants are to be converted into products
• Ea/RT


  Equal to the fraction of molecules with energy necessary for a reaction to occur

  
• Transition State Theory


  Supposes that as reactants approach each other a transitory activated complex (transition state) is formed at a potential energy maximum

  
• Integral form of the Rate Law for a first-order reaction

  1. Rate = -∆[A]/∆t = k[A]1

  2. ∫t0-d[A]/dt = ∫t0k[A]1

  3. -ln[A]t – (-ln[A]0) = k(t) – k(0)

  4. ln[A]t = –k(t) + ln[A]0
• Increase in temperature


  Leads to an increase in rate of reaction
• Activation energy


  The minimum energy required for a reaction to occur
• Reaction rates


  Directly proportional to energy, collisions, temperature and orientation

  
• As the concentration of the reactants increases


  The frequency of the molecules colliding increases, the probability of successful collisions occurring increases and thus the reaction rate increases
• Integral form of the Rate Law for a second-order reaction

  1. initial rate = -d[A]/dt = k[A]2

  2. 1/[A]t = k(t) + 1/[A]0
• Molecules at a higher temperature

  Have more average kinetic energy, and the proportion of reactant molecules with sufficient energy to react (energy greater than activation energy: E > Ea) is significantly greater thus there are more successful collisions and a faster reaction rate

  
• Using Arrhenius Equation to determine rate constant at a given temperature


  1. Assume reaction carried out at two temperatures T1 and T2 with rate constants k1 and k2

  2. ln k1 = ln A - (Ea/R)(1/T1)

  3. ln k2 = ln A - (Ea/R)(1/T2)

  4. Subtract to get ln(k1/k2) = (Ea/R)(1/T2 - 1/T1)

  
• Half-life


  The time taken for the concentration of a reactant to decrease to half its original concentration
• Rate Law


  An expression relating the rate of a reaction to the rate constant and the initial concentration of the reactants

  
• Catalyst


  A substance that accelerates the rate of a chemical reaction but remains chemically unchanged afterwards

  
• Increasing the pressure in a gaseous reaction

  Increases the frequency of collisions between reactant particles, thus giving a corresponding increase in the frequency of successful collisions and thus an increase in the rate of reaction
• Half-life for a second order reaction

  t1/2 = 1/k[A]0
• Reactants in the solid state

  React faster when they are sub-divided into smaller particles, resulting in a greater surface area per unit volume, and an increase in the contact made with the other reactant particle, thus the faster is the reaction
• Determining the order of a reactant using the Initial Rate Method

  1. Hold [A] constant, vary [B] and observe how rate changes

  2. Hold [B] constant, vary [A] and observe how rate changes

  
• Reaction Mechanism


  A step by step sequence of elementary reactions (elementary steps) that explains how the overall reaction proceeds
• Reaction Mechanisms


  • Must account for all species in a reaction
  • Must explain a rate law
  • Account for intermediates (when observed)
• Intermediate


  A substance that is formed in one step and used up in another
• Determining the order of a reactant using the Isolation Method


  1. If [A] is constant and doubling [B] doubles the rate, the order w.r.t B is 1

  2. If [B] is constant and doubling [A] quadruples the rate, the order w.r.t A is 2

  
• Rate determining step


  The slowest step in a chemical reaction
• For a zero order reaction, if concentration increases two-fold, rate does not increase
• Rate Determining Step and its use in determining rate law

  1. Since the reaction rate depends on the rate determining step, we can use reaction mechanisms as a replacement of the isolation method to determine the rate law of a reaction

  2. The reaction order for any single elementary step is equal to the coefficients for that step
• For a first order reaction, if concentration increases two-fold, rate increases two times
• For a second order reaction, if concentration increases two-fold, rate increases four times
• Formulating a reaction mechanism

  1. Measure the rate of reaction

  2. Formulate the rate law

  3. Determine any possible intermediates that may be formed during the conversion of reactants to products