Rate constants and temperature

• Rate constant


  A measure of the rate of a chemical reaction
• As temperature increases


  The rate and rate constant increase
• For many reactions, each 10°C increase in temperature doubles the rate constant and doubles the rate of the reaction
• Factors affecting the rate constant


  • Increasing the temperature shifts the Boltzmann distribution to the right, increasing the proportion of particles that exceed the activation energy E
  • As the temperature increases, particles move faster and collide more frequently
• The change in rate is mainly determined by the activation energy E
• Arrhenius equation


  An exponential relationship between the rate constant k and temperature T
• Pre-exponential factor (frequency factor) A


  Takes into account the frequency of collisions with the correct orientation
• Exponential factor


  Linked to activation energy and temperature
• Determining Ea and A graphically


  1. Plot ln k against 1/T

  2. Gradient is -Ea/R

  3. Intercept is ln A
• The Arrhenius equation can be expressed in logarithmic form: ln k = -Ea/RT + ln A
• Reaction


  • Hydrogen peroxide decomposing to form water and oxygen
• Rate constant k varies with temperature for the hydrogen peroxide decomposition reaction
• The activation energy Ea and frequency factor A can be calculated from the Arrhenius equation using the data in Table 1