Module 5

• Reaction rate


  Measured by observing changes in quantities of reactants/products over time
• Rate


  Change in concentration divided by time, units = mol dm^-3 s^-1
• Rate for a particular reactant

  Rate = k[A]^n, where n is the order with respect to that reactant
• 0 order


  Changing concentration has no effect on overall rate
• 1st order


  If concentration doubles, overall rate doubles
• 2nd order


  If concentration doubles, overall rate quadruples
• Rate equation


  rate = k[A]^n[B]^m
• Rate constant k


  Proportionality constant
• Overall order


  Sum of orders with respect to each reactant
• Continuous monitoring


  1. Continuous measurements taken during the course of a reaction

  2. Can be used to plot concentration-time graph

  
• Colorimetry


  Wavelength of light passing through a coloured solution is controlled using a filter & the amount of light absorbed is measured
• Analysis by colorimetry


  1. Prepare standard solutions of known concentration of I2

  2. Select complementary colour filter

  3. Zero colorimeter with water

  4. Measure standard absorbance

  5. Plot calibration curve of absorbance against concentration

  6. Carry out propane and iodine reaction and take absorbance readings at timed intervals

  7. Use calibration curve to measure concentration

  8. Plot graph of concentration-time to determine order
• 0 order concentration-time graph

  Straight line with negative gradient, gradient equals k
• 1st order concentration-time graph

  Downward curve with decreasing gradient

  
• 2nd order concentration-time graph

  Downward curve steeper at start, tails off more slowly

  
• Half-life


  Time taken for half of a reactant to be used up
• 1st order reactions


  Half-life is constant, exponential decay
• Calculating k for 1st order reaction

  1. From rate (calculate gradient of tangent to concentration-time graph)

  2. From half-life (k=ln2/t1/2)
• 0 order rate-concentration graph

  Horizontal straight line, 0 gradient, y-intercept=k
• 1st order rate-concentration graph

  Straight line through origin, gradient=k

  
• 2nd order rate-concentration graph

  Upward curve, increasing gradient, plot graph of rate-concentration^2 (gradient=k)

  
• Initial rate


  Instantaneous rate at the start of a reaction when time=0, found by measuring gradient at t=0 on concentration-time graph

  
• Clock reaction


  Convenient way of obtaining initial rate with single measurement
• Iodine clock


  1. Several experiments with different concentrations of one reactant, all other reactant concentrations are kept constant

  2. Colour change is delayed by adding a small amount of another chemical (e.g. Na2S2O3) which removes iodine as it forms

  3. Measure time for blue-black colour to appear, initial rate is proportional to 1/t

  4. Plot graph of 1/t against concentration to determine order with respect to each reactant
• Accuracy of initial rate measurement is better for shorter time periods, as the less the rate changes the more accurate the measurement
• Rate-determining step


  Slowest step in a multi-step reaction sequence
• Reaction mechanism


  Series of steps that make up an overall reaction
• Hydrolysis of haloalkanes can be investigated experimentally to find a reaction mechanism
• Factors contributing to increased rate and rate constant with increasing temperature


  • Shifts Boltzmann distribution to right, increasing proportion of molecules exceeding activation energy
  • Particles move faster and collide more frequently
• With increasing temperature, increased frequency of collisions is comparatively small compared to increase in proportion of molecules exceeding activation energy, so change in rate is mostly due to activation energy
• Rate constant equation


  k=Ae^(-Ea/RT), where A is the pre-exponential factor and e^(-Ea/RT) is the exponential factor
• Determining Ea and A graphically

  1. Plot graph with x-axis=1/T, y-axis=lnk, use ln(k)= -Ea/RT + ln(A)

  2. Gradient=-Ea/R, y-intercept=lnA
• Homogeneous equilibria


  Contains equilibrium species all in the same state/phase
• Heterogeneous equilibria

  Contains equilibrium species that have different states/phases, Kc only uses gases or aqueous since concentration of solids/liquids is essentially constant
• Determining Kc experimentally


  1. Mix equal moles of carboxylic acid and alcohol with half moles of HCl as a catalyst

  2. Add same mole of HCl to another flask as a control

  3. Stopper flasks and leave for a week to reach equilibrium

  4. Carry out titration of mixture with a standard solution of NaOH and repeat with control to determine amount of acid catalyst used

  5. Determine equilibrium amounts of each component from equation

  6. Find concentrations and then find Kc
• Mole fraction


  The proportion by volume of a gas to the total volume of the gases in a mixture
• Partial pressure


  Contribution that a gas makes towards the total pressure
• Amount of oxygen that can dissolve in blood is proportional to partial pressure of oxygen breathed in
• Kp


  Equilibrium constant using partial pressures instead of concentrations
• Only gases used in Kp