Rate - concentration graphs and initial rates

• Concentration-time graphs


  Graphs that can be plotted from continuous measurements taken during the course of a reaction
• Specification reference: 5.1.1
• Continuous monitoring of rate


  Measuring a property that changes over time, such as colour change, to monitor the progress of a reaction
• Monitoring rate with a colorimeter


  1. Select wavelength of light

  2. Measure absorbance of solution

  3. Relate absorbance to concentration
• Colorimeter


  • Measures the intensity of light passing through a sample
  • Wavelength of light is controlled using a filter
  • Measures the absorbance of the solution
• Analysing by colorimetry


  1. Prepare standard solutions

  2. Select complementary colour filter

  3. Dilute samples with water

  4. Measure absorbance of standards

  5. Plot calibration curve

  6. Use calibration curve to determine concentration
• The reaction between propanone and iodine, in the presence of an acid catalyst, produces an orange/brown colour that fades as the reaction proceeds
• As the reaction proceeds


  Iodine is used up and its orange/brown colour fades
• The absorbance of the colour is measured precisely by the colorimeter
• Concentration-time graphs


  • The gradient is the rate of the reaction
  • The shape can be used to deduce the order with respect to a reactant
• Zero order reaction


  • Produces a straight line with a negative gradient
  • The reaction rate does not change during the course of the reaction
  • The gradient is equal to the rate constant
• First order reaction


  • Produces a downward curve with a decreasing gradient over time
  • The time for the concentration to halve is constant (half-life)
  • The rate constant can be determined using the half-life
• Second order reaction


  Produces a downward curve, steeper at the start but tailing off more slowly
• Half-life


  The time taken for half of a reactant to be used up
• First order reactions have a constant half-life with the concentration halving every half life
• The decomposition of dinitrogen pentoxide to nitrogen dioxide and oxygen is a first order reaction
• Determination of the rate constant for a first order reaction


  1. Calculate from the rate (gradient of tangent)

  2. Calculate from the half-life
• Many drugs break down in the body by exponential decay
• Drugs used to combat asthma


  • Salbutamol (half-life 1.6 hours)
  • Salmeterol (half-life 5.5 hours)
• Doctors advise using salbutamol for asthma attacks and salmeterol late at night before sleeping
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• Exponential decay


  Drug breakdown in the body is affected by many different factors, but many drugs break down by exponential decay
• Half-life


  The average half-lives of two medicines used to combat asthma are salbutamol: 1.6 hours, salmeterol: 5.5 hours
• Salbutamol


  Used in inhalers to combat asthma
• Doctors will advise that these drugs should be used differently-salbutamol for asthma attacks and salmeterol late at night before sleeping
• Rate-concentration graphs can be plotted from measurements of the rate of reaction at different concentrations
• Rate-concentration graphs


  • They offer a route into the direct link between rate and concentration in the rate equation