Concentration - time graphs

• Continuous monitoring of rate


  Concentration-time graphs can be plotted from continuous measurements taken during the course of a reaction
• Monitoring rate with a colorimeter

  1. Prepare standard solutions of known concentration

  2. Select a filter with the complementary colour

  3. Calibrate the colorimeter with water

  4. Measure the absorbance readings of the standard solutions

  5. Plot a calibration curve

  6. Carry out the reaction and take absorbance readings

  7. Use the calibration curve to measure the concentration
• 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 of the graph is the rate of the reaction
  • The order with respect to a reactant can be deduced from the shape of the graph
• Zero order reaction


  • Produces a straight line with a negative gradient
  • The reaction rate does not change at all during the course of the reaction
  • The value of 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 of the reactant to halve is constant, this is called the half-life
  • The rate constant can be determined using the half-life value
• Second order reaction

  The graph is also 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
• Decomposition of dinitrogen pentoxide

  • 2N2O5(g) → 4NO2(g) + O2(g)
• Determination of k for a first order reaction

  1. Calculate the rate constant from the rate using the rate equation

  2. Calculate the rate constant from the half-life using the exponential relationship
• Drug breakdown in the body is often affected by exponential decay
• Asthma medicines

  • Salbutamol: 1.6 hour half-life
  • Salmeterol: 5.5 hour half-life
• Doctors advise using salbutamol for asthma attacks and salmeterol late at night before sleeping


  Due to the different half-lives