Concentration - time graphs

    Cards (15)

    • 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