5.2.7 Rate-Concentration Graphs

Order of reaction from initial rate

• The progress of the reaction can be followed by measuring the initial rates of the reaction using various initial concentrations of each reactant
• These rates can then be plotted against time in a rate-time graph
• In a zero-order reaction the rate doesn’t depend on the concentration of the reactant
  • The rate of the reaction therefore remains constant throughout the reaction
  • The graph is a horizontal line
  • The rate equation is rate = k

Rate-concentration graph of a zero-order reaction

• In a first-order reaction the rate is directly proportional to the concentration of a reactant
  • The rate of the reaction increases as the concentration of the reactant increases
  • This means that the rate of the reaction decreases as the concentration of the reactant decreases when it gets used up during the reaction
  • The graph is a straight line
  • The rate equation is rate = k [A]

Rate-concentration graph of a first-order reaction

• In a second-order reaction, the rate is directly proportional to the square of concentration of a reactant
  • The rate of the reaction increases more as the concentration of the reactant increases
  • This means that the rate of the reaction decreases more as the concentration of the reactant decreases when it gets used up during the reaction
  • The graph is a curved line
  • The rate equation is rate = k [A]²

Rate-concentration graphs of a second-order reaction