Rates of reaction

• The reaction rate is the change in the amount of reactants or products per unit time (normally per second)

• The units depend on what is being measured. For example, if the reactants are in solution, the rate will be the change in concentration per unit time (moldm^-3s^-1)

Following the rate of reaction

• Continuous monitoring is the technique of measuring how the amount of a reactant or product changes over the complete course of the reaction

• Continuous monitoring techniques include:

  • pH measurement

    • If one of the reactants or products is an acid or a base, the pH of the reaction mixture can be monitored using a pH probe.

    • The pH data can be converted into [H⁺] using the equation [H⁺] = 10^-pH

    • The units for rate will be concentration time^-1

  • Gas volume

    • If a gas is given off, it can be collected into a gas syringe, and the volume of gas in the syringe can be recorded at regular time intervals

    • The units for rate will be volume time^-1

  • Loss of mass

    • If a gas is given off, the system will lose mass, The mass can be recorded at regular intervals by using a balance.

    • The units for rate will be mass time^-1

  • Colour change

    • If one of the reactants or products are coloured, then colour change during the reaction can be monitored using a colorimeter.

    • A colorimeter measure the absorbance of a solution, so the more concentrated the colour of the solution, the higher the absorbance.

    • Absorbance data can be converted into concentrations using calibration curves

  • Titration

    • The concentration of a reactant or product in solution can be monitored by taking small samples of the reaction mixture at regular intervals and titrating them using a suitable titrant

    • When a sample of the reaction has been taken, the reaction will continue so it must be stopped otherwise the concentration will continue to change.

    • This can be achieved by quenching; adding a chemical which stops the reaction, or decreasing the temperature of the sample a lot to significantly slow the reaction down.

Concentration-time graphs

• The rate of reaction is given by the gradient at any point on the graph

• The gradient can be calculated by drawing a tangent

• Where the gradient is 0 and the concentration no longer changes is when the reaction has finished