Rates

• Average rate of reaction

  Total amount of Product produced / Total Time of reaction
• Instantaneous rate of reaction


  Slope of the tangent to the graph at a particular time
• To calculate the instantaneous rate at time t

  1. Construct the tangent to the graph at time t

  2. Ensure the point of contact of the tangent is approximately the midpoint of the tangent

  3. Construct a right angled triangle with the tangent as the hypotenuse

  4. Slope of the tangent = Opposite / Adjacent
• The rate of reaction is continually changing with time
• Types of rate of reaction

  • Average rate
  • Instantaneous rate
• Average rate of reaction

  The overall averaged out rate over the entire time of the reaction
• Instantaneous rate of reaction


  The rate of reaction at a particular time
• The instantaneous rate of reaction is usually calculated for earlier times of the reaction as this is when the rate of reaction is changing most rapidly
• As the reaction proceeds

  The number of collisions decreases, causing the reaction rate to slow down
• When all the reactant molecules have been used up


  There are no more collisions and the reaction ceases
• Slope of a graph


  Can be used to show the rate of change in a chemical reaction
• All rate of reaction graphs have the same overall shape - up and over if measuring product formed or down and over if measuring reactant used
• Rate of reaction graphs

  • The steeper the graph, the faster the reaction takes place
  • The higher the graph, the more product is produced
• When the graph first starts to flatten out is the time that the reaction is complete or has stopped
• Maintaining the temperature gives more particles an energy greater than the activation energy, so more reactants are able to overcome the energy barrier and form products
• A rise in temperature can lead to a large increase in rate
• To monitor the rate of production of oxygen from hydrogen peroxide using manganese dioxide as a catalyst

  1. Quickly add the MnO2 to the H2O2 in the conical flask

  2. Immediately insert the rubber stopper into the flask, while simultaneously starting the stopwatch

  3. It is vital that all of the reactants are added to each other at the start of the reaction
• The results of the experiment will be plotted on a graph which will have an up and over shape
• Initially the graph has a steep slope as gas is coming off at a steady rate, then the graph starts to level off as the rate of reaction slows down, and finally the graph has no slope as no more gas is given off and the reaction has stopped