Rate of reaction

• To draw and interpret graphs of the quantity of reactants or products in a chemical reaction:

  • Plot the quantity of product formed against time
  • The slope of the line gives an idea of the rate of reaction
  • A steeper slope indicates a faster reaction
  • Initially, the reaction is fast as a lot of product is formed in a short time due to a large number of reactant molecules
  • As the reaction progresses, the slope becomes less steep, indicating a slower reaction as fewer reactant molecules are available to react
  • When the slope is zero, the reaction has stopped as all reactant molecules have reacted
• In a chemical reaction, if the product is a gas, the quantity is measured in centimeters cubed
• To calculate the mean rate of a chemical reaction:

  • Use the formula: Mean rate of reaction = Quantity of product formed / Time taken
  • Example: If 60 grams of product was formed in 15 seconds, the mean rate of reaction is 4 grams per second
• Sample question:

  • 20 grams of product was formed in 40 seconds
  • Calculate the mean rate of reaction
  • Answer: 0.5 grams per second
• To calculate the mean rate of reaction from the quantity of reactant used:

  • Use the formula: Mean rate of reaction = Quantity of reactant used / Time taken
• Sample question:

  • 30 grams of reactant was used in 10 seconds
  • Calculate the mean rate of reaction
  • Answer: 3 grams per second
• Sample question for practice:

  • 150 grams of reactant was used in 75 seconds
  • Calculate the mean rate of reaction
  • Answer: 2 grams per second
• To draw tangents on the curve of a rate of reaction graph:

  • Identify the point on the curve where you want to measure the rate of reaction
  • Draw a straight line that just touches the curve at that point
  • This straight line is called a tangent
• The slope of the tangent gives us an idea of the rate of reaction at that specific point
• Scientists draw tangents on the curve to overcome the problem of the slope of the curve constantly changing
• The slope of the tangent indicates the rate of reaction at a given point
• To calculate the rate of reaction using a tangent:

  • Construct a triangle using the tangent at the desired point
  • Measure the vertical and horizontal sides of the triangle from the y-axis and x-axis respectively
  • Divide the vertical side value by the horizontal side value to calculate the rate of reaction
• In the example provided, the rate of reaction at 30 seconds was calculated by dividing 14 grams (vertical side) by 25 seconds (horizontal side), resulting in a rate of 0.56 grams per second
• The effect of the concentration of reactants or the pressure of reactant gases on the rate of a chemical reaction is crucial
• Collision Theory states that chemical reactions occur when reacting particles collide with sufficient energy
• The rate of a chemical reaction is determined by the frequency of successful collisions
• In a reaction, the rate is proportional to the concentration of reactants
• Higher concentration leads to a faster reaction rate due to more collisions per second
• This principle also applies to reactions involving gases, where higher pressure results in a faster reaction rate
• A practical investigation can be conducted to study the effect of concentration on the rate of a reaction
• Questions on the effect of concentration on the rate of reaction can be found in revision workbooks
• Rate of reaction=speed reactants become products