Rates of reaction

• Sample question

  • In a chemical reaction, 20 grams of product was formed in 40 seconds. Calculate the mean rate of reaction
• In a chemical reaction, you can also measure the quantity of reactant used
• Calculating the mean rate of a chemical reaction

  Mean rate of reaction = Quantity of product formed / Time taken
• Sample question

  • 150 grams of reactant was used in 75 seconds. Calculate the mean rate of reaction
• If the product is a gas, measure the quantity in centimeters cubed
• Drawing and interpreting graphs of the quantity of reactants or products in a chemical reaction


  Plot the quantity of product formed against time to get a graph. The slope of the line indicates the rate of reaction. A steeper slope means a faster reaction. Initially, the reaction is fast, then slows down as reactant molecules turn into product. Eventually, the slope becomes zero, indicating the reaction has stopped.
• Sample question

  • 30 grams of reactant was used in 10 seconds. Calculate the mean rate of reaction
• Sample question

  • In a chemical reaction, 60 grams of product was formed in 15 seconds. Calculate the mean rate of reaction
• If we measure the quantity of product formed by a reaction over time, we get a graph
• The slope of the tangent gives us an idea of the rate of reaction at that point
• Scientists draw tangents to overcome the problem of changing slopes to measure the rate of reaction at specific points
• Initially, the reaction is extremely fast, then slows down, and finally stops
• Practice questions on calculating the rate of reaction using tangents can be found in the revision workbook
• Calculating the rate of reaction using a tangent involves constructing a triangle with the tangent
• Measuring rate of reaction using tangents

  1. Draw tangents on the curve of the rate of reaction graph

  2. Use the slope of the tangent as a measure of the rate of reaction

  3. Calculate the gradient of a tangent as a measure of the rate of reaction at a specific point
• The slope of the graph gives us an idea of the rate of the reaction
• The slope of the curve constantly changing indicates that the rate is also constantly changing
• The tangent is a straight line that touches the curve at a specific point
• The rate of reaction is calculated by dividing the vertical side of the triangle by the horizontal side
• The slope of the tangent can be used to calculate the rate of reaction at a specific point