How to calculate the rate of reaction

• What is the purpose of using graphs in the video?

  To measure the mean rate of a reaction and the rate of reaction at a specific time
• How is the rate of reaction calculated?

  By dividing the amount of reactants used or the amount of product formed by the time taken for that change
• If 1200 cm³ of hydrogen is produced in 4 minutes, what is the rate of reaction in cm³ per second?

  240 cm³ per second
• Why does the average rate of reaction calculated over a period not reflect the actual rate throughout the reaction?

  Because the reaction is fastest at the beginning and slows down as it progresses
• What happens to the graph of hydrogen produced over time as magnesium is used up?

  The graph shows a steep curve initially, indicating a high rate of reaction, which then plateaus as magnesium is consumed
• What are the two main tasks you could be asked to do with a reaction graph?

  1. Calculate the mean rate of reaction over a certain period
  2. Calculate the actual rate of reaction at a specific time
• How do you calculate the mean rate of reaction over the first three minutes using a graph?

  By finding the volume of hydrogen produced in the first three minutes and dividing it by 180 seconds
• What is the method to find the actual rate of reaction at a specific time on the graph?

  By calculating the gradient of the tangent drawn at that point on the curve
• What is a tangent in the context of a reaction graph?

  A straight line that touches the curve at a point and has the same gradient as the curve at that point
• How do you find the gradient of the tangent line on the graph?

  By calculating the change in volume of hydrogen divided by the change in time
• What values are used to calculate the rate of reaction at two minutes?

  The change in volume of hydrogen and the change in time from the tangent line
• If the change in volume of hydrogen is 600 cm³ and the change in time is 170 seconds, what is the rate of reaction?

  1. 53 cm³ per second
• What should you keep in mind when drawing tangents for exam purposes?

  Examiners allow answers within a small range of values, so precision is not critical
• How would a graph plotting the amount of magnesium remaining against time appear?

  • Starts with the initial amount of magnesium used
  • Declines rapidly at first
  • Slows down as the reaction progresses
• How do you find the rate of reaction at one minute using the magnesium graph?

  By drawing a tangent at one minute and calculating the change in magnesium over the change in time
• If the change in magnesium is 0.72 grams and the change in time is 100 seconds, what is the rate of reaction?

  0.0072 grams per second
• What happens if you make your tangent longer or shorter when calculating the rate?

  You will still get the same answer as long as the tangent is steep enough
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