Measuring R of R

• 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?

  240 cm³ per minute
• Why does the average rate of reaction calculated over four minutes not reflect the actual reaction rate?

  Because the reaction is fastest at the beginning and slows down as it progresses
• What would happen to the graph of hydrogen production as magnesium is used up?

  The graph would plateau once all of the magnesium has been used up
• What are the two main tasks you could be asked to do with the reaction graph?

  1. Calculate the mean rate of the reaction over a certain period
  2. Calculate the actual rate at a particular time
• How do you find the mean rate of reaction in the first three minutes from the graph?

  By finding the amount of hydrogen produced in those three minutes and dividing by 180 seconds
• What is the mean rate of reaction in the first three minutes if 1200 cm³ of hydrogen is produced?

  1. 67 cm³ per second
• How do you calculate the rate of reaction at a specific time, such as two minutes?

  By calculating the gradient of the tangent to the curve at that point
• How do you find the gradient of the tangent line?

  By calculating the change in volume of hydrogen divided by the change in time
• What is a tangent in the context of the reaction graph?

  A straight line that touches the curve and has the same gradient at that point
• What is the formula for calculating the rate of reaction using the gradient?

  Rate = $\frac{\Delta y}{\Delta x}$
• 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 do if your tangent is not perfectly accurate?

  Don't worry, examiners allow answers within a small range of values
• 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 y over the change in x
• 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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