6.3 Measuring Rates of Reaction From a Graph

• What is the purpose of using graphs in measuring the rate of a reaction?

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

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

  $240 \text{ cm}^3/\text{s}$
• 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 information can be plotted on a graph to visualize the rate of reaction?

  Time on the x-axis and volume of hydrogen produced on the y-axis
• What shape does the graph of hydrogen production typically take at the beginning of a reaction?

  A steep curve indicating a high rate of reaction
• What happens to the graph as magnesium is used up in the reaction?

  The rate of hydrogen production slows down until the graph plateaus
• How do you calculate the mean rate of reaction over a specific time using a graph?

  By finding the volume of hydrogen produced during that time and dividing by the time
• How do you find the rate of reaction at a specific time on the graph?

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

  A straight line that touches the curve and has the same gradient at that point
• How do you calculate the gradient of the tangent line?

  By finding the change in y divided by the change in 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?

  $3.53 \text{ cm}^3/\text{s}$
• What should you do if your tangent line is not perfect when estimating the rate of reaction?

  It's acceptable as examiners allow a small range of values
• How would you find the rate of reaction using a graph of magnesium remaining against time?

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

  $0.0072 \text{ grams/s}$
• What happens to the graph of magnesium remaining over time?

  It starts with the initial amount and declines rapidly at first, then more slowly
• How does the length of the tangent affect the calculated rate of reaction?

  The length of the tangent does not affect the rate as long as it is steep enough