How to Calculate the Rate of Reaction - Measuring Rate of Reaction

Measuring Reaction Rates with Graphs

In this video, we explore how graphs can be used to measure the mean rate of a reaction and the rate of reaction at specific times. We previously learned that the rate of reaction can be calculated by dividing the change in amount of reactants or products by the time taken for that change.

Example of Rate Calculation

• For a reaction between magnesium and hydrochloric acid that produced 1200 cm³ of hydrogen in 4 minutes:

  • Average rate = 1200 cm³ / (4 min * 60 seconds) = 5 cm³/s

• This calculation reflects the average rate over the four-minute period, which may not accurately represent the changes in reaction rate throughout the reaction.

Using Graphs to Monitor Reaction Rates

• If we plot the volume of hydrogen produced against time on a graph:

  • X-axis: time

  • Y-axis: volume of hydrogen produced

• The graph starts with a steep curve indicating a high rate of reaction due to the abundance of reactants. As magnesium is consumed, the curve flattens when the reaction slows down, eventually plateauing when all magnesium is used.

Calculating Mean Rate from Graphs

• To calculate the mean rate over a specific period, such as the first three minutes, locate that point on the graph, trace up to the curve, and read off the corresponding volume on the Y-axis. For example:

  • At three minutes, 1200 cm³ of hydrogen is produced.

  • Mean rate = 1200 cm³ / (3 min * 60 seconds) = 6.67 cm³/s

Calculating Instantaneous Rate at a Specific Time

• To find the rate at a specific moment, such as at two minutes:

  • Trace to the curve to find the corresponding volume at two minutes, then draw a tangent at that point.

  • Calculate the gradient of the tangent line: Change in volume / Change in time.

  • Example: The volume change is 600 cm³, time change is approximately 170 seconds, yielding a rate of 600 / 170 = 3.53 cm³/s.

• Accuracy is flexible; minor deviations in drawing tangents are acceptable in exams.

Alternative Graphs

• The same method can be used to graph the remaining amount of magnesium against time:

  • Example: If starting with 1.2 grams, the graph shows a rapid decline initially, followed by a slower decrease.

  • To find the rate at one minute, repeat the tangent method, estimating changes from the graph to calculate the rate (e.g., 0.72 grams over 100 seconds leads to 0.0072 grams/s).

Conclusion

• Understanding reaction rates through graphical representation allows for better insight into the dynamics of chemical reactions. Practice utilizing these methods to gain proficiency.