48. Measuring Rates of Reaction from Graphs
Measuring Rates of Reaction from Graphs
Based on the video, here are the notes on how to use graphs to calculate both the average (mean) rate and the specific rate at any given moment in a reaction.
1. Calculating the Mean Rate
The mean rate gives the average speed of the reaction over a set period.
How to find it:
Identify the time period on the x-axis.
Trace up from the time to the curve, then across to the y-axis to find the quantity of reactant used or product formed.
Use the formula: Mean Rate = Total Quantity / Time.
Example: If 1200 cm³ of gas is produced in 3 minutes (180 seconds), the mean rate is 1200 / 180 = 6.67 cm³/s.
2. Calculating the Rate at a Specific Time
Because reactions change speed—usually starting fast and slowing down—you may need to find the rate at one exact moment.
The Method: Drawing a Tangent
Find the specific time on the x-axis and trace up to the curve.
Draw a tangent—a straight line that just touches the curve at that exact point and has the same steepness as the curve at that point.
Calculate the gradient (slope) of that tangent line.
Formula for Gradient: Rate = (Change in y) / (Change in x).
Tip: Make the tangent line as long as possible (extending it to the axes) to make the measurement easier and more accurate.
3. Different Graph Types
Volume of Product vs. Time: The curve goes up. The rate is the change in volume divided by the change in time.
Mass of Reactant vs. Time: The curve goes down. The rate is the change in mass divided by the change in time. Even though the mass is decreasing, the rate is typically expressed as a positive value.
4. Exam Tips
Time Conversion: Always check the units required. If the question asks for the rate in "per second," remember to convert minutes into seconds (minutes × 60).
Tangent Accuracy: Examiners understand that tangents are drawn by eye. They provide a "range" of acceptable values in the mark scheme, so as long as your line is a good fit, you will get the marks.