Rate of reaction on a graph (AQA GCSE Chemistry): Revision Notes
Rate of reaction on a graph
What is a rate of reaction graph?
A rate of reaction graph shows how fast a chemical reaction happens. You can plot two things on these graphs:
- Amount of product made against time
- Amount of reactant used up against time
The gradient (steepness) of the line tells you the rate of reaction. A steeper line means a faster reaction, while a flatter line indicates a slower reaction.
Practical example - marble chips and acid
Scientists often measure the rate by collecting gas produced in a reaction.
Worked Example: Measuring Gas Production
The reaction: Marble chips + dilute hydrochloric acid → carbon dioxide gas + other products
Method:
- Put marble chips and acid in a flask
- Collect the CO₂ gas in a gas syringe
- Measure the volume of gas every minute
- Record your results in a table
Sample results:
| Time (min) | Volume (cm³) |
|---|---|
| 0 | 0 |
| 1 | 87 |
| 2 | 132 |
| 3 | 178 |
| 4 | 193 |
| 5 | 204 |
How to draw the graph properly
Follow these essential steps to create an accurate rate of reaction graph:
Step 1: Choose your axes
- x-axis: time (minutes)
- y-axis: volume of gas (cm³)
Step 2: Scale your axes
- Make sure your data points use at least half the graph paper
- This makes it easier to read
Step 3: Label your axes clearly
- Include units in brackets
- Example: "Time (min)" and "Volume (cm³)"
Step 4: Plot your points carefully
- Use a sharp pencil
- Mark each point clearly
Step 5: Draw the best line
- Draw a best-fit line or smooth curve
- Don't just join dot-to-dot
- Ignore anomalous points (ones that don't fit the pattern)
Common Mistake to Avoid: Never simply connect the dots! Always draw a smooth best-fit line that shows the overall trend, even if it doesn't pass through every single point.
How the rate changes during a reaction
Looking at the graph shape tells you about the reaction rate:
At the start:
- The line is very steep
- Rate is fastest here
- Lots of reactant particles available
During the reaction:
- The line becomes less steep
- Rate slows down
- Fewer reactant particles left
At the end:
- The line becomes flat (horizontal)
- Rate is zero
- All reactant used up
This pattern occurs because as reactants are consumed, there are fewer particles available to collide and react, causing the reaction rate to decrease over time.
Measuring rate from the graph
To find the exact rate at any point, you need to calculate the gradient:
- Draw a tangent (straight line touching the curve at one point)
- Calculate the gradient of this tangent line using:
- Gradient = rate of reaction
Worked Example: Comparing Rates
If the tangent has a gradient of 90 cm³/min at 2 minutes and 45 cm³/min at 4 minutes, then the reaction is twice as fast at 2 minutes compared to 4 minutes.
Key Points to Remember:
- Gradient shows rate - steeper line means faster reaction
- Plot quantity against time - either product made or reactant used up
- Rate decreases as the reaction progresses
- Draw best-fit lines and ignore anomalous points
- Label axes properly with units in brackets