Rate of reaction on a graph (AQA GCSE Chemistry Combined Science): Revision Notes
Rate of reaction on a graph
What graphs can show us
When we study chemical reactions, we can create graphs to see how fast they happen. These graphs plot the amount of product made or the amount of reactant used up against time.
The most important thing to remember is that the steepness (gradient) of the line shows the rate of reaction. A steeper line means the reaction is happening faster.
Understanding reaction rate graphs is fundamental to chemistry. The ability to interpret these graphs helps us predict how reactions behave and how we can control them in industrial processes.
Measuring the rate with marble chips and acid
Let's look at a common practical example. When marble chips react with dilute hydrochloric acid, they produce carbon dioxide gas.
Practical Setup: Marble Chips and Acid Reaction
The setup:
- Marble chips go into a flask with acid
- The CO₂ gas produced is collected in a gas syringe
- We measure the volume of gas at different times
Typical results:
| Time (minutes) | Volume (cm³) |
|---|---|
| 0 | 0 |
| 1 | 87 |
| 2 | 132 |
| 3 | 178 |
| 4 | 193 |
| 5 | 204 |
Notice how the volume increases rapidly at first, then more slowly as time progresses.
Plotting the graph correctly
Creating accurate graphs is essential for analysing reaction rates. The quality of your graph directly affects the reliability of your conclusions.
Essential Steps for Accurate Graphing:
-
Choose your axes carefully - put quantity (volume in cm³) on the y-axis and time (minutes) on the x-axis
-
Use most of the graph paper - make your scale big enough so the plotted points fill at least half the page
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Label your axes properly - include the units (cm³ for volume, min for time)
-
Plot points carefully - take time to be accurate
-
Draw a best-fit line - this can be straight or curved, depending on your data
Watch out for anomalous points
Sometimes you get results that don't fit the pattern. These are called anomalous points. You should ignore these when drawing your best-fit line, as they're usually caused by measurement errors.
Anomalous points can occur due to various factors such as measurement errors, equipment malfunctions, or temporary changes in experimental conditions. Always examine your data critically before including points in your analysis.
How the rate changes during a reaction
The graph tells us an important story about how reactions work. Understanding this pattern is crucial for predicting reaction behaviour.
- At the start - the line is steep, showing the reaction is fast
- As time goes on - the line gets less steep, showing the reaction slows down
- Eventually - the line becomes flat, showing the reaction has stopped
This happens because as reactants get used up, there are fewer particles available to react, so the reaction slows down.
Calculating average rates
You can work out the average rate of reaction using the graph. Mathematical analysis of your experimental data provides quantitative insights into reaction kinetics.
Worked Example: Calculating Average Rate
To find the average rate over the first 2 minutes from our marble chips experiment:
Formula:
Calculation: From the example data:
This tells us that on average, 66 cm³ of carbon dioxide was produced every minute during the first 2 minutes.
Key Points to Remember:
- The gradient (steepness) of a graph line shows the rate of reaction
- Steeper lines mean faster reactions
- Rate decreases as the reaction continues because reactants get used up
- Always ignore anomalous points when drawing best-fit lines
- Label your axes with correct units and use most of the graph paper