AQA · GCSE · Combined Science
Course version: 8464 · Version from Sept 2018
Calculating rates of reactions: draw tangents to the curves on these graphs and use the slope of the tangent as a measure of the rate of reaction
The rate of reaction at a particular moment relates to the slope of the quantity-time curve.
- 4 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
The rate of reaction at a particular moment relates to the slope of the quantity-time curve. Students draw a tangent: a straight line that touches the curve at the chosen time and follows its direction. The slope of this tangent is used as a measure of the rate of reaction; a steeper tangent indicates a faster rate. For example, a tangent drawn at 20 s that is steeper than one at 40 s shows the reaction is faster at 20 s. Tangents are most reliable where the curve is smooth.
What earns the marks
- A tangent is drawn as a straight line touching the curve at the required time and matching the curve's direction at that point.
- The slope of the tangent is used as a measure of the rate of reaction at that specific time.
- A steeper tangent is interpreted as a faster rate at that time, and a near-zero slope near the plateau as a very slow or stopped reaction.
- The tangent must only touch the curve at one point, rather than crossing it as a secant line.
Common mistakes and how to put them right
Drawing the tangent through two points on the curve instead of touching it at one point.
Put it right by placing the line so it just touches the curve at the chosen time and follows its slope.
Using the whole curve's average slope when asked for the rate at a specific time.
Put it right by drawing the tangent at that time only.
Exam tips
- Use a ruler and a sharp pencil, and carefully align the ruler to match the curve's direction at the specific time.
- Remember that comparing the steepness of tangents allows you to compare rates at different times without calculating the exact gradient.
Try the lesson
The start of the Calculating rates of reactions: draw tangents to the curves on these graphs and use the slope of the tangent as a measure of the rate of reaction lesson, made from checked teaching notes for this specification point.
Have a go first
A graph shows the volume of gas produced against time for a reaction. To find the rate of reaction at a particular time, what should you draw on the curve at that time?
Listen
The rate of reaction at a particular moment relates to the slope of the quantity-time curve. Students draw a tangent: a straight line that touches the curve at the chosen time and follows its direction. The slope of this tangent is used as a measure of the rate of reaction; a steeper tangent indicates a faster rate. For example, a tangent drawn at 20 s that is steeper than one at 40 s shows the reaction is faster at 20 s. Tangents are most reliable where the curve is smooth. A strong answer earns marks for these points. A tangent is drawn as a straight line touching the curve at the required time and matching the curve's direction at that point. The slope of the tangent is used as a measure of the rate of reaction at that specific time. A steeper tangent is interpreted as a faster rate at that time, and a near-zero slope near the plateau as a very slow or stopped reaction. The tangent must only touch the curve at one point, rather than crossing it as a secant line. A common slip: Drawing the tangent through two points on the curve instead of touching it at one point. Instead, placing the line so it just touches the curve at the chosen time and follows its slope. Exam tip: Use a ruler and a sharp pencil, and carefully align the ruler to match the curve's direction at the specific time.
See it
Key words
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Tangent
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Slope of the tangent
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Steeper tangent
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Rate of reaction
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See every definition
- Tangent
- A straight line that touches the curve at the chosen time and follows its direction.
- Slope of the tangent
- Used as a measure of the rate of reaction at that specific time.
- Steeper tangent
- Interpreted as a faster rate at that time.
- Rate of reaction
- At a particular moment, relates to the slope of the quantity-time curve.
Label the diagram
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1Box 1
2Box 2
3Box 3
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