Rate Graphs
Chemistry: Cambridge International A Level, topic 1.9.3
Reading Rate Graphs
Reaction rates can be measured over time. We plot this data on graphs, and you need to be able to read them.
Definition of a rate
- A rate of reaction is defined as the change in concentration of a reactant or product over time.
- This means that if you plot concentration against time, the rate is the gradient of the graph.
A simple graph
- The above graph is a straight line graph showing the concentration against time.
- To calculate the gradient of the graph (and the rate) you find the change in concentration divided by the change in time.
- In practice: pick two points and read off the x and y values.
- Calculate the differences in concentration values and the time, values and then divide the one by the other.
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Curved graphs
- Not all reaction rates are constant - a reaction may slow down or speed up with time.
- For reactions like this, you get a curved graph.
- You can calculate the rate at any point in time by calculating the gradient of a point on the graph.
- To do this, you need to draw a tangent on the graph at this point.
Drawing tangents
- A tangent is a straight line which just touches the curve in one place.
- To draw the tangent, choose your point and draw a straight line through it. You want to have an equal distance to the curve from the line on each side of the point.
- An example is above.
Test yourself on Rate Graphs
What is the best definition of rate?