Note that the mean energy of the particles does not change. However, since the frequency of collisions increases, the frequency of successful collisions also increases.
The curved red and green lines are the reaction data. The straight dashed lines at the start of the two reactions are called tangents. They follow the curve at the beginning of the reaction. The steeper the tangent, the faster the reaction.
Tangents can also be drawn at other points on the graph. As the reaction carries on, the steepness of the tangent to the curve decreases (eg the blue dashed line) so the reaction is slowing down.
To calculate the rate at the start of a reaction, you need to find the mathematical gradient (steepness) of the gradient. Use this equation:
For example, using the green tangent below:
Rate of reaction = 12 cm3 s - 1