On a graph showing the number of particles of each substance vs time, the gradient (steepness) of the line gives the rate of the reaction. Note: The graph in this simulation is ‘number of particles vs time’ rather than ‘concentration vs time’ but the same principles apply.
- In the beginning, the curves slope very steeply, showing that the rate is highest at the start of the reaction.
- As the reaction progresses, the gradient (steepness) decreases because the reaction slows down over time because the reactants are being consumed by the reaction so their concentration decreases.
- Eventually the curve becomes completely horizontal, at which point the reaction has stopped. This happens because eventually one of the reactants is entirely consumed, so no further collisions are possible.
Determining the rate:
The rate of reaction at a given point in time can be determined by drawing a tangent to the curve at that time (a tangent is a straight line that touches the curve once and matches its gradient at that point) and then calculating its gradient. You can try this as follows:
- Click the Draw tangent button, then click on the curve wherever you want the tangent to be. If doing this on paper, draw the tangent with a ruler in pencil in case you get it wrong.
- Click the Select points on graph button to choose two points on the line for calculating the gradient. If doing this on paper, choose two points, as far apart as possible, which intersect with easy-to-read points on the graph paper.
- Click the Draw construction lines button to add in a construction triangle, and determine its height and width by reading off against the y-axis (height) and x-axis (width). This gives you the change in y and the change in x.
- Click the Calculate gradient button to calculate the gradient by dividing the change in y by the change in x, this is the rate of reaction.