How can you determine the rate of a reaction from a concentration-time graph?

How can you determine the rate of a reaction from a concentration-time graph?

• A. Final concentration.
• B. Area under the curve.
• C. Gradient at a chosen time.
• D. Time taken to reach end point.

Answer: (C)

The rate is how fast the concentration changes with time. On a concentration–time graph, that change is shown by the slope (gradient) of the curve at any moment. The steeper the slope, the faster the change. For a reactant, the concentration falls, so the slope is negative, but the rate is taken as the positive magnitude of that slope. So to determine the rate at a specific time, you read off the gradient at that time. If you want a numerical sense, you can imagine the concentration dropping from 0.60 M to 0.50 M over 2 s. The gradient would be (0.50 − 0.60) / 2 = −0.10 M/s, so the rate is 0.10 M/s. Final concentration and time to reach the end point don’t tell you how quickly the change occurred, and the area under the curve isn’t a measure of instantaneous rate for a reaction. The gradient at a chosen time directly reflects the instantaneous rate at that moment, which is why it’s the best answer.

The rate is how fast the concentration changes with time. On a concentration–time graph, that change is shown by the slope (gradient) of the curve at any moment. The steeper the slope, the faster the change. For a reactant, the concentration falls, so the slope is negative, but the rate is taken as the positive magnitude of that slope. So to determine the rate at a specific time, you read off the gradient at that time.

If you want a numerical sense, you can imagine the concentration dropping from 0.60 M to 0.50 M over 2 s. The gradient would be (0.50 − 0.60) / 2 = −0.10 M/s, so the rate is 0.10 M/s.

Final concentration and time to reach the end point don’t tell you how quickly the change occurred, and the area under the curve isn’t a measure of instantaneous rate for a reaction. The gradient at a chosen time directly reflects the instantaneous rate at that moment, which is why it’s the best answer.