Question Constellation Sign up for free AQA · GCSE · Chemistry · Higher
Tangent rate calculation
A different student investigated how the concentration of sulfuric acid affected the rate of the reaction. The student did a different experiment using sulfuric acid of concentration 0.40 mol/dm³. The student calculated the number of moles of hydrogen collected after every 20 seconds. Figure 12 shows the results.
Determine the rate of reaction at 45 seconds. You should draw a tangent on Figure 12. Give your answer in standard form.
Write your answer first. You can study the marking guidance whenever you need it.
Study the marking See what earns credit and compare it with a full-mark answer.
Marking points
- 1 Draws an appropriate tangent at 45 seconds.
- 2 Reads a valid y-step and x-step from that tangent within the official tolerance.
- 3 Uses gradient = y-step ÷ x-step.
- 4 Calculates a rate consistent with the drawn tangent and gives the unit mol/s.
- 5 Expresses the calculated rate in standard form.
Why this answer loses marks
The student joins two points on the curve and divides their changes.
That gives an average rate over an interval; the rate at the specified instant requires a tangent and its gradient.