Read a volume-time graph, find mean rates from it, compare two curves and use a tangent for the rate at one moment.
What you will learn
- Read volumes and times off a rate graph
- Find a mean rate between two times
- Compare curves and say which reaction is faster
- Higher: find a rate from the gradient of a tangent
Reading the curve
The graph shows the hydrogen given off when magnesium reacts with excess acid.
- The gradient of the curve is the rate.
- It is steepest at the start, when there are most reactant particles.
- It gets less steep as the reactants are used up.
- It goes flat when one reactant has run out: the reaction has stopped. The final volume (60 cm³ here) depends on how much of that limiting reactant there was.
- Mean rate over the first 40 s: about 52 cm³ ÷ 40 s = 1.3 cm³/s
- Mean rate from 20 s to 60 s: (57 − 38) ÷ (60 − 20) = 19 ÷ 40 = 0.48 cm³/s
The rest of this lesson and its 6 practice questions are locked.
In this lesson
- Reading the curve
- Higher: the rate at one moment
- Comparing two curves
- Try it: Concentration in the simulation
6 practice questions, marked as you go; about 20 minutes in all.
Key terms
Gradient, Steeper, Same
In this unit: Rates of reaction and equilibrium
Measuring and graphing rates, collision theory and the factors that change a rate, the rates required practical, reversible reactions and how conditions move an equilibrium.