Learning focus
Explain chemical changes using particles, collisions, equilibrium and electron transfer; interpret graphs and industrial conditions; and apply the ideas to unfamiliar reactions.
Definition
Reaction rate describes how quickly reactants are used or products are formed. It can be expressed as change in a measurable quantity divided by time. The quantity may be mass, gas volume, concentration, colour intensity or another valid measure connected to reaction progress.
Average and instantaneous rate
Average rate is calculated over a chosen time interval. Instantaneous rate is represented by the gradient of a tangent to a curve at one moment. Initial rate is the instantaneous rate at the start and is useful because starting concentrations are known.
Units
Rate units depend on the measured quantity, such as cm3/s for gas volume, g/s for mass loss or mol/dm3/s for concentration. A numerical answer without a unit is incomplete unless the question explicitly supplies one.
Why rate changes during a reaction
Most reactions slow as reactants are consumed. Particle concentration falls, so collision frequency decreases. A curve reaches a plateau when the measured change stops, usually because a limiting reactant is exhausted.
Practical or data skill
Use a supplied gas-volume table to calculate average rates for several intervals and compare them with the gradient of a tangent near the start.
Examination tip
When comparing rates from graphs, compare gradient, not simply the height of the curve.
Review questions and suggested answers
Question 1
Define reaction rate.
Suggested answer
Change in amount or another measured quantity per unit time.
Question 2
What does a steeper product-volume graph mean?
Suggested answer
A faster reaction rate.
Question 3
Why does a graph become horizontal?
Suggested answer
The measured change has stopped, usually because the reaction is complete.
