Average and Instantaneous Rate
Simple Explanation
Average rate is the overall rate calculated across a whole time interval; instantaneous rate is the rate at one specific moment in time — found from the slope of a concentration-time graph at that exact instant.
Why Do We Need It?
Reaction rate almost always changes as a reaction proceeds (usually slowing down as reactants are used up), so the average rate over a long interval can be very different from the true rate at the start or end of the reaction — instantaneous rate captures the real rate at any given moment.
Why Does This Work?
On a concentration-time graph, average rate is the slope of a straight line connecting two points; instantaneous rate is the slope of the tangent line at a single point — as the time interval used for the average rate shrinks toward zero, the average rate calculation converges on the instantaneous rate at that point.
Real-Life Example
Why a reaction seems to "slow down" over time
A reaction that starts vigorously (lots of bubbling) gradually appears to slow down and eventually stops.
This is the instantaneous rate decreasing over time as reactant concentration drops — plotting concentration against time and looking at the slope at different points shows the rate is fastest at the very start and approaches zero as reactants run out.
Practice
On a concentration-time graph, how do you find the instantaneous rate at a specific time?
MediumCommon mistake
Assuming reaction rate is constant throughout a reaction — in most reactions the rate changes continuously (typically decreasing as reactant concentration falls), which is exactly why instantaneous rate is a distinct, important concept from average rate.
Quick Review
- Average rate: overall rate across a time interval (secant line slope).
- Instantaneous rate: rate at one specific moment (tangent line slope).
- Reaction rate usually decreases over time as reactants are used up.