Measuring Reaction Rate
Simple Explanation
Reaction rate measures how quickly reactants are used up or products are formed β usually expressed as the change in concentration of a chosen substance divided by the time that change took.
Why Do We Need It?
Being able to quantify reaction rate is the entire foundation of chemical kinetics β without a number to measure, there would be no way to compare how "fast" different reactions are, or how much a given factor speeds one up.
Formula
Average Reaction Rate
rate = Ξ[concentration] / Ξt
How quickly the concentration of a reactant or product changes over a time interval.
- rate
- β average reaction rate, typically in mol/(LΒ·s)
- Ξ[concentration]
- β change in concentration (mol/L) of a chosen reactant or product over the interval
- Ξt
- β time interval over which the change is measured, in seconds (s)
When to use it: Whenever you have concentration measurements at two points in time and need the average rate of reaction between them.
Worked Example
Calculate an average reaction rate
The concentration of a reactant drops from 0.80 mol/L to 0.50 mol/L over 60 seconds. Find the average rate of reaction.
Why Does This Work?
Rate is fundamentally about "how much changes, per unit time" β dividing the total change in concentration by the time it took to happen gives exactly that: a number that captures the reaction's speed in a way that can be measured and compared.
Real-Life Example
Monitoring an industrial reactor
Chemical engineers track how quickly a reactant is consumed in a large industrial reactor.
By sampling the reactant concentration at intervals and calculating the rate, engineers can predict how long a batch reaction will take and adjust conditions to speed up production.
Practice
A product's concentration rises from 0 to 0.40 mol/L over 20 seconds. Find the average rate.
EasyCommon mistake
Forgetting that reaction rate is always taken as a positive quantity, even when measured using a reactant that is DECREASING β the negative sign of a decreasing reactant concentration is conventionally dropped so the rate is reported as a positive number.
Quick Review
- rate = Ξ[concentration] / Ξt.
- Rate can be measured from any reactant or product in the reaction.
- Rate is always reported as a positive quantity.