Rate calculations for enzyme activitySpec 1.11
In short
The rate of reaction is how fast a reaction happens, calculated as the amount of product formed divided by the time taken. Its units depend on what is measured, such as cm³/s for a volume of gas. If the time for a reaction to finish is measured, the rate is 1 ÷ time, in s⁻¹.
The rate of reaction tells you how fast a reaction is happening. It is a compound measure, because it combines an amount with a time.
The units depend on what you measure, for example cm³/s for a volume of gas, or g/min for a mass. If an experiment measures the time taken for a reaction to finish, such as the time for starch to disappear, the rate can be found as 1 ÷ time, with units of s⁻¹.
Calculating a rate
An enzyme produces 12 cm³ of gas in 40 seconds. Calculate the rate of reaction in cm³/s.
- rate = amount of product ÷ time
- 12 ÷ 40 = 0.3
Answer: 0.3 cm³/s
A longer time means a slower reaction. If the time doubles, the rate (1 ÷ time) halves.
Quick check
What is the name of the part of an enzyme that the substrate fits into?
Show answer
The active site.
Why does an enzyme stop working at a high temperature?
Show answer
The shape of the active site changes, so the substrate no longer fits. The enzyme is denatured.
Why does the rate level off as substrate concentration increases?
Show answer
All the active sites are occupied.
In the pH core practical, what colour shows that starch is still present with iodine solution?
Show answer
Blue-black.
Calculate the rate when 30 cm³ of gas is collected in 60 s.
Show answer
0.5 cm³/s.
Written and checked against the Edexcel GCSE Combined Science (1SC0) specification · Updated October 2026