What Is the Rate of Reaction?
The rate of reaction measures how quickly reactants are consumed or products are formed during a chemical reaction. It is defined as the change in concentration of a reactant or product per unit time.
Mathematically:
Where:
- = change in concentration (mol dm)
- = time interval (s)
The standard unit of reaction rate is mol dm s.
If the concentration of a reactant decreases, will be negative. However, rates are always reported as positive values, so we use the absolute value of the change.
Calculating the Rate of Reaction
For the reaction:
Over 5 seconds, the concentration of NO increases from 0 to mol dm.
To find the rate with respect to O: for every 4 mol NO produced, 5 mol O is consumed. Using the stoichiometric ratio:
Always check stoichiometry when comparing rates expressed in terms of different species — the rates are not necessarily equal unless the stoichiometric coefficients are the same.
Average Rate vs. Instantaneous Rate
There are two ways to describe the rate of a reaction at different points in time:
- Average rate: The overall rate calculated across a specific time interval. Found by computing the slope of a secant line connecting two points on a concentration–time graph.
- Instantaneous rate: The rate at a single specific moment in time. Found by calculating the slope of a tangent line drawn to the curve at that point.
Instantaneous Rate from a Graph
The concentration of H produced in a reaction is plotted over time. At s, a tangent to the curve passes through the points (10, 0.05) and (30, 0.20).
The instantaneous rate at s is 0.0075 mol dm s.
To draw accurate tangent lines on exam graphs: use a ruler, ensure the line touches the curve at only one point, and does not cross it. Read off two well-separated coordinates to minimise error in your slope calculation.
Common mistake: Students often draw a chord (secant) between two points rather than a true tangent. A tangent just grazes the curve at a single point — it does not pass through two points on the curve.