Order of Reaction with Respect to a Reactant
The order of a reaction with respect to a given reactant is the power to which its concentration is raised in the experimental rate equation. It defines the sensitivity of reaction rate to changes in that reactant's concentration.
What Is Reaction Order?
For the rate law \(\text{Rate} = k[A]^x[B]^y\):
- The order with respect to A = \(x\)
- The order with respect to B = \(y\)
- The overall order = \(x + y\)
Summary of Orders
| Order | Rate Law | Effect of Doubling [A] | Units of k |
|---|---|---|---|
| Zero (0) | Rate = k | No effect on rate | mol dm⁻³ s⁻¹ |
| First (1) | Rate = k[A] | Rate doubles (×2) | s⁻¹ |
| Second (2) | Rate = k[A]² | Rate quadruples (×4) | mol⁻¹ dm³ s⁻¹ |
Worked Example
Interpreting Reaction Orders from Rate Laws
Reaction: \(\text{NO}_2(\text{g}) + \text{CO}(\text{g}) \rightarrow \text{NO}(\text{g}) + \text{CO}_2(\text{g})\)
Experimental rate law: \(\text{Rate} = k[\text{NO}_2]^2[\text{CO}]^0 \implies \mathbf{\text{Rate} = k[\text{NO}_2]^2}\)
- Second order with respect to \(\text{NO}_2\)
- Zero order with respect to \(\text{CO}\)
- Overall order = 2 + 0 = 2
Zero Order Reactants in Mechanisms
If a reactant is zero order, it does not take part in the rate-determining step (or any step preceding it). Changes in its concentration have no effect on the overall rate, provided some amount is present.
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