You can determine the order of a reaction from an equation, but the method depends on whether the equation provided is a rate law or a chemical equation representing an elementary step.
Determining Order from a Rate Law
A rate law (or rate equation) is an experimentally determined mathematical expression that shows how the rate of a chemical reaction depends on the concentration of its reactants.
- How to Determine Order: When a rate law is given, the order of the reaction is the sum of the exponents (or powers) to which the concentrations of the species in the rate law are raised.
- Individual Order: The exponent for each reactant's concentration term in the rate law represents its individual reaction order.
- Overall Order: The sum of all individual orders in the rate law gives the overall order of the reaction.
Example:
Consider a hypothetical reaction with the following experimentally determined rate law:
Rate = k[A]^x[B]^y
- The reaction is
x
order with respect to A. - The reaction is
y
order with respect to B. - The overall order of reaction is
x + y
.
For instance, if Rate = k[NO₂]²[O₃]¹
:
- The reaction is second order with respect to NO₂.
- The reaction is first order with respect to O₃.
- The overall order of the reaction is 2 + 1 = 3 (third order).
Determining Order from a Chemical Equation for an Elementary Step
You can deduce the reaction order directly from the stoichiometric coefficients of a chemical equation only if that equation represents an elementary step.
- What is an Elementary Step? An elementary step (or elementary reaction) is a reaction that occurs in a single step, exactly as written in the chemical equation. Its molecularity (the number of reactant molecules involved) directly corresponds to its reaction order.
- How to Determine Order: For an elementary reaction, the exponents in its rate law are equal to the stoichiometric coefficients of the reactants in the balanced chemical equation.
Examples of Elementary Steps and Their Orders:
Elementary Step Equation | Rate Law (deduced) | Order w.r.t. Reactants | Overall Order |
---|---|---|---|
A → Products (Unimolecular) | Rate = k[A] |
A: 1 | 1st order |
A + B → Products (Bimolecular) | Rate = k[A][B] |
A: 1, B: 1 | 2nd order |
2A → Products (Bimolecular) | Rate = k[A]² |
A: 2 | 2nd order |
A + B + C → Products (Termolecular, rare) | Rate = k[A][B][C] |
A: 1, B: 1, C: 1 | 3rd order |
2A + B → Products (Termolecular, rare) | Rate = k[A]²[B] |
A: 2, B: 1 | 3rd order |
Important Distinction: Overall Reactions vs. Elementary Steps
It's crucial to understand that most chemical equations represent overall reactions, which often consist of a series of elementary steps (a reaction mechanism). For such multi-step reactions, the stoichiometric coefficients of the balanced chemical equation do not necessarily determine the reaction order.
For overall reactions, the reaction order must be determined experimentally. It cannot be inferred simply by looking at the balanced chemical equation unless the reaction is explicitly stated to be an elementary step. The reaction order reflects the actual mechanism, which is typically not evident from the overall balanced equation alone.