Why Divide by Stoichiometric Coefficient?
* Consistent Rate: The rate of a reaction is defined as the change in concentration of a reactant or product over time. However, the rate can vary depending on which species you're looking at. For example, in the reaction:
2A + B → 3C
The rate of disappearance of A might be twice as fast as the rate of disappearance of B. To get a consistent measure of the reaction rate, we need to account for these differences in stoichiometry.
* Relative Rates: Dividing by the stoichiometric coefficient allows us to express the rate of the reaction in terms of the rate of change of any reactant or product. This provides a more complete picture of the reaction's progress.
How It Works:
1. Stoichiometric Coefficients: These numbers represent the relative amounts of each reactant and product involved in a balanced chemical equation.
2. Rate of Change: The rate of change is the change in concentration of a species over time. It can be positive (for products, as they are formed) or negative (for reactants, as they are consumed).
3. Calculation: To get the consistent rate of reaction, we divide the rate of change of each species by its stoichiometric coefficient.
Example:
Consider the reaction: 2NO₂(g) → N₂O₄(g)
* Rate of NO₂ disappearance: -Δ[NO₂]/Δt
* Rate of N₂O₄ formation: +Δ[N₂O₄]/Δt
To find the rate of reaction, we can use either species:
* Rate of reaction: (-Δ[NO₂]/Δt) / 2 = (+Δ[N₂O₄]/Δt) / 1
Key Points:
* The rate of reaction is always positive.
* This method applies to both reactants and products.
* It is essential to use a balanced chemical equation to determine the correct stoichiometric coefficients.
In Summary:
Dividing the rate of change of a reactant or product by its stoichiometric coefficient allows us to express the rate of reaction in a consistent and meaningful way, regardless of which species is being monitored. This is a fundamental concept in chemical kinetics.
