Overview
This lecture introduces the concept of reaction rates in chemistry, showing how to define, calculate, and express rates using changes in concentration over time and stoichiometric relationships.
Defining Reaction Rate
- The reaction rate is the change in concentration of a reactant or product divided by the change in time.
- Concentration is measured in moles per liter (M), and time in seconds, so rate units are M/s.
- Rates can be based on either product appearance or reactant disappearance.
Calculating Reaction Rates
- For reactants, rate = - (final [A] - initial [A]) / (final time - initial time) (negative sign yields positive rate).
- For products, rate = (final [B] - initial [B]) / (final time - initial time).
- Example: If [A] decreases by 0.02 M in 2 seconds, rate = 0.01 M/s.
- The negative sign for reactants ensures the rate is always positive.
Reaction Rate and Stoichiometry
- In reactions with different stoichiometric coefficients, rate expressions are adjusted by dividing the change by the coefficient.
- Example reaction: 2 N₂O₅ → 4 NO₂ + O₂
- If O₂ forms at 9×10⁻⁶ M/s, NO₂ forms at 4 × 9×10⁻⁶ = 3.6×10⁻⁵ M/s, N₂O₅ disappears at -2 × 9×10⁻⁶ = -1.8×10⁻⁵ M/s.
- To standardize the rate, divide the rate of change for each species by its coefficient:
- Rate = -½ (Δ[N₂O₅]/Δt) = ¼ (Δ[NO₂]/Δt) = (Δ[O₂]/Δt)
Summary of Rate Expressions
- The rate of reaction is always a positive value, regardless of whether it's based on reactants or products.
- The standardized rate allows comparison between species with different stoichiometric coefficients.
Key Terms & Definitions
- Reaction Rate — Change in concentration of a reactant or product per unit time (M/s).
- Stoichiometric Coefficient — Number in a balanced equation that shows the ratio of reactants and products.
- Average Rate — Rate measured over a time interval, not at a single instant.
- Negative Sign for Reactants — Used so calculated rates remain positive, as reactant concentration decreases.
Action Items / Next Steps
- Practice calculating rates for simple and complex reactions using concentration and time data.
- Review the next lesson on factors affecting reaction rates.