Overview
This lecture introduces chemical kinetics, focusing on reaction rates, rate laws, reaction order, and integrated rate laws for zero, first, and second order reactions.
Introduction to Chemical Kinetics
- Chemical kinetics studies the speed of chemical reactions and changes in concentration over time.
- Reaction rate is measured by the change in concentration of reactants (decreasing) or products (increasing) per unit time.
- Units for reaction rate are typically molarity per second (M/s).
Measuring Reaction Rates
- Rate = change in concentration / change in time.
- For reactants, rate is negative due to decreasing concentration; for products, it's positive.
- The slope of a concentration vs. time graph (tangent line) gives the instantaneous rate.
- Average rate is calculated with two points: (Δconcentration) / (Δtime).
Relationship to Balanced Equations
- The rate of appearance/disappearance relates to stoichiometric coefficients.
- Rate = -(1/coefficient) × (Δ[reactant]/Δt) = +(1/coefficient) × (Δ[product]/Δt).
Rate Laws and Reaction Order
- Rate law: rate = k × [A]^n, where k is rate constant and n is order of reaction.
- Overall reaction order is the sum of exponents in the rate law.
- Zero order: Rate independent of reactant concentration.
- First order: Rate directly proportional to reactant concentration; doubling [A] doubles rate.
- Second order: Rate proportional to square of [A]; doubling [A] quadruples rate.
Integrated Rate Laws and Graphical Analysis
- First order: ln[A] vs. time yields straight line, slope = -k.
- Second order: 1/[A] vs. time yields straight line, slope = +k.
- Zero order: [A] vs. time yields straight line, slope = -k.
- Units of k:
- Zero order: M/s
- First order: 1/s
- Second order: 1/(M·s)
Half-life Concepts
- Half-life: time for concentration to halve.
- Zero order: t½ = [A]_0 / (2k)
- First order: t½ = 0.693 / k (independent of [A]_0)
- Second order: t½ = 1 / (k·[A]_0)
Experimental Applications
- Use data tables or graphs to determine initial/average/instantaneous rates.
- Add exponents in rate law to find overall reaction order.
- For multi-reactant systems, rate law may be experimentally determined and may not match stoichiometric coefficients.
Key Terms & Definitions
- Reaction Rate — Speed at which reactant/product concentration changes per unit time.
- Rate Law — Equation expressing rate as function of reactant concentrations.
- Order of Reaction — Sum of exponents in the rate law, indicating concentration dependence.
- Integrated Rate Law — Equation relating concentration and time for a particular reaction order.
- Rate Constant (k) — Proportionality factor in the rate law.
- Half-life (t½) — Time required for half the initial reactant to be consumed.
Action Items / Next Steps
- Review homework on determining rate laws and calculating rates from data tables.
- Familiarize yourself with integrated rate law equations and their graphical representations.
- Prepare for discussion on the effect of temperature on reaction rates in the next part.