Lecture 11 Summary
Chemical kinetics: how fast a reaction happens
Reaction rate = (change in concentration) / (elapsed time)
Rate law: the dependence of reaction rate on the concentration of the reactants
k is a reaction rate constant
m and n is the reaction order: the reaction order for A is m and the reaction order for B is n. The reaction order of the entire reaction is m + n
The reaction order is determined experimentally
For the reaction with rate = k[A]m
- m = 0: A concentration does not change the reaction rate
- m = 1: doubling A doubles rate
- m = 2: doubling A → quadruple
Rate constant k
- If k is small → react slowly
- If k is big → react quickly
To find out the concentration at a given time → Integrated rate law
Because over time, the concentration of your reactants is going to change
- Zero-order Reactions:
A straight line → does not depend on the concentration of the reaction
The slope of the line is the rate constant - First-order Reactions:
→ Calculating half-life: the time it takes for the concentration of a reactant to be reduced in half
For zero order reaction → all half-lives are the same
For first order reactions → half-lives may vary
Determine the order of the reaction:
- Looking at the concentrations in the trails: does not change, double or triple
- Graphically:
- Graph of concentration over time is straight line: zero order, slope of the line is the rate constant
- Graph of natural log of concentration over time is a straight line: first order → determining half lives
- Graph of 1/[A] (concentration) is a straight line: second order
Criteria for a reaction to occur:
- Must collide
- Correct orientation
- Kinetic energy (temperature)