Rate-Concentration Graphs and Advanced Concepts
This page expands on rate-concentration graphs for different reaction orders and introduces more advanced concepts in chemical kinetics.
For zero-order reactions, the rate-concentration graph is a horizontal line, as the rate is constant regardless of concentration. The rate equation for a zero-order reaction is:
Example: rate = kA⁰ = k
For first-order reactions, the rate-concentration graph is a straight line passing through the origin, with the gradient equal to the rate constant k. The rate equation for a first-order reaction is:
Example: rate = kA¹
Second-order reactions have a more complex rate-concentration relationship, and the rate constant cannot be directly determined from the graph. The rate equation for a second-order reaction is:
Example: rate = kA²
The page then introduces the concept of clock reactions, where the time from the start to a visual change is measured and assumed to represent the initial rate.
Definition: Rate Determining Step: The slowest step in a reaction mechanism, which determines the overall reaction rate.
The rate equation only includes reacting species involved in the rate-determining step.
The Arrhenius equation is introduced, relating reaction rate to temperature and activation energy:
Example: ln k = -Ea/RT + ln A
Where:
- k is the rate constant
- Ea is the activation energy
- R is the gas constant
- T is the temperature
- A is the pre-exponential factor
Highlight: The Arrhenius equation takes into account the frequency of collisions with the correct orientation and represents the proportion of molecules that exceed the activation energy and have sufficient energy for a reaction to take place.
The page concludes by showing how to determine the activation energy from an Arrhenius plot, where the gradient is equal to -Ea/R.
