• State of reactions can be looked at as equilibrium or kinetic

• Equilibrium: no information on pathways or time to get to equilibrium

• Kinetic: describe pathways of reactions and position along pathways

• Overall reactions
- give no information about kinetics
- DG provides information on tendency toward equilibrium
 
•Kinetics are more complicated to apply to natural systems

- particularly true of biologically mediated reactions
  * microbial catalysis can make reactions faster than lab rates (e.g. Fe(II) oxidation).
- ground water- mineral reactions difficult
  * controlled by surface reactions
* lab rates much faster than natural rates

 
Important questions:

(1) Is reaction fast enough to be considered equilibrium controlled?
(2) Is reaction homogeneous (all within gas, liquid or solid phase) or heterogeneous?

- Slow reversible, heterogeneous (often slow) and irreversible reactions usually kinetically controlled
 
Consider elementary first order reaction:
 

Rates of forward and reverse reactions can be:
 

Here k’s are forward and reverse reaction rate constants
 

The overall rate, R, is:

 
At equilibrium:
 

Rearranging:
 

Which is the equilibrium constant

Equilibrium constant also controlled by DG

 
Remember residence time:
 

Also write it:
 

It will be shown later that
 

And
 

 
In general:

- if reaction is reversible and rate is fast compared to residence time, then equilibrium attained
- if reaction is irreversible and rate is comparable or slower than system, then kinetics controls reactions
 
Typical system in Partial Equilibrium

- some reactions faster than residence times
- other reactions slower than residence times

Computer models compare chemical state of a system to its hypothetical equilibrium state
- This information coupled with information on rates, residence times, aqueous composition and minerals determine if equilibrium possible
 
Elementary and Overall Reactions:

 
Rate Laws: