Summary of thermodynamic and kinetic effects

At this level, a simple approach will suffice for most occasions. Thus, in brief, reversible reactions are governed by thermodynamic considerations, while irreversible reactions are controlled by kinetic factors. 

The disorder of a system is increased if the system produces gaseous products, when the reagents were solids or liquids, or the number of components increases. Conversely, the production of a charged species in a polar solvent reduces the entropy, because it imposes order on the solvent molecules. The effect of a change in entropy is enhanced by raising the temperature. This can be used to control which particular reaction is favoured. 

Hydrogens that are accessible are likely to be abstracted first. Note also any statistical weighting. Selectivity can be introduced by using a hindered base, which has a large steric demand, so that it is only able to abstract readily approachable hydrogens. 

The overall rate of such a multi-step reaction is determined by the rate of the slowest step, which is called the rate determining (or rate limiting) step. 

A catalyst is a component of the reaction mixture that affects the rate of the reaction, while not being consumed itself. A catalyst does not affect the equilibrium position of a reaction. It may be heterogeneous or homogeneous, depending on whether it is in a different or similar physical state to the reaction mixture. Catalysts can be present in trace or stoichiometric amounts. A catalyst increases the rate of a reaction by lowering the activation energy barrier of the main reaction. 

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