The Handy Chemistry Answer Book (2014)

KINETICS

What is the transition state for a reaction?

The transition state of a chemical reaction is the highest energy structure through which the reactant molecule(s) must pass to complete the reaction. Since this is the highest energy point along the path of the reaction, this configuration is the most “difficult” to reach along the reaction path and thus the energy barrier to reach the transition state limits how quickly the reaction can proceed.

What is the rate constant for a reaction?

The rate constant for a chemical reaction is a quantity that describes how rapidly the reaction proceeds. Rate constants can have different units, depending on how many molecules are involved in the reaction. Consider a simple reaction where a single molecule of a species A becomes a molecule of species B. The rate of the reaction will depend on the concentration of species A (denoted [A]) present, and the rate constant (k) for this reaction. The rate equation for this reaction would be:

Reaction rate = k[A]

This tells us that the reaction rate depends only on the concentration of A, and that the reaction rate will increase as the concentration of A is increased. In truth, the reaction rate also depends on the temperature, pressure, and perhaps other factors as well, but these are all bundled into the rate constant, k.

How is the rate of a reaction affected by temperature?

The rate of a chemical reaction will generally increase with increasing temperature. This is because a higher temperature translates into a higher average energy per molecule, which makes it easier for molecules to surmount the energetic barrier to the reaction. In terms of how this fits into the rate equation, the rate constant k depends on temperature, and k almost always (but there are exceptions) increases with increasing temperature.