Chemical Kinetics and Thermodynamics Fundamentals

Hess's Law and Enthalpy Diagrams

Hess's Law: If a reaction occurs at several stages, whether real or theoretical, the total enthalpy change is equal to the sum of the enthalpies of reaction of these intermediate reactions.

An enthalpy diagram represents the energies at play in a chemical reaction using activated complex theory. According to this theory, when the reactants approach, there is an intermediate state of high energy and short duration, which is the activated complex.

In these diagrams, we can represent the activation energy, which is the energy needed for reactant molecules to absorb to form the activated complex. A slow reaction has a high activation energy, while a quick reaction will have a low activation energy.

Entropy and Molecular Disorder

Entropy (S) is a state function that measures the molecular disorder of a system. It increases when the system becomes disordered and decreases with increasing molecular order.

Measuring Reaction Speed

The average speed of a reaction is measured from the decrease of concentration of a reactant or the increase of the concentration of a product in a time interval.

The instantaneous speed is expressed by the derivative of concentration with time of a reactant or product, divided by the corresponding stoichiometric coefficient and converted to a positive quantity. This is the value of the velocity at a given time.

The Function of Catalysts

A catalyst is a substance that, by being present in a chemical reaction, produces a change in its speed without being consumed during its course. It participates in the reaction but is not altered by it. There are positive catalysts that increase the speed of reaction and negative catalysts that reduce the rate of reaction.

Collision Theory and Activated Complexes

According to the theory of collisions, for a chemical reaction to take place, a collision of reactant particles must occur, whether they are atoms, molecules, or ions. This collision must be effective, and for it to be so, the particles must:

• Have enough kinetic energy for the rearrangement of bonds and the formation of a new substance.
• Collide with the proper orientation.

The Activated Complex Theory

When the molecules of the reactants approach, they experience a deformation where the collision leads to an intermediate of high energy and short duration: the activated complex. The activated complex is a combination of molecules that can lead to products or return to produce the initial reactants. This state can only be reached if the reactant molecules have enough activation energy, which is the additional energy the reactants must absorb so that, upon colliding, they form the activated complex.