Chemical Reaction Rates and Mechanisms
Chemical Reaction Rates
The speed of a chemical process is defined as the change in the concentration of reactants or products with respect to time. The overall order of a reaction is the sum of the partial orders for each of the reagents.
Zero-Order Reactions
In these reactions, the reaction rate is independent of the concentration. Zero-order reactions are often found in heterogeneous catalysis, when the reaction is carried out on a surface saturated with reagent, and in reactions catalyzed by a substrate with sufficient excess to saturate the catalyst. Formula: 
First-Order Reactions
In these reactions, the rate is dependent upon the concentration of reactants and corresponds to unimolecular elementary processes. Formula: 
Second-Order Reactions
Half-Life Periods
Half-life of order 0: 
Half-life of order 1: 
Half-life of order 2: 
The reaction mechanism is the set of stages or states that make up a chemical reaction. Reaction mechanisms are linked to chemical kinetics.
The molecularity is the number of molecules that take part as reactants in an elementary process, i.e., the sum of each reactant molecule before forming the activated complex to become the products. It is a theoretical concept indicating the number of individual particles participating in an elementary step. The rate-limiting step determines the speed of reaction and is the slowest step of all that make up the reaction mechanism.
Collision Theory: This theory belongs to the field of chemical kinetics, but also has applications in physics. It involves the collision of molecules, which have a certain potential energy, vibrational energy, kinetic energy, and so on. All these energies must be large enough to break the barrier of energy called “activation energy”. If this barrier is broken, the collisions will be effective and a reaction between the molecules involved will occur. On the other hand, if the shock is not effective, the reaction will not occur. When it comes to an “activation energy”, the reaction is at a peak of energy, and depending on the stability of the molecule formed, the reaction may be exothermic (the reaction product is more stable than the separated reactants) or endothermic (if the product of the reaction is more unstable than the separated reactants). Conditions for the shock to be effective: the reactant molecules collide with enough energy to break or weaken their links. Active molecule and activation energy / reactant molecules collide with the proper orientation.
Temperature 
Influence of a Catalyst: A catalyst is a substance that varies the speed of a chemical reaction without being consumed at the end of the reaction. It appears unchanged. When we talk about increasing the speed of reaction, it is a positive catalyst or catalyst directly, and when we talk about reducing the speed of reaction, it is a negative catalyst or inhibitor. The role of a catalyst is believed to be diminishing the value of the activation energy (Ea) that is unique to each reaction. This effect of the catalyst serves as an explanation of positive catalysts. But it cannot be expressed with certainty that inhibitors have the opposite effect (increased Ea), as it is not exactly known how the latter act.
In reactions with a catalyst, by lowering the Ea, an intermediate compound and unstable activated complex is formed before the product. This is called homogeneous catalysis when the catalyst and the substances involved in the reaction have the same state of aggregation. It is called heterogeneous catalysis when the state of aggregation of the catalyst is different from the substances involved.