Reaction Process

A reaction mechanism is a step by step sequence of reactions that show an overall chemical change The same reaction can occur by different pathways under different conditions. A species that appears in some steps but not in the overall equation is called an intermediate

. A simple looking reaction like I 2 + H 2  2HI is more complex than it appears. This reaction has 2 possible reaction pathways. Step 1I 2 ↔ 2I Step 22I + H 2 ↔ 2HI I 2 + H 2 ↔ 2HI

A second pathway is as follows Step 1I 2 ↔ 2I Step 2I + H 2 ↔ H 2 I Step 3H 2 I + I ↔ 2HI I 2 + H 2 ↔ 2HI

Homogenous reaction – reaction exists in a single phase Heterogenous reaction – reaction is made up of materials in different phases

Collision theory is an attempt to understand what happens to the individual molecules during a reaction. Molecules must have the proper energy and orientation for a reaction to occur. You need enough activation energy to create enough high energy collisions to get a reaction started. Once an exothermic reaction starts, it will sustain itself

An activated complex is an intermediate material. You need enough activation energy to create the activated complex.

Reaction rates Reaction rate is the change in concentration of the reactants per unit of time Factors that influence reaction rates – presence of a catalyst, surface area, temperature, concentration A catalyst is a substance that speeds up a reaction without being consumed itself

Experiments are conducted to find rate laws The slowest step in any reaction is called the rate determining step A rate law is an equation the relates the reaction rate to the reactants