Collision Theory and Potential Energy Diagrams Study Guide

Chemical reactions occur when reactant particles physically collide with one another.
It is important to note that collisions are not always effective; not every collision between particles results in a chemical reaction.
For a collision to be considered effective and result in a reaction, it must meet two specific criteria: 1. The Reactants must have the correct orientation during the collision. 2. The particles must collide with sufficient collision energy.
Ineffective Collisions: If particles collide with the wrong orientation or insufficient energy, no reaction occurs. An example provided depicts an ineffective collision between $O$ , $N$ , and $Br$ particles which results in "No Reaction".

ACTIVATION ENERGY ( $E_a$ ) is defined as the minimum energy with which particles must collide in order for a chemical reaction to occur.
This energy serves as a threshold or barrier that reactants must overcome to transform into products.

A Potential Energy Diagram is a graphical representation of the energy changes that occur during a chemical reaction.
Axes of the Diagram: - Vertical Axis (Y-axis): Potential Energy ( $E_p$ ). - Horizontal Axis (X-axis): Reaction Coordinate, which represents the progression of the reaction from start to finish.
Key Points on the Diagram: - Reactants: Located at the beginning of the reaction coordinate at a specific potential energy level. - Products: Located at the end of the reaction coordinate at a specific potential energy level. - Activation Energy ( $E_a$ ): The energy difference between the potential energy of the reactants and the peak of the energy barrier.

The peak or top of the activation energy barrier on a potential energy diagram represents the TRANSITION STATE.
The TRANSITION STATE is characterized as the change-over point of the reaction.
The specific chemical species that exists at the exact moment of the transition state is the ACTIVATED COMPLEX.
Characteristics of the Activated Complex: - It is a temporary, transitional species. - It is considered neither a reactant nor a product. - It is highly unstable due to its high potential energy.
Case Study: Dissociation of Nitrosyl Bromide - Chemical Reaction: $2BrNO(g) \rightarrow 2NO(g) + Br_2(g)$ - Reactant: $2BrNO$ - Transition State/Activated Complex representation: The partial bonding state where bonds are breaking and forming, visualized as $Br---NO$ and $Br---NO$ . - Products: $2NO + Br_2$

The Heat of Reaction ( $\Delta H$ ) represents the change in enthalpy between the reactants and the products.
On a potential energy diagram, $\Delta H$ is the vertical distance between the potential energy level of the reactants and the potential energy level of the products.
Formula for Enthalpy Change: $\Delta H = H_{products} - H_{reactants}$

Reaction Equation: $CO_2 + NO \rightarrow CO + NO_2$
Given Data: - Forward Activation Energy ( $E_{a,fwd}$ ): $360\,kJ$ - Heat of Reaction ( $\Delta H$ ): $+226\,kJ$
Analysis and Calculations: - Because $\Delta H$ is positive ( $+226\,kJ$ ), the reaction is endothermic, meaning the products have higher potential energy than the reactants. - To find the Reverse Activation Energy ( $E_{a,rev}$ ), which is the energy required for the reverse reaction ( $CO + NO_2 \rightarrow CO_2 + NO$ ) to reach the transition state, the following relationship is used: - $\Delta H = E_{a,fwd} - E_{a,rev}$ - $226\,kJ = 360\,kJ - E_{a,rev}$ - $E_{a,rev} = 360\,kJ - 226\,kJ$ - $E_{a,rev} = 134\,kJ$
Diagram Requirements: - Label individual axes as Potential Energy and Reaction Coordinate. - Mark the potential energy of the reactants, the products (at a higher level), and the peak (transition state). - Indicate $\Delta H = +226\,kJ$ as the gap between reactants and products. - Indicate $E_{a,fwd} = 360\,kJ$ as the height from reactants to the peak. - Indicate $E_{a,rev} = 134\,kJ$ as the height from products to the peak.