Lesson 9 - Collision Theory and Potential Energy Diagrams Flashcards
Collision Theory
The theory that proposes that for a reaction to occur, reacting particles must collide with one another. However not all collisions are successful. Only a fraction leads to a chemical reaction or successful/effective collision.
What needs to happen for a successful collision to occur?
For a successful collision;
1. The collision must occur at the right orientation (collision geometry). An effective collision is the one where the same angle is formed by the reactant as the product.
2. The collision must occur with sufficient energy. Usually only a small fraction of reactants possess sufficient activation energy, which depends on the kinetic energy of colliding particles.
Activation Energy
The minimum kinetic energy needed to break bonds and trigger a reaction. Or the maximum potential energy.
Maxwell-Boltzman Distribution Curve
A graph that demonstrates the percentage of collisions with sufficient activation energy at a given temperature. Only particles at or above the Ea threshold are able to participate in effective collisions.
Potential Energy Diagrams/Reaction Profiles
Represent the changes in potential energy during a chemical reaction.
Describe the potential energy diagram of an exothermic and endothermic Reaction
In an exothermic reaction, the products have less potential energy than the reactants, energy has been lost and ΔH is negative.
In an endothermic reaction, the products have more potential energy than the reactants, energy has been gained and ΔH is positive.
Moving left to right in a potential energy diagram
- Reactants move closer together and the potential energy increases
- If the collision has sufficient kinetic energy, the reactants will change configuration and enter a transition state
Henry Eyring’s extension of collision theory
Eyring focused on the intermediate stage of the reactants and products called the transition state or the activated complex. (AB+C -> A+BC)
A-B C (reactants)
A- - B- - C (activated complex)
A B-C (products)
How is Ea determined?
Ea is determined by analyzing reaction rates at varying temperatures.
Relationship between Ea and rate of reaction
Reactions with a higher Ea proceed slowly at room temperature and Reactions with a lower Ea proceed quickly at room temperature
Equation of reversible reactions, ΔHr =
ΔHr = Ea(fwd)-Ea(rev)
An activated complex is neither reactant nor product but…
An activated complex is neither reactant nor product but an unstable and temporary chemical species containing partial bonds
Describe activation energy and relate it to enthalpy
Activation energy is the minimum kinetic energy needed in order to break bonds and trigger a reaction. Enthalpy is a measure of the total energy in a system. In a chemical reaction, activation energy is required to overcome the energy barrier that separates the reactants from the products. The change in enthalpy for a reaction is the difference between the enthalpy of the products and the reactants. Thus they are related in that they both play a role in the energy changes that occur during a chemical reaction, governed by the first law of thermodynamics.
