Lecture Twenty Three - Reaction kinetics IV Flashcards
How is the half life of a reaction calculated?
How is the half life calculated for a first order reaction?
Radio activity is first order.
Doesn’t matter what units the decay is in, as the units cancel.
What is the Arrhenius equation (explain with Ea)?
The Activation Energy (Ea) is the minimum amount of energy required for a collision between molecules to result in a chemical reaction.
Reacting molecules must have enough energy to overcome electrostatic repulsion and a minimum amount of energy to break chemical bonds so that new ones may be formed.
Molecules that collide with less than the activation energy bounce off one mother chemically unchanged, with only their direction of travel and their speed altered by the collision.
The Boltzmann distribution of kinetic energy:
The area under each curve is the same for a specific number of molecules, but a higher temperature allows for more molecules to be present with sufficient kinetic energy for a reaction to occur (increased RR).
(Curve with fraction of collisions with a given kinetic energy Vs kinetic energy).
Molecules must have the correct orientation for the reaction to proceed.
The Arrhenius Equation is:
K = rate constant.
A = frequency factor (constant).
Ea = activation energy.
R = molar gas constant = 8.314 J/K/mol.
T = temperature (K).
What is a catalyst?
Alters the rate of reaction without appearing in any of the products. Provides a new pathway which may have a different activation energy.
Heterogeneous catalyst’s:
At least one of the reactants interacts with the solid surface (in a physical process called adsorption) in such a way that a chemical bond in the reactant becomes weak and then breaks.
Homogenous catalyst’s:
The catalyst is in the same phase as the reactant(s); the number f collisions between reactants and catalyst is at a maximum because the catalyst is uniformly dispersed throughout the reaction mixture.
Autocatalysis:
One of the products catalyses the reaction.
What are elementary reactions (intermediates, rate determining step)?
The complete pathways from reactants to products which may involve a series of steps each of which i called an elementary reaction.
We can predict the reaction mechanism using elementary reaction.
We then test the prediction with experimentally determined kinetics data.
Intermediates: Species which are formed in one step and consumed in another are intermediates; they do not appear in the balanced chemical equation for the reaction, but they may be vital in determining the rate of reaction.
Rate determining step: Not all of these steps affect the rate of the reaction, normally it is only the slowest elementary step that affect the reaction rate.
Usually the rate determining step has the highest Ea.
Molecularity: The molecularity of an elementary step is the number of molecules or atoms of reactant taking part.
- Unimolecular.
- Bimolecular.
- Trimolecular (or termolecular).
For an elementary step, the molecularity is the same as reaction order.
