Topic 9 + 16: Kinetics I, II Flashcards
What is rate of reaction?
Change in concentration / time
What is activation energy
This minimum amount of energy required for the reaction to take place
What is collision theory
Particles must collide with sufficient energy and in the correct orientation in order for a successful reaction to occur
What factors affect the rate of reaction
- Nature of reactants - particle size
- Temperature
- Concentration
- Presence of a catalyst
Describe the effect of particle size on rate of reaction
In a solid molecules can only react at the surface (where molecules are exposed). If there is a higher surface area (i.e. smaller particle size) then more molecules are able to react and the frequency of collisions increases.
Describe the affect of temperature on reaction rates
Increasing the temperature means more particles have a higher kinetic energy. Therefore more collisions will have sufficient energy and there will be more frequent collisions. Therefore there are more frequent successful collisions and a faster rate of reaction.
The reverse would be true for decreasing
Describe the affect of concentration on rate of reaction
If the solution is more concentrated then there are more molecules in the same volume. This means there are more molecules closer together. Therefore there are more frequent collisions. Also more particles with the required minimum energy are added to the system.
Describe the affect of catalysts on rate of reaction.
Decrease the activation energy so that a higher proportion of molecules have sufficient energy for successful collisions. Therefore faster rate of reaction (more frequent successful collisions).
What is the most probable energy on a Maxwell-Bolztmann distribution?
The energy at the peak (maximum point) of the graph.
What is the average energy on a Maxwell-Bolztmann distribution
The average energy lies slightly to the right of the most probable energy (the peak).
What does the area under the curve of a Maxwell-Bolztmann distribution represent?
The number of molecules
Explain the shape of a Maxwell-Bolztmann Distribution
Due to many collisions taking place between molecules, there is a spread of molecular energies and velocities.
* No particles have zero energy/velocity
* Some have very low and some have very high energies
* Most have intermediate energies
How does increasing the temperature change the Maxwell-Bolztmann Distribution?
- Shift to higher energies
- Curve gets broader and flatter due to greater spread of values
- Area under the curve stays constant
The opposite is true for decreasing the temperature
What is the affect on the Maxwell-Bolztmann distribution of increasing concentration
Area under the curve increases. Distribution of energies stays roughly the same (it almost looks like another curve the same just copied slightly above the original).
Homogeneous catalyst
Catalyst is in the same phase as the reactants
Heterogeneous catalyst
The reactants and the catalyst are in different phases
Give examples of heterogeneous catalysts
- Zeolite in cracking
- Iron in the Haber process
- Ni/Pt/Pd in hydrogenation of alkenes
Give the 3 stages of heterogeneous catalysis
- Adsorption (molecules bond to active sites on catalyst)
- Reaction (weakened bonds allow for molecules to react with each other forming new bonds and breaking old ones)
- Desorption (products released from surface of the catalyst).
What are the units of rate of reaction
moldm^-3s^-1
What method would you use to measure the rate of reaction of a reaction which produces a gas
- measure volume of gas produced using a gas syringe
- weigh reaction mixture on a balance at time intervals -> look for change in mass
What method would you use to measure the rate of reaction of a reaction which has an acidic or basic reactant or product
- Titration; sample reaction mixture at time intervals, quench and titrate samples
- measure the pH using a pH probe meter
What method would you use to measure the rate of reaction of a reaction which has a coloured reactant or product
Colorimetry -> spectroscopic method
Coloured compounds frequently include transtition metal compounds, aqueous bromine/iodine, etc.
Why is the initial reaction rate often quoted
As the reaction proceeds,
* the concentration of reactants decreases and it can be difficult to measure the changes accurately
* reverse reaction may occur introducing error
What is initial rate of reaction
Rate of reaction when reactants are first mixed