Kinetics Flashcards
What is the Maxwell-Boltzmann distribution curve
graph that shows the distribution of energies at a certain temperature
What does the graph show
that only a small proportion of molecules in the sample have enough energy for an effective collision and for a chemical reaction to take place
What happens when you increase temperature
particles gain more kinetic energy
This causes the particles to move around faster resulting in more frequent collisions
Furthermore, the proportion of successful collisions increases, meaning a higher proportion of the particles possess the minimum amount of energy (activation energy) to cause a chemical reaction
With higher temperatures, the Boltzmann distribution curve flattens and the peak shifts to the right
what does an increase in temperature causes
more effective collisions as the particles have more kinetic energy, making them move around faster
A greater proportion of the molecules having kinetic energy greater than the activation energy
Effect of adding catalyst
increases the rate of a reaction by providing the reactants with an alternative reaction pathway which is lower in activation energy than the uncatalysed reaction
Types of catalyst
Homogeneous means that the catalyst is in the same phase as the reactants
Heterogeneous means that the catalyst is in a different phase to the reactants
Adding catalyst boltzmann graph
By lowering Ea, a greater proportion of molecules in the reaction mixture have the activation energy, and therefore have sufficient energy for an effective collision
This is shown by the combined number of particles in the light and dark shaded areas
As a result of this, the rate of the catalysed reaction is increased compared to the uncatalysed reaction
Example of heterogeneous catalyst reaction
Ammonia production via the Haber Process
Steps of Haber process
Hydrogen and nitrogen are obtained from natural gas and air respectively and pumped into the compressor
The gases are compressed to about 200 atmospheres
The pressurised gases, at around 450 oC, are pumped into a tank containing beds of solid iron which is a heterogeneous catalyst, where some of the hydrogen and nitrogen react to form ammonia
Unreacted hydrogen, nitrogen and the ammonia product pass into a cooling tank where the ammonia is liquefied and removed
The unreacted hydrogen and nitrogen are recycled back into the system and start over again
Temperature on equilibrium
why not a higher or lower temperature
A higher temperature favours the reverse reaction as it is endothermic (takes in heat) so a higher yield of reactants would be made
Lower temperature favours the forward reaction as it is exothermic (releases heat) so a higher yield of products will be made
However, at a lower temperature the rate of reaction is very slow
So 450ºC is a compromise temperature between having a lower yield of products but being made more quickly
pressure on equilibrium
why not lower or high pressure
Lower pressure favours the reverse reaction as the system will try to increase the pressure by creating more molecules (4 molecules of gaseous reactants) so a higher yield of reactants will be made
Higher pressure favours the forward reaction as it will try to decrease the pressure by creating less molecules (2 molecules of gaseous products) so a higher yield of products will be made
However, high pressures can be dangerous and very expensive equipment is needed
So, 20 MPa is a compromise pressure between a lower yield of products being made safely and economically
adsorption reaction and desorption
Diffusion of the nitrogen and hydrogen gas to the iron surface
Adsorption of the reactant molecules onto the iron surface by forming bonds between the iron and reactant atoms
These bonds are so strong that they weaken the covalent bonds between the nitrogen atoms in N2 and hydrogen atoms in H2
But they are weak enough to break when the catalysis has been completed
The reaction takes place between the adsorbed nitrogen and hydrogen atoms which react with each other on the iron surface to form NH3
Desorption occurs when the bonds between the NH3 and iron surface are weakened and eventually broken
The formed NH3 diffuses away from the iron surface
Benefit of catalyst
Catalyst use lowers production costs. By allowing for the reaction to take place at a lower temperature and pressures, which decreases energy demands; as well as by allowing more product to be formed in less time.
If industrial processes can operate at lower temperatures and pressures, less fossil fuels have to be burned and CO2 emissions are reduced.
Using catalysts improves at the atom economy of the reaction hence producing less unwanted by-products.
What does the rate of chemical reaction depend on
collision frequency
collision energy
activation energy
what is a successful and unsuccessful collision
successful / effective collision is where the particles collide in the correct orientation and with sufficient energy for a chemical reaction to occur
An unsuccessful / ineffective collision is when particles collide in the wrong orientation or when they don’t have enough energy and bounce off each other without causing a chemical reaction
