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
How can collision frequency be altered
Changing the concentration of the reactants
Increasing the concentration will mean that there are more particles available to react in the same volume / amount of space leading to more frequent, successful collisions
Changing the total pressure
Increasing the pressure means that there will be the same number of particles but in a smaller volume leading to more frequent, successful collisions
Changing the temperature
This will increase the kinetic energy of the reacting particles, ultimately, resulting in more frequent, successful collisions
Changing the size of the reacting particles
This is achieved by increasing the surface area which means that there are more particles available to react in the same volume / amount of space leading to more frequent, successful collisions
activation energy in exo and endo reactions
In exothermic reactions, the reactants are higher in energy than the products
In endothermic reactions, the reactants are lower in energy than the products
the Ea in endothermic reactions is relatively larger than in exothermic reaction
Will reaction take place if don’t posses activation energy and have correct orientation
Even though particles collide with each other in the same orientation, if they don’t possess a minimum energy that corresponds to the Ea of that reaction, the reaction will not take place
Therefore, for a collision to be effective the reactant particles must collide in the correct orientation AND possess a minimum energy equal to the Ea of that reaction
do catalyst have any effect on yield
no only increase rate of reverse reactions
how can effectiveness of heterogeneous catalyst be decreased
presence of impurities
Adsorb onto catalyst surface and occupy active sites
Prevent bond weakening in the reactants
‘Take up’ the surface area of catalyst by forming strong bonds to surface of catalyst so are less likely to desorb from surface of catalyst
what is an unsuccessful collision
colliding species do not have enough energy to break the necessary bonds. The collision will not result in a chemical reaction
What do reactants need to overcome
activation energy (Ea)
In exothermic reactions, the reactants are higher in energy than the products
In endothermic reactions, the reactants are lower in energy than the products
Therefore, the Ea in endothermic reactions is relatively larger than in exothermic reaction
will reaction take place if same orientation but not same minimum energy
for a collision to be effective the reactant particles must collide in the correct orientation AND possess a minimum energy equal to the Ea of that reaction
how can you measure rates of reaction.
mass loss
gas production
what are the three things that the rate of a chemical reaction depends on
collision frequency
collision energy
activation energy
what is a successful collision
where the particles collide in the correct orientation and with sufficient energy for a chemical reaction to occur / possess a minimum energy equal to the Ea of that reaction
what is an unsuccessful collision
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
how can the collision frequency be altered
Concentration
Increasing the concentration will mean that there are more particles available to react in the same volume
Pressure
Increasing the pressure means that there will be the same number of particles but in a smaller volume leading to more frequent, successful collisions
Temperature
increase the kinetic energy of the reacting particles, ultimately, resulting in more frequent, successful collisions
Size of the reacting particles
increasing the surface area which means that there are more particles available to react in the same volume
in which type of reaction is the Ea higher
Ea in endothermic reactions is relatively larger than in exothermic reaction
explain how impurities in gaseous reactants could make catalyst less effective
impurities adsorb onto (catalyst) surface
(1)
impurities prevent bond weakening in the reactants
less surface area (of catalyst) / fewer active sites available for
(1)
impurities form strong bonds (to surface)
impurities less likely to desorb (from surface)
explain how using catalyst increases the rate constant
the catalyst provides an alternative pathway of lower activation energy
(1)
* so a greater proportion of molecules / more molecules have energy greater than the activation energy (so faster reaction)
