8. Kinetics Flashcards
What are the optimum conditions for a fast reaction rate?
We need plenty of rapidly moving particles in a small volume.
For a reaction to take place between two particles what conditions must be fulfilled?
They must bond have enough energy (more than the activation energy.
They must both be orientated correctly.
List the factors which increase the rate of reaction
- Increasing the temperature
- Increasing the concentration of a solution
- Increasing the pressure of a gas reaction
- Increasing the surface area of solid reactants
- Using a catalyst
How does increasing the temperature increase the rate of reaction?
This increases the speed if the molecules which in turn increases both their energy and the number of collisions.
How does increasing the concentration of a solution increase the rate of reaction?
If there are more particles present in a given volume, collisions are more likely. However, as a reaction proceeds the reactants are up and their concentration falls so it means the rate drops as the reaction continues.
How does increasing the pressure of a gas reaction speed up the rate of reaction?
There are more molecules or atoms in a given volume so collisions are more likely
How does increasing the surface area of solid reactants increase the rate of reaction?
The greater the total surface area of a solid, the more of its particles are available to collide with molecules in a gas or liquid.
How does using a catalyst increase the rate of reaction?
It can change the rate if a chemical reaction without being chemically changed itself.
What kind of proportion of collisions result in a reaction?
Only a very small proportion.
What is the minimum energy called that a molecule must have to start a reaction?
The activation energy
Abbreviation: Ea
Reactions which need large activation energies take place…
Extremely slowly at room temperature because very few collisions will have sufficient energy to bring about a reaction.
Reactions which need small activation energies take place…
Rapidly at room temperature because many collisions will have enough energy to bring about a reaction.
What is the species that exists at the top of the curve of an enthalpy diagram called?
A transition state or activated complex
Where is the activation energy measure from?
The activation energy is measured from the reactants to the top of the curve.
What do we end up with if we plot a graph of ‘energy’ against ‘fraction of molecules’?
The Maxwell-Boltzmann distribution
What does the Maxwell-Boltzmann distribution tell us about the distribution of energy amongst particles?
- no particles have zero energy
- most particles have intermediate energies - the peak of the curve
- a few have very high energies (Infact there is no upper limit)
NB: the average energy is not the same as the most probable energy
If we mark Ea on a MB distribution what does the area to the right of the line under the graph represent?
The number of particles with enough energy to react.
What does the need of an activation energy to be present before a reaction can take place explain?
The fact that exothermic reactions do not occur spontaneously at room temperature.
At higher temperatures how is the shape of the Maxwell-Boltzmann distribution changed?
The peak of the curve is lower and moves to the right. The number of particles with very high energy increases.
The area under the curve stays…
The same for each temperature.
The total number of collisions in a given time increases a little as the particles move faster. However, this is not as important to the…
rate of reaction as the increase in the number of effective collisions due to a higher proportion of molecules have energy above the activation energy.
How do catalysts work?
Catalysts work because they provide a different pathway for the reaction, one with a lower activation energy. Thus they reduce the activation energy that is needed to start the reaction.
Do catalysts change the shape of the Maxwell-Boltzmann distribution?
No, they just lower the Ea so more molecules have enough energy to start a reaction.
What two things do catalysts not effect?
The enthalpy change of the reactions
The position of equilibrium in a reversible reaction (both forward and reactions are speed up equally as much)
How were most catalysts discovered?
By trial and error
What two categories are catalysts divided into?
Heterogeneous catalysts
Homogeneous catalysts
What is a heterogenous catalyst?
A catalyst which is in a different phase to the reactants - usually a solid catalyst and liquid or gaseous reactants
What is a homogeneous catalyst?
Where catalyst and reactants are in the same phase
See catalytic materials notes
This is in unit 2 as well as unit 1
What reactions take place on catalytic converters?
- The gases first form weak bonds (the strength is critical) with the metal atoms of the catalyst, this process is called adsorption. This means the gases are in just the right position to react together.
- The products break away from the metal atoms (desorption). This frees up room on the catalyst surface for more gases to take their place and react.
What are widely used as catalysts in the petrochemical industry?
Zeolites - minerals which molecules fit into which changes their structure and reactivity.
Margarines are hardened by…
Bubbling hydrogen through them
Using a nickel catalyst
CFCs were used in chemical applicants. They escaped to the upper atmosphere because they are relatively unreactive due to the strength of the carbon-halogen bond. What do the CFCs eventually decompose into?
Chlorine atoms which act as catalysts to break down the ozone layer which protects from ultraviolet radiation
Nitrogen monoxide acts in a similar way to chlorine atoms.
