Kinetics Flashcards
Collision theory
-for a reaction to take place two particles must collide with enough energy to break bonds
-collision must take place in the part of the particle that are going to react
-particles need to be moving fast to have enough energy so need lots of particles in a small volume
Factors effecting rate of reaction
- increasing temperature
-increasing concentration of a solution
-increasing pressure of a gas reaction
-increasing the surface are of a solid reactant
-using a catalyst
Factors effecting rate of reaction - increasing temperature
increases the kinetic energy of the molecules increasing their energy and therefore the number of successful collisions
Increasing concentration of a solution -
if there are more particles present in a given volume collisions are more likely to occur so therefore more successful collisions so a faster rate.
However the concentration of the reactants will decrease as time goes on so the rate will therefore drop
Increasing the pressure of a gas reaction
Same effect as increasing concentration as there are more particles in a given volume so successful collisions are more likely.
Increasing surface area of solid reactants
the greater the total surface area the more particles are available to collide this means that breaking a solid into smaller pieces increases the rate as there are more sites for the reaction to occur
Using a catalyst
A substance that will increase the rate of a reaction without being chemically changed itself
Activation energy
The minimum energy needed to start a reaction to be able to break bonds
What is the name of the species that is labeled on the top curve of an enthalpy diagram?
Transition state or activated complex
-some bonds are in the process of being made and some are being broken
What is a maxwell-boltzmann distribution graph?
Shows the distribution of energy amongst particles.
Features shown on a mb distribution graph
- no particles have zero energy
- most particles have intermediate energies around the peak of the curve
- a few have very high energies
Points on a mb distribution graph
-most probable energy = peak
- average
-there is no upper limit so line will never touch x-axis
Activation energy on a mb distribution graph
-where the activation energy is marked on the graph the area under the graph to the right of where activation energy is plotted is the number of particles with enough energy to react
why do not all exothermic reactions take place at room temperature?
activation energy must be present before a reaction can occur
How does the mb distribution graph change with temperature?
Total area under the graph remains the same
At higher temperatures:
-peak is lower
-peak moves to the right
-number of particles with very high energy increases
-more particles have activation energy providing evidence for how it increases rate of reaction
At lower temperatures:
What are catalysts used for?
- to speed up reactions
-used in industry as it is cheaper
-product is not used up
How do catalysts work?
-provide an alternative pathway for a reaction of lower activation energy reducing the activation energy of a reaction
-there will be more particles making successful collisions so on the mb distribution graph there will be a larger area under the section of the graph.+-
-they do not effect the enthalpy change of a reaction or affect the position of equilibrium
Catalytic converters
-reduce pollutants from car fumes
-honeycomb made of a ceramic material coated in platinum and rhodium metals
-shape provides a large surface area
-gases pass over the catalyst and form less harmful products
Reaction occurring in a catalytic converter
carbon monoxide + nitrogen oxides -> nitrogen + carbon dioxide
hydrocarbons + nitrogen oxides -> nitrogen + carbon dioxide + water
Explanation of the reactions in a catalytic converter
1) gases first from weak bonds with the metal atoms of the catayst - proccess called adsorption
-this holds gases in the correct position for them to react together, gases then react on the surface
2) the products then break away from the metal atoms - process called desorption
-this frees up room on the catalyst surface for more gases to react
The strength of the weak bonds holding the gases onto the metal is critical as they need to strong enough to hold the gases for long enough to react but weak enough to release products easily
Types of catalyst
-zeolites which are minerals of which have a very open pore structure that molecules can fit into (synthetic ones are used in industry)
-nickel (used for the hardening of fats)
