P1: Kinetics Flashcards
Explain the 3 key points of ‘collision theory’
- For a reaction to take place between two particles, they must collide with enough energy to break bonds, and be in the correct orientation
- To get a lot of collisions, you must have a lot of particles in a small volume, and must be moving fast to break the bonds.
- Most collisions between molecules or other particles do not lead to a reaction.
What are the 5 factors affecting rate of reaction and explain them?
- INCREASING TEMPERATURE: increases both energy and number of collisions, due to a speed increase.
- INCREASE IN CONC OF SOLUTION: more particles are present, so collisions are more likely, reaction is faster. ( same for an ^ pressure in gas).
- INCREASE SURFACE AREA: more particles are available to collide e.g cube vs granulated sugar
- CATALYST: lowers the activation energy without being used up itself, speeds it up, increases proportion of successful collisions.
These increase the frequency of collisions
Define activation energy (Ea)
The minimum energy required for a reaction to occur.
What does the ‘Maxwell Boltzmann Distribution’ tell us?
The distribution of energy amongst the particles
What are some key points to note about the ‘Maxwell Boltzmann Distribution’?
- No particles have zero energy
- Most particles have intermediate energy (around the peak of the curve)
- A few have very high energies, there is no upper limit
- Average energy is not the same as the most probable energy.
How would you work out the total number of particles using the ‘Maxwell Boltzmann Distribution’?
Work out the area under the graph
What happens to the ‘Maxwell Boltzmann Distribution’ if we increase the temperature?
-The curve will get lower as there are more particles with greater energy
-Also a higher number of particles with greater energy than the activation energy.
-Will skew slightly more to the right of the original curve.
-area under the curve is the same.
Define what a Catalyst is
They provide a quicker alternate pathway or route for the reaction, one with a lower activation energy, and doesn’t get used up or changed itself.
What catalyst is used in The Haber Process and what is the reaction?
Iron
N2 (g). +. 3H2 (g). »_space; 2NH3 (g)
What catalyst is used to harden fats with hydrogen and what is the reaction?
Nickel
H2C=CH2 + H2»_space;> CH3CH3
What are the catalysts in a catalytic converter in car exhausts?
Platinum and Rhodium
What catalysts are used to crack hydrocarbon chains from crude oil?
Zeolite, aluminium oxide, and silicon dioxide.
What does a catalytic converter do?
Reduces the number of polluting gases coming out of petrol-engine car exhausts.
What is the structure of a catalytic converter?
-A honeycomb ceramic material, coated with platinum and rhodium metals.
-Honeycomb shape provides a large surface area.
What is the word equation for a catalytic converter?
Carbon monoxide + nitrogen oxides»_space;> nitrogen + carbon dioxide
What is Adsorption?
1st reaction that takes place on the surface of the catalyst.
-The gases first form weak bonds with the metal atoms of the catalyst: this holds the gases loosely, to react together.
What is Desorption?
2nd reaction to take place on the surface of the catalyst.
-Products break away from metal atoms, freeing up room on the catalyst surface for more gases to take their place and react.
Define rate of reaction
Measurement of change in concentration or amount of products or reactants in unit time.
What is a transition state?
The top of the curve in and energy level diagram, bonds are being broken and made.
Discuss an exothermic reaction with regards to activation energy.
A large activation energy means a slower reaction.
Smaller activation energy means faster reaction as it requires less energy to get going.
Talk about endothermic energy
Products have more energy than reactants.
Why don’t most collisions between gas-phase reactants not lead to a reaction?
The molecules do not have enough energy (sufficient).
Why does a small increase in temp lead to a large increase in rate of reaction between colliding particles?
MANY more particles will have energy greater than the activation energy, so more successful collisions.
Explain how a catalyst works
Creates an alternate route for reaction, lowering the activation energy= faster rate of reaction.
How do you work out rate?
Rate= 1/ time
What is a Heterogeneous catalyst?
Where the catalyst is in a different state/phase to the reactants.
e.g reactant: aq
catalyst: solid
What is a Homogeneous catalyst?
Where the catalyst is in the same state/phase as the reactants.
e.g reactant: aq
catalyst: aq
How do catalysts speed up reactions? (Homogeneous)
They lower the activation energy, so more particles can react as the have sufficient energy, therefore a faster reaction. They create an intermediate.
How do Heterogeneous catalysts work?
The reaction occurs on the surface of the catalyst through adsorption, binds to active site, then desorption.
If two molecules collide but don’t have enough energy to react, what happens?
They will bounce off each other, and one may gain kinetic energy from the other
Where is the highest point of energy in a reaction profile?
The transition state (the peak)
What happen to the Maxwell Boltzmann Distribution if you decrease the temperature?
-The peak skews to the left of the original curve
-The peak is higher
-The activation energy is lower
What are the labels on the X and Y axis of a reaction profile?
X= Progress of reaction
Y= Energy
What are the X and Y axis of a Maxwell Boltzmann Distribution ?
X= Energy
Y= Number of particles/molecules
Where should the most probable energy, and average energy be on a Maxwell Boltzmann Distribution?
Most probable: The centre of the peak
Average: Just to the right of the most probable
What 2 factors increase the proportion of successful collisions, therefore increasing rate of reaction?
Kinetic energy: the faster they move the more successful collisions as they have enough energy to break bonds
Orientation: some may repel each other and will not collide
Briefly describe the required practical for rate of reaction
-Place 10cm3 of 0.05 mol/dm-3 of sodium thiosulfate into a test tube and place over the cross in water
-note the starting temperature
-add 1cm3 of hydrochloric acid and start timing
-stop timer when you can no longer see the cross and record temp
-take an average of the time
-repeat with different temps
-plot on graph
