Kinetics I Flashcards
(16 cards)
A reaction won’t take place between 2 particles unless…
- They collide in the right direction
- They collide with at least a certain minimum amount of kinetic energy
What do you get when you plot a graph of the number of molecules in a substance with different kinetic energies?
A Maxwell-Boltzmann Distribution
What are the important parts of the Maxwell-Boltzmann distribution curve?
- It starts at (0,0) because no molecules have 0 energy
- x-axis = KE and y-axis = number of particles
- The highest point = the modal, and a little to the right of this is the mean
- Activation energy is at the right end of a curve
- The area under the curve from the activation energy to the end of the curve = the number of particles with sufficient energy to react
- Only a few molecules are moving slowly
How does temperature increase rate of reaction?
- Particles will have more KE and will move faster
- So a greater proportion of particles will have sufficient energy to react
- The particles will collide more often, so there will be more successful collisions
How does a temperature increase change the shape of the Maxwell-Boltzmann distribution curve?
Peak will be lower (up until the intersection) and shifted to the right
Area under curve stays the same
How does increasing concentration increase the rate of reaction?
There will be more particles in a given volume of the solution, so the particles will collide more frequently. Which means they’ll have more chance to react.
How does increasing pressure speed up the reaction?
There will be more particles in a given volume of the gas, so the particles will collide more frequently. Which increases the frequency of successful collisions.
How do catalysts increase the rate of reactions?
They lower the activation energy by providing a different reaction pathway. So more particles have enough energy to react.
Reaction rate definition?
The change in amount of reactant/product per unit time.
To work out the initial RoR, draw a tangent at time = _____
Time = 0
Catalyst definition
A substance that increases the RoR by providing an alternative reaction pathway with a lower activation energy, so greater proportion of collisions result in a reaction. The catalyst is chemically unchanged at the end of the reaction.
What is heterogenous catalysis? Include an example
- A catalyst that’s n a different phase from the reactants.
- E.g. in the Haber process, gases are passed over a solid iron catalyst
- Increasing the SA of the catalyst increases the number of molecules that can react at the same time, increasing the RoR.
How do solid heterogenous catalysts work?
- Reactant molecules arrive at the surface and bond with the solid catalyst - adsorption
- Bonds between the reactants atoms are weakened and break up. This forms radicals, which get together and make new molecules.
- The new molecules are then detached from the catalyst - desorption
What is homogenous catalysis?
Catalysts in the same physical state as the reactants.
- Usually it’s an aqueous catalyst for a reaction between 2 aqueous solutions.
- During homogenous catalysis, the reactants combine with the catalyst to make an immediate species, which then reacts to form the products and reform the catalyst.
On a Maxwell-Distribution Curve, is the Ea with a catalyst to the right or left of the Ea without a catalyst?
Left
Other than lowering the cost, what other pro of catalysts is there?
- They can change the properties of a product to make it more useful.
- E.g. polyethene can be made more dense, rigid and have a higher MPT with a catalyst
