Kinetics I Flashcards
Homogeneous catalyst
Catalyst in the same state as the reactant
Heterogeneous catalyst
catalyst in a different state to the reactant
Advantages of heterogeneous catalysts?
-More frequently used in industry because they’re easier to separate (e.g. if they are solid) so they can be reused + they don’t get mixed up with the product
Economic Benefits of catalysts?
Because catalysts lower activation energy it means less input energy is required so it costs less.
If the reaction can occur at lower temperatures then less money goes into protecting the reaction chamber from heat loss to the surroundings
Because most industrial processes are exothermic (eg. Haber), at lower temperatures, the equilibrium shifts right and increases the yield
How to: ??????
1) Find initial rate
2) Find rate of reaction between two points if it is a proper curve
3) Find AVERAGE rate of reaction between two given points
1) Draw a tangent against the very FIRST plotted point
2) Draw separate tangents at intervals between the two time points
3) Draw tangent to curve at those two given points, add their gradients and divide by 2.
What does the collision theory state?
A reaction will not take place between two particles unless they collide in the correct orientation and at least a certain minimum amount of energy (activation energy)
What does the Maxwell-Boltzmann graph show?
What does the area under the graph show?
How many molecules have a given amount of energy at a particular time.
Total number of molecules in the reaction mixture
What to note about energy of molecules at a given temperature?
energy is always dispersed UNEQUALLY. Eg. at 50 degrees not ever single molecule would have 40J
On a Maxwell Boltzmann graph what can you say about molecules either side of the activation energy?
How is this depicted?
All molecules under the curve to the left of the activation energy will not successfully react (because they haven’t reached the activation energy required) whereas
all the molecules to the right will react (because they have energies equal to or greater than the activation energy required)
This will be depicted because the one with the lower activation energy will have a greater area under the curve to the right of the activation energy (because more molecules have the energy equal to or greater than the activation energy required)
How does a catalyst work?
Provides a surface over which particles can successfully collide to react.
This provides an alternate reaction pathway which lowers activation energy.
The catalyst has an active site and provides this area for particles to bind to and when the product is formed it is released from the catalyst.
Effects of increasing
concentration SA catalyst temperature pressure
X
know this - fill in later
How does a Maxwell - Boltzmann curve showing temperature at T1 (where T1 is a lower temp) change when temperature increases?
imagine the tallish curve on the left which usually represents T1
The curve decreases in height and moves to the right (this is the T2 peak)
If a Maxwell-Boltzmann is moving left or right (it looks like this as a result of the width of the curve changing), why does its height also change?
Because when a curve moves, the reaction itself is the same (the energies of molecules change - not the reaction itself) so there still needs to be the same number of particles in the reaction mixture and therefore the same area under the graph.
[area under graph = no. molecules in reaction mixture]
Maxwell Boltzmann graph can only be applied to
GASES
How to change the position of the activation energy on a Maxwell-Boltzmann curve?
add a catalyst (Ea moves left) meaning more pa than the Ea.
What to remember about the particles on the LHS of Ea?
don’t think they never react because they don’t have enough energy - this isn’t true becaus eone instant later that particle could suddenly have enough energy to react
How to get my head around the larger area under the curve and after Ea when it shifts to the right
you have two curves on top of each other, exactly the same shape (imagine standard MB curve)
imagine a vertical line crossing down through both curves near to when they are going down to become parallel with the x axis
if one curve moves right now more of its thickness will be on the RHS past the vertical line
Explain why the two Maxwell-Boltzmann temperature curves (high vs low temp) are different shapes?
the higher temp curve has moved right and flattened because the number of particles with energy equal to or exceeding activation energy has increased but there are still the same number of particles in the reaction mixture and so the surface area under the graph must be the same
activation energy
minimum collision energy needed for particles to react in a collision. If the particles have an Ea greater than or equal then they will also have to be in the correct orientation to react
What is the order of vertical lines on a Maxwell-Boltzmann diagram?
first line (nearer to LHS but still on far right) - line for if there is a catalyst (Ecat) second line (Far right) - activation energy (Ea)
What to remember about the MZ graph on effect of temperature?
two graphs, one activation energy
What to remember about the MZ graph on effect of a catalyst?
one graph, two activation energies
Imagine the vertical Ea line on an MB graph - how is this represented in a reaction profile diagram?
horizontal line reactant, horizontal line product to the right and below
in between a small curve jumping from both lines (its height starting from the reactant line is the Ea.
What does the reaction profile for a catalyst look like?
How many curves?!
TWO CURVES!
horizontal line reactant (1), horizontal line product to the right and below(2)
IN BETWEEN - a small horizontal line(3) in between and just above the first line (this is the intermediate)
small curve going from (1) to (2) [its height from the reactant line is the Ea]
another small curve going from (2) all the way down to (3)
see sergy boy’s notes
chemguide version of what a catalyst reaction profile looks like
horizontal line reactant (1), horizontal line product to the right and below(2)
first tall curve from (1) to (2) - [line from top of tall curve to the reactant line is the Ea without a catalyst]
second shorter curve from (1) to (2) - [line from top of short curve to the reactant line - Ea with a catalyst]
what do catalysts provide? and what happens to their bonds?
a surface on which the reactants molecules can bind, as they do, their bonds get weaker
example of a heterogenous catalyst
solid iron catalyst in Haber process is in a different phase to the nitrogen and hydrogen
two examples of reactions (other than Haber) that use heterogenous catalysts
oxidation of ammonia by air catalysed by platinum
manufacture of hydrogen from steam and methane catalysed by nickel
typical reaction route with a catalyst
gaseous reactants onto metal to form adsorbed reactants, go on to make adsorbed products (slow step), go on to become gaseous products and the catalyst surface is free again
explain why producing ammonia in the Haber process would be an issue if there was no iron catalyst
the strength of the nitrogen-nitrogen triple bond means the reaction has a very high activation energy. Without the iron catalyst this would mean it would have to occur at super high temperatures and therefore because the reaction is exothermic, favor the backwards reaction, producing less ammonia. This is unfavorable so we use an iron catalyst so the reaction can happen at a reasonable temperature and we can still get a high yield.
