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Catalysts
Learning intention
Learn how a catalyst speeds up reaction rate by lowering the activation energy, and how to represent this on a potential energy diagram.
Catalysts at WorkHeterogeneous
Homogeneous
Enzymes are biological catalysts, and are protein moleculesthat work by homogeneous catalysis. E.g. invertase and lactase.Enzymes are used in many industrial processes
When the catalyst and reactants are in different statesyou have ‘Heterogeneous Catalysis’. They work by the adsorption ofreactant molecules.
E.g. Ostwald Process (Pt) for making nitric acid and the HaberProcess (Fe) for making ammonia and the Contact Process (Pt)for making Sulphuric Acid.
When the catalyst and reactants are in the same stateyou have ‘Homogeneous Catalysis’. E.g. making ethanoic acid from methanol and CO using a soluble iridium complex.
How a heterogenous catalyst works
Heterogenous Catalysis are thought to work in three stages...
Adsorption Reaction DesorptionAdsorption Reaction Desorption
Higher Chemistry Eric Alan and John Harris
For an explanation of what happens click on the numbers in turn, starting with
How a heterogenous catalyst works
Adsorption (STEP 1)Incoming species lands on an active site and forms bonds with the catalyst. It may use some of the bonding electrons in the molecules thus weakening them and making a subsequent reaction easier.
How a heterogenous catalyst works
Adsorption (STEP 1)Incoming species lands on an active site and forms bonds with the catalyst. It may use some of the bonding electrons in the molecules thus weakening them and making a subsequent reaction easier.
Reaction (STEPS 2 and 3)Adsorbed gases may be held on the surface in just the right orientation for a reaction to occur.This increases the chances of favourable collisions taking place.
How a heterogenous catalyst works
Desorption (STEP 4)There is a re-arrangement of electrons and the products are then released from the active sites
Adsorption (STEP 1)Incoming species lands on an active site and forms bonds with the catalyst. It may use some of the bonding electrons in the molecules thus weakening them and making a subsequent reaction easier.Reaction (STEPS 2 and 3)Adsorbed gases may be held on the surface in just the right orientation for a reaction to occur.This increases the chances of favourable collisions taking place.
How a heterogenous catalyst works
Metals Ni, Pt hydrogenation reactions Fe Haber Process Rh, Pd catalytic converters
Oxides Al2O3 dehydration reactions
V2O5 Contact Process
Format FINELY DIVIDED increases the surface areaprovides more collision sites
IN A SUPPORT MEDIUM maximises surface area and reduces
costs
Examples of heterogenous catalysts
In some cases the choice of catalyst can influence the products
C2H5OH ——> CH3CHO + H2 C2H5OH ——> C2H4 + H2O
Ethanol undergoes different reactions depending on the metal used as the catalyst.The distance between active sites and their similarity with the length of bondsdetermines the method of adsorption and affects which bonds are weakened. Alumina DehydrationCopper Dehydrogenation (oxidation)
How a heterogenous catalyst works
Poisoning
Impurities in a reaction mixture can also adsorb onto the surface of a catalyst thus removing potential sites for gas molecules and decreasing efficiency.
expensive because... the catalyst has to replaced the process has to be shut
down
examples Sulphur Haber processLead catalytic converters in cars
How a heterogenous catalyst works
Investigating Catalysis
The problem
Transition metals and their compounds are said to be effective catalysts
You are asked to link the transition metal compounds to their ability to act as a catalyst for the decomposition of hydrogen peroxide
Materials available
hydrogen peroxide solution (5-10 vol)cobalt(II) chloride (aq)copper(II) sulphate (aq)manganese(II) chloride (aq)nickel(II) nitrate (aq)bench dilute sodium hydroxide
(approximately 1 mol l-1).
Decomposition of hydrogen peroxide
Hydrogen → water + oxygen2H2O2 → 2H2O + O2
What will you see during the reaction?
What is the test for oxygen?
Testing the catalysts
Collect 5 test tubes in a rack
To each add 5 cm3 of hydrogen peroxide
Add 3 cm3 of a different catalyst to 4 test tubes, leaving the last one as a control
Note the results
Which type of catalysis?
Did the experiment involve heterogeneous or homogeneous catalysis?
Which catalyst was the most effective?
Changing the solubility of the catalysts
The transition metal ion in the compound has the catalytic effect.
The ion can be found in solution (aqueous) or in a solid state.
How could you use the chemicals provided to create transition metal ions in the solid state i.e. a heterogeneous catalyst?
Investigate the effect of changing the state of the
catalysts
Which was the best catalyst overall?
http://media.rsc.org/Creative%20Problem%20Solving/CPS-17.pdf
An Example of a homogenous catalyst
Dissolve 4 spatulas full of potassium sodium tartrate in a small beaker with about 2.5 cm depth of water. Repeat with another beaker (the control)
To one beaker, add enough 10% cobalt chloride solution (catalyst) for the solution to be pink.
Add 10 ml hydrogen peroxide to the beakers.
Heat the control to around 60 - 80 oC and note observations
Now heat the one with the catalyst and note observations
An Example of a homogenous catalyst
Higher Chemistry Eric Alan and John Harris
Catalysts work by providing…
“AN ALTERNATIVE REACTION PATHWAY WHICH HAS A LOWER ACTIVATION ENERGY”
Catalysts and Potential energy diagrams
Potential energy graphs and catalysts
P.E.
50 -
25 -
Reaction path
75 -
60 -
Reactants
Products
Uncatalysed
Catalysed
Uncatalysed
Catalysed
Potential energy graphs and catalysts
Catalysts lower the activation energy needed for a successful collision.
Activation energy Ea for the forward uncatalysed reaction
P.E.
Reactants
Products
Reaction path
Activation energy Eafor the forward catalysed reaction
50 -
25 -
75 -
60 -
Potential energy graphs and catalysts
Catalysts lower the activation energy needed for a successful collision.
Activation energy Ea for the reverse uncatalysed reaction reaction
P.E.
Reactants
Products
Reaction path
Activation energy Ea for the reverse catalysed reaction
50 -
25 -
75 -
60 -
Potential energy graphs and catalysts
P.E.
50 -
25 -
Reaction path
75 -
60 -
∆H Activation energy
Effect of catalyst – forward reaction No change Lowered
Effect of catalyst – reverse reaction No change Lowered
Catalysts lower the activation energy needed for a successful collision.
Reactants
Products
CatalystsA catalyst speeds up the reaction by lowering the activation energy.
A catalyst does not effect the enthalpy change for a reaction
A catalyst speeds up the reaction in both directions and therefore does not alter the position of equilibrium or the yield of product, but does decrease the time taken to reach equilibrium.
Energy distribution and catalysts
Total number of collisions (area underthe graph) with sufficient K.E. energy to create new products.
Catalysed reactionEa is reduced
No ofCollisionswith a givenK.E.
Kinetic energy
Ea
Ea
Un-catalysed reaction
Concentrated solutions of hydrogen peroxide are used in the propulsion systems of torpedoes. Hydrogen peroxide decomposes naturally to form water and oxygen:
2H2O2(aq) → 2H2O(ℓ) + O2(g) ΔH = −196∙4 kJ mol–1
Transition metal oxides act as catalysts in the decomposition of the hydrogen peroxide.
Unfortunately, there are hazards associated with the use of hydrogen peroxide as a fuel in torpedoes. It is possible that a leak of hydrogen peroxide solution from a rusty torpedo may trigger an explosion.
Using your knowledge of chemistry, comment on why this could happen.