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.

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Reaction path

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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

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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

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Potential energy graphs and catalysts

P.E.

50 -

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Reaction path

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∆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.