Types of Catalysis
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What are the three main types of catalysis?
Homogenous catalysis Heterogeneous catalysis Autocatalysis
Define homogenous catalysis
A catalysis in which both the reactants and the catalyst are in the same phase
Define a phase.
A specific state of matter that has uniform chemical and physical properties
What is an example of homogeneous catalysis?
The ‘iodine snake’/ the reaction between concentrated potassium iodide solution and hydrogen peroxide solution
Why is the ‘iodine snake’ considered a homogenous catalysis?
As both liquids are in the same phase
Define heterogeneous catalysis.
Catalysis in which the reactants and the catalyst are in different phases
What is an example of heterogeneous catalysis?
The reaction between methanol vapour and oxygen gas to form methanal using platinum as the catalyst
Describe the 9 steps in the reaction between methanol vapour and oxygen gas using platinum as a catalyst to form methanal
• 25 cm cubed of methanol is placed in a conical flask • A piece of cardboards in the shape of a ‘T’ is covered in aluminium foil and suspended in the conical flask • The methanol is heated to 60 degrees Celsius in a fume cupboard and is then placed on a heat resistant mat • A spiral of platinum wire is heated until red hot over a Bunsen burner using tongs • The hot spiral of platinum wire is quickly hung inside the conical flask • The platinum now undergoes a cycle of glowing red hot and cooling again as the reaction proceeds • ‘Pops’ will be heard as one of the products, hydrogen explodes • The smell of methanal will be noticed • A flame may be observed in the flask
Why is the T covered in aluminium foil during the reaction between methanol vapour and oxygen gas using platinum as a catalyst to form methanal?
To prevent the cardboard from burning
Why does the platinum glow during the reaction between methanol vapour and oxygen gas using platinum as a catalyst to form methanal?
The platinum glows as the reaction taking place is exothermic (gives out heat)
Why will the platinum cease to glow for a while before becoming red hot again during the reaction between methanol vapour and oxygen gas using platinum as a catalyst to form methanal?
This occurs as carbon monoxide is formed due to incomplete combustion of the methanol which poisons the catalyst but once this gas leaves the platinum it will begin to glow again
What causes the flame inside the conical flask during the reaction between methanol vapour and oxygen gas using platinum as a catalyst to form methanal?
The hot wire igniting the methanol vapour causes the flame
What extinguishes the flame which may form as a result of the methanol igniting during the reaction between methanol vapour and oxygen gas using platinum as a catalyst to form methanal?
When the flame uses up the oxygen in the flask the flame will be extinguished
What happens to methanol in the absence of sufficient oxygen?
Methanol oxidises to methanal and hydrogen gas
Define autocatalysis
Catalysis in which one of the products of the reaction act as a catalyst for the reaction
Describe the rate of reaction throughout an autocatalytic reaction.
An autocatalytic reaction starts slowly but speeds up as the catalyst product starts to form
Give an example of an autocatalytic reaction.
The reduction of manganate (VII) ions
How is the reduction of manganate (VII) ions autocatalytic?
As when the first few drops of the purple MNO4- solution are added to the Fe2+ solution they are decolourised slowly but subsequent drops are decolourised rapidly i.e. the Mn2+ formed by the initial slow reaction catalyses the subsequent reaction