5: Catalysis Flashcards
What is a catalyst?
A substance that speeds up rate of reaction without being used up, by providing an alternative reaction pathway with a lower activation energy
What is a heterogenous catalyst?
A catalyst that is in a different state to the reactants
What are homogenous catalysts?
Catalysts that are in the same state as reactants
In heterogenous catalysis, what state are reactants and catalysts in?
Solid catalyst, gaseous reactant
Name the heterogenous catalyst used in the Haber process.
Iron
Give an equation for the Haber process.
N2 (g) + 3H2 (g) = 2NH3 (g)
Name the catalyst used in the contact process.
V2O3
Give an equation for the contact process.
SO2 (g) + 0.5 O2 (g) -> SO3 (g)
Where do heterogenous catalysts work during a reaction?
The reactant binds to the catalysts surface.
Explain the process of how heterogenous catalysts work.
- Reactants adsorb onto reactant surface
- Reaction take places
- Products desorbed
Give 2 issues that could happen if the heterogenous catalyst is too strong.
Reactants cannot move around the surface.
Products cannot desorb
Give 1 thing that could happen if the heterogenous catalyst is too weak.
Reactants may never adsorb.
Explain how catalytic converters are made efficient.
The catalyst is heterogenous, so is spread across the surface. Inside the catalytic converter is a honeycomb structure, increasing the surface area for attachment of the catalyst.
What is catalyst poisoning?
The active site of the catalyst becomes blocked, preventing the catalyst from binding to reactants, ruining its ability to speed up rate.
Describe how catalysts become poisoned.
Active site becomes blocked by substances that will not desorb.
Name an element that could poison the iron catalyst in the Haber process.
Sulfur
Name an element that could poison the catalyst in a catalytic converter.
Lead
What state are homogenous catalysts usually?
Aqueous
Briefly explain how homogenous catalysts work.
They form an intermediate species which reacts with products and then regenerates the original catalyst.
Describe the shape of an energy diagram showing the use of a homogenous catalyst.
The first peak shows the activation energy for the main reaction. The second peak shows the activation energy for reaction of intermediates.
Explain why the reaction of the equation shown is slow.
2I- + S2O8 2- -> I2 + 2SO4 2-
Both reactants have a negative charge and repel each other, making it unlikely that they will collide and react.
Give the ionic equation when you add an Fe2+ catalyst to this reaction.
Explain how this catalyst works to speed up rate of reaction.
2I- + S2O8 2- -> I2 + 2SO4 2-
2Fe2+ + S2O8 2- -> 2Fe3+ + 2SO4 2-
Addition of a positive ion means that there is no repulsion so particles collide and react.
Give the equation involving 2I- and 2Fe 3+ in the second stage of homogenous catalysis of S2O8 2-.
Explain what happens.
2I- + 2Fe 3+ -> 2Fe 2+ + I2
The iodide ions react with the new catalyst intermediate to produce iodine.
What is autocatalysis?
Where the product of a reaction acts as a catalyst.
How does rate of reaction change over progression of a reaction using autocatalysis?
As the reaction progresses, more product is produced. As more product is produced, this means that more catalyst is produced. Therefore, as the reaction goes on, it becomes faster as more and more catalyst is formed.
Give a redox equation for the autocatalysis involving 5MnO4- and 5C2O4 2-
2MnO4- + 5C2O4 2- + 16H+ -> 2Mn2+ + 8H2O + 10CO2
Give an equation for the formation of Mn3+ ion intermediates during autocatalysis of this reaction.
2MnO4- + 5C2O4 2- + 16H+ -> 2Mn2+ + 8H2O + 10CO2
4Mn 2+ + MnO4- + 8H+ -> 5Mn3+ + 4H2O
Give an equation for the reaction of Mn3+ intermediates with C2O4 2- ions during autocatalysis of this equation.
2MnO4- + 5C2O4 2- + 16H+ -> 2Mn2+ + 8H2O + 10CO2
2Mn 3+ + C2O4 2- -> 2CO2 + 2Mn 2+
What is an autocatalyst?
The product of a reaction acts as a catalyst
A student determines the concentration of ethanedioate ions in an acidified solution by
titration with potassium manganate(VII) solution.
2 MnO4
– + 5C2O4
2– + 16H+ → 2 Mn2+ + 10CO2 + 8H2O
The mixture is warmed before the addition of potassium manganate(VII) solution
because the reaction is slow at first. When more potassium manganate(VII) solution
is added, the mixture goes colourless quickly due to the presence of an autocatalyst.
Explain the meaning of the term autocatalyst.
Explain, using equations where appropriate, why the reaction is slow at first and then
goes quickly
M1 autocatalyst: product of the reaction catalyses the reaction
M2 slow: negative ions repel / ions of same charge repel
M3 high Ea
M4 attraction between oppositely charged ions / negative reactant
ion(s) and positive catalyst / Mn2+ / Mn3+
M5 4 Mn2+ + MnO4
– + 8 H+ → 5 Mn3+ + 4 H2O
M6 2 Mn3+ + C2O4
2– → 2 Mn2+ + 2 CO2
Platinum acts as a heterogeneous catalyst in the reaction between ammonia and
oxygen. It provides an alternative reaction route with a lower activation energy.
Describe the stages of this alternative route.
Reactant(s) adsorbed onto the
platinum surface
Reaction occurs on the surface (bond making and breaking)
Product desorbed
