Catalysis Flashcards
What is a catalyst?
speeds up a chemical reaction by providing an alternate reaction pathway with lower activation energy
Why are there more successful collisions in the presence of a catalyst?
a greater proportion of reactant particles will have energy that is above the lowered activation energy
-increase in percentage of successful collisions, so increase in rate of reaction
What is a heterogeneous catalyst?
a catalyst that acts in a different phase to the reactants. occurs on surfaces
What is a homogeneous catalyst?
a catalyst that acts in the same phase to the reactants. occurs through an intermediate species
How do heterogeneous catalysts work?
-surface of solid catalyst
-when a reactant approaches the solid there is a tendency for them to react, forming weak temporary bonds - adsorption
1- adsorption of a reactant
2- reaction
3- desorption of a product
Name three things that could happen as a result of adsorption
-reactant more likely to collide with another reactant, increasing collision frequency
-reactant held in a more favourable configuration so activation energy is decreased
-reactant may undergo internal bond breaking/rearrangement, so the reactive fragments react more readily/AE is lowered
What is the heterogeneous catalyst in the Haber process?
Fe
What is the heterogeneous catalyst in the Contact process?
V2O5
What happens when the adsorption is too strong?
-keeps molecules immobile
-fails to regenerate active sites
-poisoned by the product
What happens when the adsorption is too weak?
reactants aren’t encouraged to come together and aren’t given a good opportunity to react with each other
How is the cost of catalysts reduced?
expensive catalysts are spread very thinly onto an inert support medium in order to increase the surface to mass ratio
What is the impact of impurities on a catalyst?
blocks the active sites, reducing the efficiency of the catalyst and increases the cost of the process
What poisons the iron catalyst in the Haber process?
sulphur impurities
What is the equation for the Contact process?
2SO2(g) + O2(g) ⇌ 2SO3(g)
Explain the Contact process
1) SO2 diffuses onto the catalyst surface and a redox reaction occurs
V2O5(s) + SO2(g) –> V2O4(s) + SO3(g)
2) vanadium oxide(IV) formed then reacts with oxygen to regenerate vanadium (V) oxide
2V2O4(s) + O2(g) –> 2V2O5(s)
What is the overall equation for the oxidation of iodide ions by peroxodisulphate ions and the two steps that can occur in any order?
overall:
S2O8 2- + 2I- –> 2SO4 2- + I2
two steps:
2Fe3+ + 2I- –> 2Fe2+ + I2
2Fe3+ + S2O8 2- –> 2Fe3+ + 2SO4 2-
What is autocatalysis?
where the product of a particular reaction catalyses the reaction
What is the overall equation for the reaction between ethanedioate ions and manganate(VII) ions?
2MnO4- + 16H+ + 5C2O4 2- –> 2Mn2+ + 10CO2 + 8H2O
Why is the reaction between C2O4 2- and MnO4 - so slow?
both negative ions and so they repel
Describe how autocatalysis is involved in the reaction between C2O4 2- and MnO4 -?
-Mn2+ ions act as an autocatalyst
-they react readily with the MnO4- ions to make Mn3+ ions
4Mn2+ + MnO4- + 8H+ –> 5Mn3+ + 4H2O
-these can then react with C2O4 2- ions to regenerate Mn2+ ions
2Mn3+ + C2O4 2- –> 2CO2 + 2Mn2+
Why does the colour change from purple to colourless not happen straightaway?
-rate of reaction is initially slow
-as some Mn2+ ions start to form, further addition of the potassium manganate(VII) leads to rapid decolourisation
-until the light pink end point is reached
What is the half equation for the reaction of potassium manganate ions?
MnO4- + 8H+ + 5e- –> Mn2+ + 4H2O
What makes an appropriate acid?
-strong -goes brown if there is not an excess of H+
-not an oxidising agent
-not a reducing agent
DILUTE SULPHURIC ACID
How do you do redox titration calculations?
-calc the moles of manganate or dichromate using mol = conc x vol
-use molar ratios for finding out moles of Fe2+ (x5 for Mn, x6 for Cr)
-calc mass of Fe2+ present - 55.8 x moles Fe2+
-calc % by mass by doing mass Fe2+ / mass of sample x100
