Transition Metals Part 3 Flashcards
Catalyst
Substance which increases rate of a chemical reaction without being changed in chemical composition/amount
What do catalysts provide
An alternative reaction route with a lower activation energy
What is not changed in presence of a catalyst
Delta H
Why does rate of reaction increase in presence of a catalyst (MB)
Shaded area bigger- more molecules have energy greater than or equal to activation energy- more successful collisions
Why can transition metals act as catalysts in a wide range of reactions
They have variable oxidation states
Heterogeneous catalyst
A catalyst in a different state/phase to the reactants
Examples of heterogeneous catalysts
- iron in Haber processs
- vanadium (V) oxide in contact process
Why is a very thin layer of metal used on a support medium when transition metals act as catalysts
- increases SA also less metal used
- molecules absorb, bonds weaken, lowers EA
- reaction proceeds, products desorb, leaving space for new reactants
How to heterogeneous catalysts work
By adsorbing reactants to active sites on catalyst surface
How can heterogeneous catalysts be poisoned
Impurities in reactants May also bind to active sites and block reactants
Why does catalyst being poisoned reduce efficiency of catalyst
It blocks active sites and reduced SA available for reaction- slowing it down
How can poisoning of catalyst be minimised
Purifying the reactants
What is the contact process used industrially for
To manufacture sulphuric acid
What acts as a heterogeneous catalyst in contact process
Vanadium (V) oxide
Step 1 contact process;
Sulfur dioxide diffuses onto active sites and redox reaction occurs
Reaction for oxidation of sulfur dioxide to sulfur trioxide by vanadium 5 oxide
V2O5 + 5O2 > V2O4 + SO3
Step 2 contact process
Vanadium 4 oxide reacts with oxygen to regenerate vanadium 5 oxide
Equation vanadium for oxide and oxygen to vanadium 5 oxide
V2O4 > 1/2O2 > V2O5
Overall equation contact process
SO2 + 1/2 O2> SO3
Homogeneous catalyst is
A catalyst in the same state/phase as the reactants
Why may reaction between peroxodisulfate ions and iodide ions be slow
Reactants have same charge and so are less likely to collide
What may be use to catalyst me reaction of peroxodisulfate ions with iodide ions
Iron 2/3
Step 1 catalysis iodide and peroxodisulfate with iron 2
2Fe2+ + S2O82- > 2SO42- + 2Fe3+
Step 2 catalysis iodide and peroxodisulfate with iron 2
2Fe3+ + 2I- > I2 + 2Fe2+
Intermediate species in catalysis of iodide peroxodisulfate with iron 2
Iron 3
Why is iron 3 just as effective in catalysing this reaction as iron 2
Step 2 could occur before step 1
Why is reaction between manganate 7 ions and ethanedioate ions initially very slow
Many reactants, both negative
When does the rate of the reaction between manganate 7 and ehtanedioate ions increase rapidly
As conc of Mn2+ produced increases
What does the Mn2+ act as in reaction between manganate 7 and ethanedioate
An auto catalyst
Autocatalysis
When the catalyst for the reaction is a reaction product
Step 1 autocatalysis manganate 7 and ethanedioate
2MnO4- + 5C2O42- + 16H+ > 2Mn2+ + 8H2O + 10CO2
Step 2 autocatalysis
4Mn2+ + MnO4- + 8H+ > 5Mn3+ + 4H2O
Step 3 autocatalysis
2Mn3+ + C2O42- > 2CO2 + 2Mn2+
Intermediate species in autocatalysis
mn3+
