Transition Metals As Catalysts
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Define a heterogeneous catalyst (ctlyst. diff. phscl. phse. rctnts.)
A catalyst which is in a different physical phase from the reactants
Which type of transition metal catalysis involves active sites?
Heterogeneous transition metal catalysis
What is the active of a heterogeneous catalyst? (ste. sfce. htrgns. ctlyst. rctnt. adsb. + rctn. ocrs.)
A site on the surface of a heterogeneous catalyst where reactants adsorb and the reaction occurs
How can the surface area of a solid catalyst be increased? (trn. pwdr./cot. onto. sprt. mdm.)
By turning it into a powder or by coating it onto a support medium
Why is increasing surface area important in heterogeneous catalysis? (inc. nmbr. AS - inc. ROR + red. csts. - lss. ctlyst. rqrd.)
Increases the number of active sites, increases the rate of reaction and reduces costs as less catalyst is required
What catalyst is used in the Contact Process?
Vanadium (V) oxide (V₂O₅)
What type of catalyst is vanadium (V) oxide in the Contact Process, and how does it work? (prvds. act. sts. + use. vrble. ox. sts. - prvd. rctn. pthwys.)
A heterogeneous catalyst providing active sites and uses its variable oxidation states to provide an alternative reaction pathways
What reaction occurs in the Contact Process?
Conversion of sulfur dioxide (SO₂) to sulfur trioxide (SO₃)
Explain the importance of variable oxidation states in transition metal catalysts (TM - chnge. ox. stes. - alw. rct. w/ diff. spcs. - sep. stps, frm. intrmdte. spcs. - rgen. end. rctn.)
Transition metals can change oxidation states allowing them to react with different species in separate steps. They form intermediate species which are regenerated at the end of the reaction
Why do variable oxidation states lower activation energy? (ctlyst. rct. mltple. stps. - stp. = lwr. EA. - unctly. rctn. prvd. alt. rctn. pthway.)
The catalysts react in multiple steps, each step has a lower activation energy than the uncatalysed reaction providing an alternative reaction pathway
Write the first step V₂O₅ in the Contact Process
V₂O₅ + SO₂ → V₂O₄ + SO₃
Write the second step in the Contact Process of V₂O₅
V₂O₄ + ½O₂ → V₂O₅
What happens to sulfur’s oxidation state in the Contact Process?
Sulfur is oxidised from +4 in SO₂ to +6 in SO₃
Why is SO₃ not reacted directly with water to produce sulfuric acid and produced in two steps? (rctn. too exo. + difclt. cntrl. - 2 stp. rctn. alw. sfr. + >esly. cntrld. rctn.)
The reaction is too exothermic and difficult to control, so the two step reaction allows for a safer and more easily controlled reaction
What is the first step in converting SO₃ into sulfuric acid?
SO₃(g) + H₂SO₄(l) → H₂S₂O₇(l)
What is the second step in forming sulfuric acid?
H₂S₂O₇(l) + H₂O(l) → 2H₂SO₄(l)
What is catalyst poisoning? (ocrs. imp. bnd. act sts. + prvnt. ctlyst. frm. fnctng.)
Occurs when impurities bind to active sites and prevent the catalyst from functioning
How is the iron catalyst poisoned in the Haber process?
Sulfur impurities form iron sulfide on the surface
What reaction is catalysed by Fe²⁺ ions?
Reaction between iodide ions (I⁻) and peroxodisulfate ions (S₂O₈²⁻)
Why is the activation energy lower when Fe²⁺/Fe³⁺ ions catalyse the reaction between S₂O₈²⁻ and I⁻ ? (opp. chrgs. atrct. - specific ones? - red. rplsn. - ult. lwrng. EA - btwn S + I)
The opposite charges attract, with Fe²⁺ attracts S₂O₈²⁻ and Fe³⁺ attracting I⁻, this reduces the repulsion, ultimately lowering the activation energy between S₂O₈²⁻ and I⁻ directly
What are the first and second steps of the mechanism between I⁻ and S₂O₈²⁻ catalysed by Fe²⁺/Fe³⁺?
• S₂O₈²⁻ + 2Fe²⁺ → 2Fe³⁺ + 2SO₄²⁻ • 2Fe³⁺ + 2I⁻ → 2Fe²⁺ + I₂
What is an autocatalysis? (ctlyst. prod. rctn. ctylsed.)
A catalyst that is one of the products of the reaction it catalyses
Describe the rate graph for an autocatalytic reaction (rte. inc. slwly. ctlyst. frms, rte. inc. rpdly. >ctylst. prod. + slwly. dec. rctnts. used.)
The rate initially increases slowly as the catalyst forms. The rate then increases rapidly as more catalyst is produced and then slowly decreases as the reactants are used up
How does Mn²⁺ autocatalyse the reaction? (Mn²⁺ prod. drng. rctn, Mn²⁺ rcts. w/ MnO₄⁻ - frm. Mn³⁺ intmrdts, Mn³⁺ ox. C₂O₄²⁻ - CO₂ - alw. Mn²⁺ regen.)
Mn²⁺ is produced during the reaction. It then reacts with MnO₄⁻ to form Mn³⁺ intermediates. The Mn³⁺ ion oxidise C₂O₄²⁻ to CO₂, allowing Mn²⁺ is regenerated