Transition Metals: Catalysts

Question 1

What kind of catalysts can transition metals and their compounds act as?

Answer 1

• Heterogenous catalysts
• Homogenous catalysts

Question 2

Why are transition metals good catalysts?

Answer 2

• TMs can exist in variable oxidation states so can provide alternative pathways with lower E_a easily

Question 3

What is a heterogenous catalyst?

Answer 3

• A heterogeneous catalyst is in a different phase from the reactants
• Reaction occurs at active sites on the surface

Question 4

What does the efficiency of heterogenous catalysts depend on?

Answer 4

• Strength of adsorption

• How strong reactants stick to surface of TM (compound)

• Strength of desorption

• How well products unstick to surface of TM (compound)

Question 5

What is a homogenous catalyst?

Answer 5

• A homogenous catalyst is in the same phase as the reactants

Question 6

What is used to minimise the cost of heterogenous catalysts?

Answer 6

• Support medium
• Used to increase surface area of catalyst

Question 7

What can reduce the surface area of a catalyst? How can this be tackled?

Answer 7

• Catalyst poisoning
• Impurities may adsorb onto surface of heterogenous catalysts and block active sites from reactants
• Purifying reactants

Question 8

Which heterogenous catalysts must you know about?

Answer 8

• Vanadium(V) oxide( V₂O₅) in the Contact process
• Fe in the Haber process

Question 9

Give the overall equation for the Contact Process

Answer 9

• SO_{2 (g)} + ½O_{2 (g)} → SO_{3 (g)}
• Above → is V₂O_5(s) catalyst

Question 10

Give all of the equations involved in the Contact Process and the changes in oxidation states of vanadium

Answer 10

• V₂O₅ + SO₂ → V₂O₄ + SO₃
• Vanadium reduced from +5 to +4
• ½O₂ + V₂O₄ → V₂O₅
• Vanadium oxidised from +4 to +5
• SO_{2 (g)} + ½O_{2 (g)} → SO_{3 (g)}

Question 11

Give the equation for the Haber process

Answer 11

• N_{2 (g)} + 3H_{2 (g)} → 2NH_{3 (g)}
• Above → is Fe_(s)

Question 12

In reactions with homogenous catalysts, what must reactions proceed through?

Answer 12

• Reactions must proceed through an intermediate species where the TM ion temporarily changes oxidation state
• Reactions combine with catalyst to make an intermediate species, which then reacts to form the products and reform the catalyst

Question 13

Which homogenous catalysts must you know about?

Answer 13

• Fe²⁺ catalysing reaction between S₂O₈^2- (peroxydisulfate) and I^-
• Mn²⁺ autocatalysing reaction between MnO^₄- and C₂O₄^2-

Question 14

Give the overall reaction between S₂O₈^2- and I^-

Answer 14

• S₂O₈^2- _(aq) + 2I^- _(aq) → I_{2 (aq)} + 2SO₄^2- _(aq)

Question 15

Why does the reaction between S₂O₈^2- and I^- occur slowly? How does iron(II) or iron (II) act as a catalyst?

Answer 15

• Both ions are negatively charged so repel
• Low collision frequency and high E_a
• Positively charged iron ions are attracted to the negatively charged reactants resulting in a reaction with lower E_a

Question 16

Give all the equations involved in the reaction between S₂O₈^2- and I^-

Answer 16

• S₂O₈^2- _(aq) + 2Fe²⁺ _(aq) → 2Fe³⁺ _(aq) + 2SO₄^2- _(aq)
• 2I^‒ _(aq) + 2Fe³⁺ _(aq) → I_{2 (aq)} + 2Fe²⁺ _(aq)

_- S₂O₈^2–_(aq) + 2I^–_(aq) → 2SO₄^2–_(aq) + I_2(aq)

Question 17

What does autocatalysis mean?

Answer 17

• When one of the products of a reaction acts as a catalyst for the reaction

Question 18

How does Mn²⁺ autocatalyse the reaction between MnO₄^- and C₂O₄^2-?

Answer 18

• Mn²⁺ isn’t present at start of reaction but after it’s produced it acts as a catalyst

Question 19

Give the overall equation for the reaction between MnO₄^‒ and C₂O₄^2‒

Answer 19

• 2MnO₄^‒_(aq) + 16H⁺_(aq) + 5C₂O₄^2‒_(aq) → 2Mn²⁺_(aq) + 8H₂O_(l) + 10CO_2(g)

Question 20

Give all the reactions involved in the reaction between MnO₄^‒ and C₂O₄^2‒

Answer 20

• MnO₄^–_(aq) + 8H⁺_(aq) + 5e^– → Mn²⁺_(aq) + 4H₂O_(l)
• C₂O₄^2–_(aq) → 2CO_2(g) + 2e^–
• 2MnO₄^‒_(aq) + 16H⁺_(aq) + 5C₂O₄^2‒_(aq) → 2Mn²⁺_(aq) + 8H₂O_(l) + 10CO_2(g)

Question 21

Draw a concentration-time graph for the reaction between MnO₄^‒ and C₂O₄^2‒ and explain its shape

Answer 21

• Curve starts with small gradient (low rate)
• Negative ions collide so E_a high
• Curve gets steeper
• Because autocatalyst (Mn²⁺) is formed
• Curve levels out approaching time axis
• Because MnO₄^- ions used up