AP Chemistry

Unit 5.11

CATALYSIS

Unit 5.11

Enduring Understanding:

• The speed at which a reaction occurs can be influenced by a catalyst

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Learning Objective:

• Explain the relationship between the effect of a catalyst on a reaction and changes in the reaction mechanism

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Catalysts

• Catalysts increase reaction rates but are not consumed by the reaction
• They work in one of two ways:
• Alter the rate of a reaction by increasing the frequency of effective collisions
• Altering the chemical pathway to one that requires less energy
• Especially for the rate determining step
• Two types:
• Heterogeneous catalysts
• Homogeneous catalysts

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Heterogeneous Catalysis

• In heterogeneous catalysis the catalyst (heterogeneous catalyst) exists in a different phase from the reactants and products in the reaction being catalyzed
• Most heterogeneous catalysts are solids with gaseous reactants
• This means that the catalyst cannot mix with the reactants
• So the rate-limiting step occurs at the solid surface
• Also called surface catalysis

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A(g) + B(g) D(g)

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C(s)

Heterogeneous Catalysis

• In general, heterogeneous catalysis speeds up the chemical reaction by altering the orientation of the molecules
• Or by providing an alternate reaction pathway that requires less E_A
• This is accomplished by providing a surface (usually solid) that absorbs (binds) with the reactants
• The reactants diffuse on the surface, come together, react, and the created product desorbs from the surface into the gas phase

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Heterogeneous Catalysis

• The detailed mechanisms of heterogeneous catalysts isn’t fully understood
• It appears that certain sites on the catalyst, called active sites, are able to weaken or break bonds in reactant molecules
• Important examples include the Haber process (synthesis of NH₃ from N₂ and H₂) and within catalytic converters

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Heterogeneous Catalysis

• For example:
• Hydrogenation of ethane (C₂H₄) by a metal catalyst, Pt
• Both the hydrogen and ethane bond weakly to the metal’s surface (this process is called absorption)
• The H₂ dissociate to H and bond to Pt
• These H atoms can move across the metal surface and combine with ethane to complete the reaction
• The E_A for this reaction is ΔH = -160 kJ/mol
• If this reaction occurred without a metal surface the E_A would be ΔH = 436 kJ/mol

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Heterogeneous Catalysis

Homogeneous Catalysis

• In homogeneous catalysis the catalyst (homogeneous catalyst) exists in the same phase as the reactants and products in the reaction being catalyzed
• The number of collisions between the reactants and the catalyst is at a maximum because the catalyst can mix and disperse evenly throughout the reaction mixture

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A(g) + B(g) D(g)

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C(g)

Homogeneous Catalysis

• Typically a homogeneous catalyst provides a different chemical pathway, lowering the E_A for the rate determining step

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Homogeneous Catalysis

• The catalyst is used in one (or more) elementary step but is regenerated by the end of the reaction
• Not consumed; can be used to catalyze more reactions
• Enzymes are a type of homogeneous catalyst
• They bond to one or more reactants allowing for a more favorable orientation or a lower amount of energy to react
• This creates a new intermediate that wasn’t in the uncatalyzed pathway

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Homogeneous Catalysis

• Acid-base catalysis
• A new intermediate is created through the formation of covalent bonds between the catalyst and reactant
• A proton (H⁺) is transferred between the species
• Results in new intermediates and new elementary steps that aren’t in the uncatalyzed reaction

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Practice: I Do

1. The rate of decomposition of hydrogen peroxide can be increased by adding a homogeneous catalyst. The reaction mechanism occurs in two steps as shown below:

Step 1: H₂O₂ (aq) + I^- (aq) → IO^- (aq) + H₂O (l)

Step 2: H₂O₂ (aq) + IO^- (aq) → I^- (aq) + H₂O (l) + O₂ (g)

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• Write the overall reaction
• Identify the catalyst
• Identify the intermediate(s)

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Practice: I Do

• The rate of decomposition of hydrogen peroxide can be increased by adding a homogeneous catalyst. The reaction mechanism occurs in two steps as shown below:

Step 1: H₂O₂ (aq) + I^- (aq) → IO^- (aq) + H₂O (l)

Step 2: H₂O₂ (aq) + IO^- (aq) → I^- (aq) + H₂O (l) + O₂ (g)

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• Write the overall reaction

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Overall reaction: 2H₂O₂ (aq) → 2H₂O (l) + O₂ (g)

Practice: I Do

• The rate of decomposition of hydrogen peroxide can be increased by adding a homogeneous catalyst. The reaction mechanism occurs in two steps as shown below:

Step 1: H₂O₂ (aq) + I^- (aq) → IO^- (aq) + H₂O (l)

Step 2: H₂O₂ (aq) + IO^- (aq) → I^- (aq) + H₂O (l) + O₂ (g)

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b. Identify the catalyst

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Catalyst: Iodide ion (I^-)

Practice: I Do

• The rate of decomposition of hydrogen peroxide can be increased by adding a homogeneous catalyst. The reaction mechanism occurs in two steps as shown below:

Step 1: H₂O₂ (aq) + I^- (aq) → IO^- (aq) + H₂O (l)

Step 2: H₂O₂ (aq) + IO^- (aq) → I^- (aq) + H₂O (l) + O₂ (g)

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c. Identify the intermediate(s)

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Intermediate: IO^- (hypoiodite ion)