This question is about catalysis - AQA - A-Level Chemistry - Question 5 - 2022 - Paper 3
Question 5
This question is about catalysis.
Zeolites are used as heterogeneous catalysts in the catalytic cracking of alkanes.
Tetradecane (C14H30) can be cracked to form oc... show full transcript
Worked Solution & Example Answer:This question is about catalysis - AQA - A-Level Chemistry - Question 5 - 2022 - Paper 3
Step 1
Give an equation for this reaction.
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Answer
The cracking of tetradecane can be represented by the following equation:
C14H30→C8H18+C6H12
This shows tetradecane cracking into octane (C8H18) and a cycloalkane (C6H12).
Step 2
State the meaning of the term heterogeneous.
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The term 'heterogeneous' refers to a catalyst that is in a different phase (state) to the reactants. In this case, the solid zeolite catalyst is in a different phase than the gaseous alkanes.
Step 3
Explain the meaning of the term autocatalyst.
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An autocatalyst is a product of the reaction that also acts as a catalyst. It speeds up the reaction after its formation, meaning that as the reaction proceeds, the rate of reaction increases due to the presence of this product.
Step 4
Explain, using equations where appropriate, why the reaction is slow at first and then goes quickly.
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The reaction starts slowly because the initial reactants (ethanoate ions and manganese ions) are less reactive. As the reaction progresses, the concentration of the product (Mn²⁺ ions) increases, which acts as an autocatalyst:
Initially, the reaction can be represented as:
2MnO4−+5C2H3O2−+16H+→2Mn2++10CO2+8H2O
As Mn²⁺ accumulates, it catalyzes further reduction of MnO₄⁻, speeding up the reaction.
Thus, the introduction of additional potassium manganate(VII) later enhances the reaction rate significantly.
Step 5
Use the electrode potential data to suggest how Co²⁺ catalyses the reaction.
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Answer
Co²⁺ ions can act as catalysts because they participate in redox reactions. The relevant half-reactions are:
For Co²⁺:
Co2++2e−↔Co
The standard electrode potential shows that Co²⁺ can easily get reduced, providing a pathway for the reaction that requires lower activation energy, thus facilitating the reaction between peroxodisulfate and iodide ions.
